S-expressions: Difference between revisions
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</pre> |
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=={{header|Nim}}== |
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<lang nim>import strutils |
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const Input = """ |
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((data "quoted data" 123 4.5) |
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(data (!@# (4.5) "(more" "data)"))) |
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""" |
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type |
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TokenKind = enum |
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tokInt, tokFloat, tokString, tokIdent |
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tokLPar, tokRPar |
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tokEnd |
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Token = object |
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case kind: TokenKind |
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of tokString: stringVal: string |
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of tokInt: intVal: int |
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of tokFloat: floatVal: float |
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of tokIdent: ident: string |
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else: discard |
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proc lex(input: string): seq[Token] = |
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var pos = 0 |
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template current: char = |
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if pos < input.len: input[pos] |
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else: '\x00' |
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while pos < input.len: |
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case current |
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of ';': |
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inc(pos) |
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while current notin {'\r', '\n'}: |
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inc(pos) |
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if current == '\r': inc(pos) |
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if current == '\n': inc(pos) |
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of '(': inc(pos); result.add(Token(kind: tokLPar)) |
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of ')': inc(pos); result.add(Token(kind: tokRPar)) |
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of '0'..'9': |
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var |
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num = "" |
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isFloat = false |
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while current in Digits: |
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num.add(current) |
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inc(pos) |
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if current == '.': |
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num.add(current) |
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isFloat = true |
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inc(pos) |
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while current in Digits: |
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num.add(current) |
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inc(pos) |
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result.add(if isFloat: Token(kind: tokFloat, floatVal: parseFloat(num)) |
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else: Token(kind: tokInt, intVal: parseInt(num))) |
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of ' ', '\t', '\n', '\r': inc(pos) |
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of '"': |
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var str = "" |
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inc(pos) |
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while current != '"': |
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str.add(current) |
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inc(pos) |
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inc(pos) |
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result.add(Token(kind: tokString, stringVal: str)) |
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else: |
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const BannedChars = {' ', '\t', '"', '(', ')', ';'} |
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var ident = "" |
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while current notin BannedChars: |
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ident.add(current) |
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inc(pos) |
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result.add(Token(kind: tokIdent, ident: ident)) |
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result.add(Token(kind: tokEnd)) |
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type |
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SExprKind = enum |
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seInt, seFloat, seString, seIdent, seList |
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SExpr = ref object |
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case kind: SExprKind |
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of seInt: intVal: int |
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of seFloat: floatVal: float |
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of seString: stringVal: string |
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of seIdent: ident: string |
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of seList: children: seq[SExpr] |
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ParseError = object of CatchableError |
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proc `$`*(se: SExpr): string = |
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case se.kind |
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of seInt: result = $se.intVal |
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of seFloat: result = $se.floatVal |
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of seString: result = '"' & se.stringVal & '"' |
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of seIdent: result = se.ident |
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of seList: |
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result = "(" |
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for i, ex in se.children: |
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if ex.kind == seList and ex.children.len > 1: |
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result.add("\n") |
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result.add(indent($ex, 2)) |
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else: |
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if i > 0: |
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result.add(" ") |
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result.add($ex) |
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result.add(")") |
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var |
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tokens = lex(Input) |
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pos = 0 |
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template current: Token = |
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if pos < tokens.len: tokens[pos] |
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else: Token(kind: tokEnd) |
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proc parseInt(token: Token): SExpr = |
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result = SExpr(kind: seInt, intVal: token.intVal) |
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proc parseFloat(token: Token): SExpr = |
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result = SExpr(kind: seFloat, floatVal: token.floatVal) |
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proc parseString(token: Token): SExpr = |
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result = SExpr(kind: seString, stringVal: token.stringVal) |
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proc parseIdent(token: Token): SExpr = |
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result = SExpr(kind: seIdent, ident: token.ident) |
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proc parse(): SExpr |
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proc parseList(): SExpr = |
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result = SExpr(kind: seList) |
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while current.kind notin {tokRPar, tokEnd}: |
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result.children.add(parse()) |
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if current.kind == tokEnd: |
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raise newException(ParseError, "Missing right paren ')'") |
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else: |
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inc(pos) |
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proc parse(): SExpr = |
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var token = current |
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inc(pos) |
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result = |
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case token.kind |
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of tokInt: parseInt(token) |
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of tokFloat: parseFloat(token) |
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of tokString: parseString(token) |
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of tokIdent: parseIdent(token) |
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of tokLPar: parseList() |
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else: nil |
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echo parse()</lang> |
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{{out}} |
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<pre> |
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( |
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(data "quoted data" 123 4.5) |
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(data |
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(!@# (4.5) "(more" "data)"))) |
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</pre> |
</pre> |
||
Revision as of 18:19, 27 July 2019
You are encouraged to solve this task according to the task description, using any language you may know.
S-Expressions are one convenient way to parse and store data.
- Task
Write a simple reader and writer for S-Expressions that handles quoted and unquoted strings, integers and floats.
The reader should read a single but nested S-Expression from a string and store it in a suitable datastructure (list, array, etc).
Newlines and other whitespace may be ignored unless contained within a quoted string.
“()” inside quoted strings are not interpreted, but treated as part of the string.
Handling escaped quotes inside a string is optional; thus “(foo"bar)” maybe treated as a string “foo"bar”, or as an error.
For this, the reader need not recognize “\” for escaping, but should, in addition, recognize numbers if the language has appropriate datatypes.
Languages that support it may treat unquoted strings as symbols.
Note that with the exception of “()"” (“\” if escaping is supported) and whitespace there are no special characters. Anything else is allowed without quotes.
The reader should be able to read the following input <lang lisp>((data "quoted data" 123 4.5)
(data (!@# (4.5) "(more" "data)")))</lang>
and turn it into a native datastructure. (see the Pike, Python and Ruby implementations for examples of native data structures.)
The writer should be able to take the produced list and turn it into a new S-Expression. Strings that don't contain whitespace or parentheses () don't need to be quoted in the resulting S-Expression, but as a simplification, any string may be quoted.
- Extra Credit
Let the writer produce pretty printed output with indenting and line-breaks.
Ada
Uses Ada 2005 (Ada.Containers).
Specification of package S_Expr:
<lang Ada>with Ada.Strings.Unbounded; private with Ada.Containers.Indefinite_Vectors;
generic
with procedure Print_Line(Indention: Natural; Line: String);
package S_Expr is
function "-"(S: String) return Ada.Strings.Unbounded.Unbounded_String
renames Ada.Strings.Unbounded.To_Unbounded_String;
function "+"(U: Ada.Strings.Unbounded.Unbounded_String) return String
renames Ada.Strings.Unbounded.To_String;
type Empty_Data is tagged null record; subtype Data is Empty_Data'Class; procedure Print(This: Empty_Data; Indention: Natural); -- any object form class Data knows how to print itself -- objects of class data are either List of Data or Atomic -- atomic objects hold either an integer or a float or a string
type List_Of_Data is new Empty_Data with private; overriding procedure Print(This: List_Of_Data; Indention: Natural); function First(This: List_Of_Data) return Data; function Rest(This: List_Of_Data) return List_Of_Data; function Empty(This: List_Of_Data) return Boolean;
type Atomic is new Empty_Data with null record;
type Str_Data is new Atomic with record
Value: Ada.Strings.Unbounded.Unbounded_String;
Quoted: Boolean := False;
end record;
overriding procedure Print(This: Str_Data; Indention: Natural);
type Int_Data is new Atomic with record
Value: Integer;
end record;
overriding procedure Print(This: Int_Data; Indention: Natural);
type Flt_Data is new Atomic with record
Value: Float;
end record;
overriding procedure Print(This: Flt_Data; Indention: Natural);
private
package Vectors is new Ada.Containers.Indefinite_Vectors
(Index_Type => Positive,
Element_Type => Data);
type List_Of_Data is new Empty_Data with record
Values: Vectors.Vector;
end record;
end S_Expr;</lang>
The implementation of S_Expr:
<lang Ada>with Ada.Integer_Text_IO, Ada.Float_Text_IO;
package body S_Expr is
function First(This: List_Of_Data) return Data is
begin
return This.Values.First_Element;
end First;
function Rest(This: List_Of_Data) return List_Of_Data is
List: List_Of_Data := This;
begin
List.Values.Delete_First;
return List;
end Rest;
function Empty(This: List_Of_Data) return Boolean is
begin
return This.Values.Is_Empty;
end Empty;
procedure Print(This: Empty_Data; Indention: Natural) is
begin
Print_Line(Indention, "");
end Print;
procedure Print(This: Int_Data; Indention: Natural) is
begin
Print_Line(Indention, Integer'Image(This.Value));
end Print;
procedure Print(This: Flt_Data; Indention: Natural) is
begin
Print_Line(Indention, Float'Image(This.Value));
end Print;
procedure Print(This: Str_Data; Indention: Natural) is
begin
if This.Quoted then
Print_Line(Indention, """" & (+This.Value) & """");
else
Print_Line(Indention, +This.Value);
end if;
end Print;
procedure Print(This: List_Of_Data; Indention: Natural) is
begin
Print_Line(Indention, " ( ");
for I in This.Values.First_Index .. This.Values.Last_Index loop
This.Values.Element(I).Print(Indention + 1);
end loop;
Print_Line(Indention, " ) ");
end Print;
end S_Expr;</lang>
Specification and Implementation of S_Expr.Parser (a child package of S_Expr):
<lang Ada>generic -- child of a generic package must be a generic unit package S_Expr.Parser is
function Parse(Input: String) return List_Of_Data; -- the result of a parse process is always a list of expressions
end S_Expr.Parser;</lang>
<lang Ada>with Ada.Integer_Text_IO, Ada.Float_Text_IO;
package body S_Expr.Parser is
function Parse(Input: String) return List_Of_Data is
procedure First_Token(S: String;
Start_Of_Token, End_Of_Token: out Positive) is
begin
Start_Of_Token := S'First;
while Start_Of_Token <= S'Last and then S(Start_Of_Token) = ' ' loop
Start_Of_Token := Start_Of_Token + 1; -- skip spaces
end loop;
if Start_Of_Token > S'Last then
End_Of_Token := Start_Of_Token - 1;
-- S(Start_Of_Token .. End_Of_Token) is the empty string
elsif (S(Start_Of_Token) = '(') or (S(Start_Of_Token) = ')') then
End_OF_Token := Start_Of_Token; -- the bracket is the token
elsif S(Start_Of_Token) = '"' then -- " -- begin quoted string
End_Of_Token := Start_Of_Token + 1;
while S(End_Of_Token) /= '"' loop -- " -- search for closing bracket
End_Of_Token := End_Of_Token + 1;
end loop; -- raises Constraint_Error if closing bracket not found
else -- Token is some kind of string
End_Of_Token := Start_Of_Token;
while End_Of_Token < S'Last and then
((S(End_Of_Token+1) /= ' ') and (S(End_Of_Token+1) /= '(') and
(S(End_Of_Token+1) /= ')') and (S(End_Of_Token+1) /= '"')) loop -- "
End_Of_Token := End_Of_Token + 1;
end loop;
end if;
end First_Token;
procedure To_Int(Token: String; I: out Integer; Found: out Boolean) is
Last: Positive;
begin
Ada.Integer_Text_IO.Get(Token, I, Last);
Found := Last = Token'Last;
exception
when others => Found := False;
end To_Int;
procedure To_Flt(Token: String; F: out Float; Found: out Boolean) is
Last: Positive;
begin
Ada.Float_Text_IO.Get(Token, F, Last);
Found := Last = Token'Last;
exception
when others => Found := False;
end To_Flt;
function Quoted_String(Token: String) return Boolean is
begin
return
Token'Length >= 2 and then Token(Token'First)='"' -- "
and then Token(Token'Last) ='"'; -- "
end Quoted_String;
Start, Stop: Positive;
procedure Recursive_Parse(This: in out List_Of_Data) is
Found: Boolean;
Flt: Flt_Data;
Int: Int_Data;
Str: Str_Data;
Lst: List_Of_Data;
begin
while Input(Start .. Stop) /= "" loop
if Input(Start .. Stop) = ")" then
return;
elsif Input(Start .. Stop) = "(" then
First_Token(Input(Stop+1 .. Input'Last), Start, Stop);
Recursive_Parse(Lst);
This.Values.Append(Lst);
else
To_Int(Input(Start .. Stop), Int.Value, Found);
if Found then
This.Values.Append(Int);
else
To_Flt(Input(Start .. Stop), Flt.Value, Found);
if Found then
This.Values.Append(Flt);
else
if Quoted_String(Input(Start .. Stop)) then
Str.Value := -Input(Start+1 .. Stop-1);
Str.Quoted := True;
else
Str.Value := -Input(Start .. Stop);
Str.Quoted := False;
end if;
This.Values.Append(Str);
end if;
end if;
end if;
First_Token(Input(Stop+1 .. Input'Last), Start, Stop);
end loop;
end Recursive_Parse;
L: List_Of_Data;
begin
First_Token(Input, Start, Stop);
Recursive_Parse(L);
return L;
end Parse;
end S_Expr.Parser;</lang>
The main program Test_S_Expr:
<lang Ada>with S_Expr.Parser, Ada.Text_IO;
procedure Test_S_Expr is
procedure Put_Line(Indention: Natural; Line: String) is
begin
for I in 1 .. 3*Indention loop
Ada.Text_IO.Put(" ");
end loop;
Ada.Text_IO.Put_Line(Line);
end Put_Line;
package S_Exp is new S_Expr(Put_Line); package S_Par is new S_Exp.Parser;
Input: String := "((data ""quoted data"" 123 4.5)" &
"(data (!@# (4.5) ""(more"" ""data)"")))";
Expression_List: S_Exp.List_Of_Data := S_Par.Parse(Input);
begin
Expression_List.First.Print(Indention => 0); -- Parse will output a list of S-Expressions. We need the first Expression.
end Test_S_Expr;</lang>
- Output:
(
(
data
"quoted data"
123
4.50000E+00
)
(
data
"quoted data"
123
4.50000E+00
data
(
!@#
(
4.50000E+00
)
"(more"
"data)"
)
)
)
ALGOL 68
<lang algol68># S-Expressions # CHAR nl = REPR 10;
- mode representing an S-expression #
MODE SEXPR = STRUCT( UNION( VOID, STRING, REF SEXPR ) element, REF SEXPR next );
- creates an initialises an SEXPR #
PROC new s expr = REF SEXPR: HEAP SEXPR := ( EMPTY, NIL );
- reports an error #
PROC error = ( STRING msg )VOID: print( ( "**** ", msg, newline ) );
- S-expression reader - reads and returns an S-expression from the string s #
PROC s reader = ( STRING s )REF SEXPR:
BEGIN
PROC at end = BOOL: s pos > UPB s;
PROC curr = CHAR: IF at end THEN REPR 0 ELSE s[ s pos ] FI;
PROC skip spaces = VOID: WHILE NOT at end AND ( curr = " " OR curr = nl ) DO s pos +:= 1 OD;
PROC end of list = BOOL: at end OR curr = ")";
INT s pos := LWB s;
INT t pos;
[ ( UPB s - LWB s ) + 1 ]CHAR token; # token text - large enough to hold the whole string if necessary #
# adds the current character to the token #
PROC add curr = VOID: token[ t pos +:= 1 ] := curr;
# get an s expression element from s #
PROC get element = REF SEXPR:
BEGIN
REF SEXPR result = new s expr;
skip spaces;
# get token text #
IF at end THEN
# no element #
element OF result := EMPTY
ELIF curr = "(" THEN
s pos +:= 1;
skip spaces;
IF NOT end of list
THEN
REF SEXPR nested expression = get element;
REF SEXPR element pos := nested expression;
element OF result := nested expression;
skip spaces;
WHILE NOT end of list
DO
element pos := next OF element pos := get element;
skip spaces
OD
FI;
IF curr = ")" THEN
s pos +:= 1
ELSE
error( "Missing "")""" )
FI
ELIF curr = ")" THEN
s pos +:= 1;
error( "Unexpected "")""" );
element OF result := EMPTY
ELSE
# quoted or unquoted string #
t pos := LWB token - 1;
IF curr /= """" THEN
# unquoted string #
WHILE add curr;
s pos +:= 1;
NOT at end AND curr /= " " AND curr /= "("
AND curr /= ")" AND curr /= """"
AND curr /= nl
DO SKIP OD
ELSE
# quoted string #
WHILE add curr;
s pos +:= 1;
NOT at end AND curr /= """"
DO SKIP OD;
IF curr /= """" THEN
# missing string quote #
error( "Unterminated string: <<" + token[ : t pos ] + ">>" )
ELSE
# have the closing quote #
add curr;
s pos +:= 1
FI
FI;
element OF result := token[ : t pos ]
FI;
result
END # get element # ;
REF SEXPR s expr = get element;
skip spaces;
IF NOT at end THEN
# extraneuos text after the expression #
error( "Unexpected text at end of expression: " + s[ s pos : ] )
FI;
s expr
END # s reader # ;
- prints an S expression #
PROC s writer = ( REF SEXPR s expr )VOID:
BEGIN
# prints an S expression with a suitable indent #
PROC print indented s expression = ( REF SEXPR s expr, INT indent )VOID:
BEGIN
REF SEXPR s pos := s expr;
WHILE REF SEXPR( s pos ) ISNT REF SEXPR( NIL ) DO
FOR i TO indent DO print( ( " " ) ) OD;
CASE element OF s pos
IN (VOID ): print( ( "()", newline ) )
, (STRING s): print( ( s, newline ) )
, (REF SEXPR e): BEGIN
print( ( "(", newline ) );
print indented s expression( e, indent + 4 );
FOR i TO indent DO print( ( " " ) ) OD;
print( ( ")", newline ) )
END
OUT
error( "Unexpected S expression element" )
ESAC;
s pos := next OF s pos
OD
END # print indented s expression # ;
print indented s expression( s expr, 0 )
END # s writer # ;
- test the eader and writer with the example from the task #
s writer( s reader( "((data ""quoted data"" 123 4.5)"
+ nl
+ " (data (!@# (4.5) ""(more"" ""data)"")))"
+ nl
)
)</lang>
- Output:
(
(
data
"quoted data"
123
4.5
)
(
data
(
!@#
(
4.5
)
"(more"
"data)"
)
)
)
AutoHotkey
<lang AutoHotkey>S_Expressions(Str){ Str := RegExReplace(Str, "s)(?<![\\])"".*?[^\\]""(*SKIP)(*F)|((?<![\\])[)(]|\s)", "`n$0`n") Str := RegExReplace(Str, "`am)^\s*\v+") , Cnt := 0 loop, parse, Str, `n, `r { Cnt := A_LoopField=")" ? Cnt-1 : Cnt Res .= tabs(Cnt) A_LoopField "`r`n" Cnt := A_LoopField="(" ? Cnt+1 : Cnt } return Res } tabs(n){ loop, % n Res .= "`t" return Res }</lang> Examples:<lang AutoHotkey>Str = ( ((data da\(\)ta "quot\\ed data" 123 4.5)
("data" (!@# (4.5) "(mo\"re" "data)")))
) MsgBox, 262144, , % S_Expressions(Str)</lang>
- Output:
( ( data da\(\)ta "quot\\ed data" 123 4.5 ) ( "data" ( !@# ( 4.5 ) "(mo\"re" "data)" ) ) )
C
<lang c>#include <stdio.h>
- include <stdlib.h>
- include <ctype.h>
- include <string.h>
enum { S_NONE, S_LIST, S_STRING, S_SYMBOL };
typedef struct { int type; size_t len; void *buf; } s_expr, *expr;
void whine(const char *s) { fprintf(stderr, "parse error before ==>%.10s\n", s); }
expr parse_string(const char *s, char **e) { expr ex = calloc(sizeof(s_expr), 1); char buf[256] = {0}; int i = 0;
while (*s) { if (i >= 256) { fprintf(stderr, "string too long:\n"); whine(s); goto fail; } switch (*s) { case '\\': switch (*++s) { case '\\': case '"': buf[i++] = *s++; continue;
default: whine(s); goto fail; } case '"': goto success; default: buf[i++] = *s++; } } fail: free(ex); return 0;
success: *(const char **)e = s + 1; ex->type = S_STRING; ex->buf = strdup(buf); ex->len = strlen(buf); return ex; }
expr parse_symbol(const char *s, char **e) { expr ex = calloc(sizeof(s_expr), 1); char buf[256] = {0}; int i = 0;
while (*s) { if (i >= 256) { fprintf(stderr, "symbol too long:\n"); whine(s); goto fail; } if (isspace(*s)) goto success; if (*s == ')' || *s == '(') { s--; goto success; }
switch (*s) { case '\\': switch (*++s) { case '\\': case '"': case '(': case ')': buf[i++] = *s++; continue; default: whine(s); goto fail; } case '"': whine(s); goto success; default: buf[i++] = *s++; } } fail: free(ex); return 0;
success: *(const char **)e = s + 1; ex->type = S_SYMBOL; ex->buf = strdup(buf); ex->len = strlen(buf); return ex; }
void append(expr list, expr ele) { list->buf = realloc(list->buf, sizeof(expr) * ++list->len); ((expr*)(list->buf))[list->len - 1] = ele; }
expr parse_list(const char *s, char **e) { expr ex = calloc(sizeof(s_expr), 1), chld; char *next;
ex->len = 0;
while (*s) { if (isspace(*s)) { s++; continue; }
switch (*s) { case '"': chld = parse_string(s+1, &next); if (!chld) goto fail; append(ex, chld); s = next; continue; case '(': chld = parse_list(s+1, &next); if (!chld) goto fail; append(ex, chld); s = next; continue; case ')': goto success;
default: chld = parse_symbol(s, &next); if (!chld) goto fail; append(ex, chld); s = next; continue; } }
fail: whine(s); free(ex); return 0;
success: *(const char **)e = s+1; ex->type = S_LIST; return ex; }
expr parse_term(const char *s, char **e) { while (*s) { if (isspace(*s)) { s++; continue; } switch(*s) { case '(': return parse_list(s+1, e); case '"': return parse_string(s+1, e); default: return parse_symbol(s+1, e); } } return 0; }
void print_expr(expr e, int depth) {
- define sep() for(i = 0; i < depth; i++) printf(" ")
int i; if (!e) return;
switch(e->type) {
case S_LIST:
sep();
puts("(");
for (i = 0; i < e->len; i++)
print_expr(((expr*)e->buf)[i], depth + 1);
sep();
puts(")");
return;
case S_SYMBOL:
case S_STRING:
sep();
if (e->type == S_STRING) putchar('"');
for (i = 0; i < e->len; i++) {
switch(((char*)e->buf)[i]) {
case '"':
case '\\':
putchar('\\');
break;
case ')': case '(':
if (e->type == S_SYMBOL)
putchar('\\');
}
putchar(((char*)e->buf)[i]); } if (e->type == S_STRING) putchar('"'); putchar('\n'); return; } }
int main() { char *next; const char *in = "((data da\\(\\)ta \"quot\\\\ed data\" 123 4.5)\n" " (\"data\" (!@# (4.5) \"(mo\\\"re\" \"data)\")))";
expr x = parse_term(in, &next);
printf("input is:\n%s\n", in); printf("parsed as:\n"); print_expr(x, 0); return 0; }</lang>
- Output:
<lang>input is
((data da\(\)ta "quot\\ed data" 123 4.5)
("data" (!@# (4.5) "(mo\"re" "data)")))
parsed as: (
(
data
da\(\)ta
"quot\\ed data"
123
4.5
)
(
"data"
(
!@#
(
4.5
)
"(mo\"re"
"data)"
)
)
)</lang>
Ceylon
<lang ceylon>class Symbol(symbol) {
shared String symbol; string => symbol;
}
abstract class Token()
of DataToken | leftParen | rightParen {}
abstract class DataToken(data)
of StringToken | IntegerToken | FloatToken | SymbolToken
extends Token() {
shared String|Integer|Float|Symbol data; string => data.string;
}
class StringToken(String data) extends DataToken(data) {} class IntegerToken(Integer data) extends DataToken(data) {} class FloatToken(Float data) extends DataToken(data) {} class SymbolToken(Symbol data) extends DataToken(data) {}
object leftParen extends Token() {
string => "(";
} object rightParen extends Token() {
string => ")";
}
class Tokens(String input) satisfies {Token*} {
shared actual Iterator<Token> iterator() => object satisfies Iterator<Token> {
variable value index = 0;
shared actual Token|Finished next() {
while(exists nextChar = input[index], nextChar.whitespace) {
index++;
}
if(index >= input.size) {
return finished;
}
assert(exists char = input[index]);
if(char == '(') {
index++;
return leftParen;
}
if(char == ')') {
index++;
return rightParen;
}
if(char == '"') {
value builder = StringBuilder();
while(exists nextChar = input[++index]) {
if(nextChar == '"') {
index++;
break;
}
if(nextChar == '\\') {
if(exists nextNextChar = input[++index]) {
switch(nextNextChar)
case('\\') {
builder.append("\\");
}
case('t') {
builder.append("\t");
}
case('n') {
builder.append("\n");
}
case('"') {
builder.append("\"");
}
else {
throw Exception("unknown escaped character");
}
} else {
throw Exception("unclosed string");
}
} else {
builder.appendCharacter(nextChar);
}
}
return StringToken(builder.string);
}
value builder = StringBuilder();
while(exists nextChar = input[index], !nextChar.whitespace && nextChar != '(' && nextChar != ')') {
builder.appendCharacter(nextChar);
index++;
}
value string = builder.string;
if(is Integer int = Integer.parse(string)) {
return IntegerToken(int);
} else if(is Float float = Float.parse(string)) {
return FloatToken(float);
} else {
return SymbolToken(Symbol(string));
}
}
};
}
abstract class Node() of Atom | Group {}
class Atom(data) extends Node() {
shared String|Integer|Float|Symbol data; string => data.string;
} class Group() extends Node() satisfies {Node*} {
shared variable Node[] nodes = []; string => nodes.string; shared actual Iterator<Node> iterator() => nodes.iterator();
}
Node buildTree(Tokens tokens) {
[Group, Integer] recursivelyBuild(Token[] tokens, variable Integer index = 0) {
value result = Group();
while(exists token = tokens[index]) {
switch (token)
case (leftParen) {
value [newNode, newIndex] = recursivelyBuild(tokens, index + 1);
index = newIndex;
result.nodes = result.nodes.append([newNode]);
}
case (rightParen) {
return [result, index];
}
else {
result.nodes = result.nodes.append([Atom(token.data)]);
}
index++;
}
return [result, index];
}
value root = recursivelyBuild(tokens.sequence())[0]; return root.first else Group();
}
void prettyPrint(Node node, Integer indentation = 0) {
void paddedPrint(String s) => print(" ".repeat(indentation) + s);
if(is Atom node) {
paddedPrint(node.string);
} else {
paddedPrint("(");
for(n in node.nodes) {
prettyPrint(n, indentation + 2);
}
paddedPrint(")");
}
}
shared void run() {
value tokens = Tokens("""((data "quoted data" 123 4.5)
(data (!@# (4.5) "(more" "data)")))""");
print(tokens);
value tree = buildTree(tokens); prettyPrint(tree);
} </lang>
- Output:
(
(
data
quoted data
123
4.5
)
(
data
(
!@#
(
4.5
)
(more
data)
)
)
)
CoffeeScript
<lang coffeescript>
- This code works with Lisp-like s-expressions.
- We lex tokens then do recursive descent on the tokens
- to build our data structures.
sexp = (data) ->
# Convert a JS data structure to a string s-expression. A sexier version
# would remove quotes around strings that don't need them.
s =
if Array.isArray data
children = (sexp elem for elem in data).join ' '
'(' + children + ')'
else
return JSON.stringify data
parse_sexp = (sexp) ->
tokens = lex_sexp sexp i = 0
_parse_list = ->
i += 1
arr = []
while i < tokens.length and tokens[i].type != ')'
arr.push _parse()
if i < tokens.length
i += 1
else
throw Error "missing end paren"
arr
_guess_type = (word) ->
# This is crude, doesn't handle all forms of floats.
if word.match /^\d+\.\d+$/
parseFloat(word)
else if word.match /^\d+/
parseInt(word)
else
word
_parse_word = ->
token = tokens[i]
i += 1
if token.type == 'string'
token.word
else
_guess_type token.word
_parse = ->
return undefined unless i < tokens.length
token = tokens[i]
if token.type == '('
_parse_list()
else
_parse_word()
exp = _parse() throw Error "premature termination" if i < tokens.length exp
lex_sexp = (sexp) ->
is_whitespace = (c) -> c in [' ', '\t', '\n']
i = 0
tokens = []
test = (f) ->
return false unless i < sexp.length
f(sexp[i])
eat_char = (c) ->
tokens.push
type: c
i += 1
eat_whitespace = ->
i += 1
while test is_whitespace
i += 1
eat_word = ->
token = c
i += 1
word_char = (c) ->
c != ')' and !is_whitespace c
while test word_char
token += sexp[i]
i += 1
tokens.push
type: "word"
word: token
eat_quoted_word = ->
start = i
i += 1
token =
while test ((c) -> c != '"')
if sexp[i] == '\\'
i += 1
throw Error("escaping error") unless i < sexp.length
token += sexp[i]
i += 1
if test ((c) -> c == '"')
tokens.push
type: "string"
word: token
i += 1
else
throw Error("end quote missing #{sexp.substring(start, i)}")
while i < sexp.length
c = sexp[i]
if c == '(' or c == ')'
eat_char c
else if is_whitespace c
eat_whitespace()
else if c == '"'
eat_quoted_word()
else
eat_word()
tokens
do ->
input = """
((data "quoted data with escaped \\"" 123 4.5 "14")
(data (!@# (4.5) "(more" "data)")))
"""
console.log "input:\n#{input}\n"
output = parse_sexp(input)
pp = (data) -> JSON.stringify data, null, ' '
console.log "output:\n#{pp output}\n"
console.log "round trip:\n#{sexp output}\n"
</lang>
- Output:
<lang> > coffee sexp.coffee input: ((data "quoted data with escaped \"" 123 4.5 "14")
(data (!@# (4.5) "(more" "data)")))
output: [
[
"data",
"quoted data with escaped \"",
123,
4.5,
"14"
],
[
"data",
[
"!@#",
[
4.5
],
"(more",
"data)"
]
]
]
round trip: (("data" "quoted data with escaped \"" 123 4.5 "14") ("data" ("!@#" (4.5) "(more" "data)"))) </lang>
Common Lisp
Parsing S-Expressions
Like most Lisp dialects, Common Lisp uses s-expressions to represent both source code and data. Therefore, a properly formatted s-expression is easily "read" then "evaluated" within the top-level REPL (read-eval-print-loop).
CL-USER> (read-from-string "((data \"quoted data\" 123 4.5) (data (!@# (4.5) \"(more\" \"data)\")))") ((DATA "quoted data" 123 4.5) (DATA (!@# (4.5) "[more" "data]"))) 65 CL-USER> (caar *) ;;The first element from the first sublist is DATA DATA CL-USER> (eval (read-from-string "(concatenate 'string \"foo\" \"bar\")")) ;;An example with evaluation "foobar" CL-USER>
Unfortunately, our pointy-haired boss has asked you to write a parser for an unusual s-expression syntax that uses square brackets instead of parenthesis. In most programming languages, this would necessitate writing an entire parser. Fortunately, the Common Lisp reader can be modified through the use of macro-characters to accomplish this task. When the reader parses a macro-character token, a function associated with the macro-character is called. As evidenced below, modifying the behavior of the Lisp reader by setting macro-character functions to handle additional sytax requires far less work than writing a complete parser from scratch.
<lang lisp>(defun lsquare-reader (stream char)
(declare (ignore char)) (read-delimited-list #\] stream t))
(set-macro-character #\[ #'lsquare-reader) ;;Call the lsquare-reader function when a '[' token is parsed (set-macro-character #\] (get-macro-character #\) nil)) ;;Do the same thing as ')' when a ']' token is parsed</lang> Unit test code: <lang lisp>;;A list of unit tests. Each test is a cons in which the car (left side) contains the
- test string and the cdr (right side) the expected result of reading the S-Exp.
(setf unit-tests
(list
(cons "[]" NIL)
(cons "[a b c]" '(a b c))
(cons "[\"abc\" \"def\"]" '("abc" "def"))
(cons "[1 2 [3 4 [5]]]" '(1 2 (3 4 (5))))
(cons "[\"(\" 1 2 \")\"]" '("(" 1 2 ")"))
(cons "[4/8 3/6 2/4]" '(1/2 1/2 1/2))
(cons "[reduce #'+ '[1 2 3]]" '(reduce #'+ '(1 2 3)))))
(defun run-tests ()
(dolist (test unit-tests) (format t "String: ~23s Expected: ~23s Actual: ~s~%"
(car test) (cdr test) (read-from-string (car test)))))</lang>
- Unit test output:
CL-USER> (run-tests)
String: "[]" Expected: NIL Actual: NIL
String: "[a b c]" Expected: (A B C) Actual: (A B C)
String: "[\"abc\" \"def\"]" Expected: ("abc" "def") Actual: ("abc" "def")
String: "[1 2 [3 4 [5]]]" Expected: (1 2 (3 4 (5))) Actual: (1 2 (3 4 (5)))
String: "[\"(\" 1 2 \")\"]" Expected: ("(" 1 2 ")") Actual: ("(" 1 2 ")")
String: "[4/8 3/6 2/4]" Expected: (1/2 1/2 1/2) Actual: (1/2 1/2 1/2)
String: "[reduce #'+ '[1 2 3]]" Expected: (REDUCE #'+ '(1 2 3)) Actual: (REDUCE #'+ '(1 2 3))
Error testing:
(read-from-string "[\"ab\"\"]") ;;Error: Object ends with a string. (read-from-string "[)") ;;Error: An object cannot start with ')' (read-from-string "(]") ;;Error: An object cannot start with ']'
- Task output:
CL-USER> (setf task "[[data \"quoted data\" 123 4.5] [data [!@# [4.5] \"[more\" \"data]\"]]]")
"[[data \"quoted data\" 123 4.5] [data [!@# [4.5] \"[more\" \"data]\"]]]"
CL-USER> (read-from-string task)
((DATA "quoted data" 123 4.5) (DATA (!@# (4.5) "[more" "data]")))
65
CL-USER> (setf testing *) ;; * is the previous result in SLIME
((DATA "quoted data" 123 4.5) (DATA (!@# (4.5) "[more" "data]")))
CL-USER> (car testing)
(DATA "quoted data" 123 4.5)
CL-USER> (caar testing)
DATA
CL-USER> (cdar testing)
("quoted data" 123 4.5)
CL-USER> (cadar testing)
"quoted data"
CL-USER>
Writing S-Expressions
The next step in this task is to write a standard Lisp s-expression in the square bracket notation. <lang lisp>(defun write-sexp (sexp)
"Writes a Lisp s-expression in square bracket notation." (labels ((parse (sexp)
(cond ((null sexp) "") ((atom sexp) (format nil "~s " sexp)) ((listp sexp) (concatenate 'string (if (listp (car sexp)) (concatenate 'string "[" (fix-spacing (parse (car sexp))) "] ") (parse (car sexp))) (parse (cdr sexp)))))) (fix-spacing (str) (let ((empty-string "")) (unless (null str) (if (equal str empty-string) empty-string (let ((last-char (1- (length str)))) (if (eq #\Space (char str last-char)) (subseq str 0 last-char) str)))))))
(concatenate 'string "[" (fix-spacing (parse sexp)) "]")))</lang>
Unit test code: <lang lisp>(setf unit-tests '(((1 2) (3 4)) (1 2 3 4) ("ab(cd" "mn)op") (1 (2 (3 (4)))) ((1) (2) (3)) ()))
(defun run-tests ()
(dolist (test unit-tests) (format t "Before: ~18s After: ~s~%"
test (write-sexp test))))</lang>
- Unit test output:
CL-USER> (run-tests)
Before: ((1 2) (3 4)) After: "[[1 2] [3 4]]"
Before: (1 2 3 4) After: "[1 2 3 4]"
Before: ("ab(cd" "mn)op") After: "[\"ab(cd\" \"mn)op\"]"
Before: (1 (2 (3 (4)))) After: "[1 [2 [3 [4]]]]"
Before: ((1) (2) (3)) After: "[[1] [2] [3]]"
Before: NIL After: "[]"
- Finally, round trip output for the original task example:
CL-USER> task "[[data \"quoted data\" 123 4.5] [data [!@# [4.5] \"[more\" \"data]\"]]]" CL-USER> (read-from-string task) ((DATA "quoted data" 123 4.5) (DATA (!@# (4.5) "[more" "data]"))) 65 CL-USER> (write-sexp *) ; * is the previous return value "[[DATA \"quoted data\" 123 4.5] [DATA [!@# [4.5] \"[more\" \"data]\"]]]" CL-USER>
C#
Implementation of S-expression serializer & deserializer in C# 6.0 language.
Git repository with code and tests can be found here: https://github.com/ichensky/SExpression/tree/rosettacode
<lang csharp> using System; using System.Collections.Generic; using System.Text;
public class SNode
{
private List<SNode> _items;
public string Name { get; set; }
public IReadOnlyCollection<SNode> Items { get { return _items.AsReadOnly(); } }
public SNode()
{
this._items = new List<SNode>();
}
public SNode(string name):this()
{
this.Name=name;
}
public void AddNode(SNode node)
{
this._items.Add(node);
}
}
public class SNodeFull : SNode
{
private bool _isLeaf;
public bool IsLeaf { get => _isLeaf; }
public SNodeFull(bool isLeaf) : base()
{
this._isLeaf = isLeaf;
}
public SNodeFull(string name, bool isLeaf) : base(name)
{
this._isLeaf = isLeaf;
}
public SNodeFull RootNode { get; set; }
public void AddNode(SNodeFull node)
{
base.AddNode(node);
node.RootNode = this;
}
}
</lang> <lang csharp> using System; using System.Collections.Generic; using System.Linq; using System.Text;
namespace SExpression {
public partial class SExpression
{
public const string ErrorStrNotValidFormat = "Not valid format.";
}
public partial class SExpression : ISExpression
{
public String Serialize(SNode root)
{
if (root == null)
{
throw new ArgumentNullException();
}
var sb = new StringBuilder();
Serialize(root, sb);
return sb.ToString();
}
private void Serialize(SNode node, StringBuilder sb)
{
sb.Append('(');
if (node.Items.Count > 0)
{
int x = 0;
foreach (var item in node.Items)
{
if (x>0)
{
sb.Append(' ');
}
else
{
x++;
}
if (item.Items.Count > 0)
{
Serialize(item, sb);
}
else
{
SerializeItem(item, sb);
}
}
}
sb.Append(')');
}
private void SerializeItem(SNode node, StringBuilder sb)
{
if (node.Name == null)
{
sb.Append("()");
return;
}
node.Name = node.Name.Replace("\"", "\\\"");
if (node.Name.IndexOfAny(new char[] { ' ', '"', '(', ')' }) != -1 || node.Name == string.Empty)
{
sb.Append('"').Append(node.Name).Append('"');
return;
}
sb.Append(node.Name);
}
}
public partial class SExpression
{
public SNode Deserialize(string st)
{
if (st==null)
{
return null;
}
st = st.Trim();
if (string.IsNullOrEmpty(st))
{
return null;
}
var begin = st.IndexOf('(');
if (begin != 0)
{
throw new Exception();
}
var end = st.LastIndexOf(')');
if (end != st.Length - 1)
{
throw new Exception(ErrorStrNotValidFormat);
}
st = st.Remove(st.Length-1).Remove(0, 1).ToString();
var node = new SNodeFull(false);
Deserialize(ref st, node);
return node;
}
private void Deserialize(ref string st, SNodeFull root)
{
st = st.Trim();
if (string.IsNullOrEmpty(st))
{
return;
}
SNodeFull node = null;
SNodeFull r = root;
do
{
while (st[0] == ')')
{
st = st.Remove(0, 1).Trim();
if (st.Length==0)
{
return;
}
r = root.RootNode;
if (r==null)
{
throw new Exception(ErrorStrNotValidFormat);
}
}
node = DeserializeItem(ref st);
st = st.Trim();
r.AddNode(node);
if (!node.IsLeaf)
{
Deserialize(ref st,node);
}
}
while (st.Length > 0);
}
private SNodeFull DeserializeItem(ref string st)
{
if (st[0] == '(')
{
st = st.Remove(0, 1);
return new SNodeFull(false);
}
var x = 0;
var esc = 0;
for (int i = 0; i < st.Length; i++)
{
if (st[i] == '"')
{
if (esc == 0)
{
esc = 1;
}
else if(esc == 1 && (i> 0 && st[i - 1] == '\\'))
{
throw new Exception(ErrorStrNotValidFormat);
}
else
{
esc = 2;
break;
}
}
else if (esc==0 && " ()".Contains(st[i]))
{
break;
}
x++;
}
if (esc == 1)
{
throw new Exception(ErrorStrNotValidFormat);
}
var head = esc==0? st.Substring(0, x): st.Substring(1,x-1);
st = st.Remove(0, esc ==0 ? x: x + 2);
return new SNodeFull(head, true);
}
}
} </lang> <lang csharp> using System; using System.Collections.Generic;
namespace SExpression.Test {
class Program
{
static void Main(string[] args)
{
var str =
@"((data ""quoted data"" 123 4.5) (data(!@# (4.5) ""(more"" ""data)"")))";
var se = new SExpression();
var node = se.Deserialize(str);
var result = se.Serialize(node);
Console.WriteLine(result);
}
}
} </lang>
- Output:
((data "quoted data" 123 4.5) (data (!@# (4.5) "(more" "data)")))
D
<lang d>import std.stdio, std.conv, std.algorithm, std.variant, std.uni,
std.functional, std.string;
alias Sexp = Variant;
struct Symbol {
private string name;
string toString() @safe const pure nothrow { return name; }
}
Sexp parseSexp(string txt) @safe pure /*nothrow*/ {
static bool isIdentChar(in char c) @safe pure nothrow {
return c.isAlpha || "0123456789!@#-".representation.canFind(c);
}
size_t pos = 0;
Sexp _parse() /*nothrow*/ {
auto i = pos + 1;
scope (exit)
pos = i;
if (txt[pos] == '"') {
while (txt[i] != '"' && i < txt.length)
i++;
i++;
return Sexp(txt[pos + 1 .. i - 1]);
} else if (txt[pos].isNumber) {
while (txt[i].isNumber && i < txt.length)
i++;
if (txt[i] == '.') {
i++;
while (txt[i].isNumber && i < txt.length)
i++;
auto aux = txt[pos .. i]; //
return aux.parse!double.Sexp;
}
auto aux = txt[pos .. i]; //
return aux.parse!ulong.Sexp;
} else if (isIdentChar(txt[pos])) {
while (isIdentChar(txt[i]) && i < txt.length)
i++;
return Sexp(Symbol(txt[pos .. i]));
} else if (txt[pos] == '(') {
Sexp[] lst;
while (txt[i] != ')') {
while (txt[i].isWhite)
i++;
pos = i;
lst ~= _parse;
i = pos;
while (txt[i].isWhite)
i++;
}
i = pos + 1;
return Sexp(lst);
}
return Sexp(null);
}
txt = txt.find!(not!isWhite); return _parse;
}
void writeSexp(Sexp expr) {
if (expr.type == typeid(string)) {
write('"', expr, '"');
} else if (expr.type == typeid(Sexp[])) {
'('.write;
auto arr = expr.get!(Sexp[]);
foreach (immutable i, e; arr) {
e.writeSexp;
if (i + 1 < arr.length)
' '.write;
}
')'.write;
} else {
expr.write;
}
}
void main() {
auto pTest = `((data "quoted data" 123 4.5)
(data (!@# (4.5) "(more" "data)")))`.parseSexp;
writeln("Parsed: ", pTest);
"Printed: ".write;
pTest.writeSexp;
}</lang>
- Output:
Parsed: [[data, quoted data, 123, 4.5], [data, [!@#, [4.5], (more, data)]]] Printed: ((data "quoted data" 123 4.5) (data (!@# (4.5) "(more" "data)")))
EchoLisp
The (read-from-string input-string) function parses a string into an s-expression, which is the native representation of program/data in EchoLisp and the majority of Lisps . <lang lisp> (define input-string #'((data "quoted data" 123 4.5)\n(data (!@# (4.5) "(more" "data)")))'#)
input-string
→ "((data "quoted data" 123 4.5) (data (!@# (4.5) "(more" "data)")))"
(define s-expr (read-from-string input-string)) s-expr
→ ((data "quoted data" 123 4.5) (data (!@# (4.5) "(more" "data)")))
(first s-expr)
→ (data "quoted data" 123 4.5)
(first(first s-expr))
→ data
(first(rest s-expr))
→ (data (!@# (4.5) "(more" "data)"))
</lang>
Factor
Factor's nested sequences are close enough to s-expressions that in most cases we can simply eval s-expression strings after some minor whitespace/bracket/parenthesis transformations. However, if we wish to support symbols, this approach becomes complicated because symbols need to be declared before use. This means we need to go into the string and identify them, so we may as well parse the s-expression properly while we're there.
We have a nice tool at our disposal for doing this. In its standard library, Factor contains a domain-specific language for defining extended Backus-Naur form (EBNF) grammars. EBNF is a convenient, declarative way to describe different parts of a grammar where later rules build on earlier ones until the final rule defines the entire grammar. Upon calling the word defined by EBNF:, an input string will be tokenized according to the declared rules and stored in an abstract syntax tree.
To get an idea of how this works, look at the final rule. It declares that an s-expression is any number of objects (comprised of numbers, floats, strings, and symbols) and s-expressions (the rule is recursive, allowing for nested s-expressions) surrounded by parenthesis which are in turn surrounded by any amount of whitespace. This implementation of EBNF allows us to define actions: the quotation after the => is called on the rule token just before being added to the abstract syntax tree. This is convenient for our use case where we need to parse different types of objects into our sequence structure.
Factor has a comprehensive prettyprinter which can print any Factor object in a readable way. Not only can we leverage it to easily print our native data structure, but we can also call unparse to convert it to a string. This leaves us with a string reminiscent of the original input, and we are able to take it the rest of the way with two simple regular expressions.
<lang factor>USING: formatting kernel math.parser multiline peg peg.ebnf regexp sequences prettyprint words ; IN: rosetta-code.s-expressions
STRING: input ((data "quoted data" 123 4.5)
(data (!@# (4.5) "(more" "data)")))
EBNF: sexp>seq [=[
ws = [\n\t\r ]* => drop ignore digits = [0-9]+ number = digits => [[ string>number ]] float = digits:a "." digits:b => [[ a b "." glue string>number ]] string = '"'~ [^"]* '"'~ => "" like symbol = [!#-'*-~]+ => [[ "" like <uninterned-word> ]] object = ws ( float | number | string | symbol ) ws sexp = ws "("~ ( object | sexp )* ")"~ ws => [[ { } like ]]
]=]
- seq>sexp ( seq -- str )
unparse R/ {\s+/ "(" R/ \s+}/ ")" [ re-replace ] 2bi@ ;
input [ "Input:\n%s\n\n" printf ] [
sexp>seq dup seq>sexp "Native:\n%u\n\nRound trip:\n%s\n" printf
] bi</lang>
- Output:
Input:
((data "quoted data" 123 4.5)
(data (!@# (4.5) "(more" "data)")))
Native:
{
{ data "quoted data" 123 4.5 }
{ data { !@# { 4.5 } "(more" "data)" } }
}
Round trip:
((data "quoted data" 123 4.5)
(data (!@# (4.5) "(more" "data)")))
Go
<lang go>package main
import (
"errors" "fmt" "reflect" "strconv" "strings" "unicode"
)
var input = `((data "quoted data" 123 4.5)
(data (!@# (4.5) "(more" "data)")))`
func main() {
fmt.Println("input:")
fmt.Println(input)
s, err := parseSexp(input)
if err != nil {
fmt.Println("error:", err)
return
}
fmt.Println("\nparsed:")
fmt.Println(s)
fmt.Println("\nrepresentation:")
s.dump(0)
}
// dynamic types for i are string, qString, int, float64, list, and error. type sexp struct {
i interface{}
} type qString string type list []sexp
func (s sexp) String() string {
return fmt.Sprintf("%v", s.i)
}
func (q qString) String() string {
return strconv.Quote(string(q))
}
func (l list) String() string {
if len(l) == 0 {
return "()"
}
b := fmt.Sprintf("(%v", l[0])
for _, s := range l[1:] {
b = fmt.Sprintf("%s %v", b, s)
}
return b + ")"
}
// parseSexp parses a string into a Go representation of an s-expression. // // Quoted strings go from one " to the next. There is no escape character, // all characters except " are valid. // // Otherwise atoms are any string of characters between any of '(', ')', // '"', or white space characters. If the atom parses as a Go int type // using strconv.Atoi, it is taken as int; if it parses as a Go float64 // type using strconv.ParseFloat, it is taken as float64; otherwise it is // taken as an unquoted string. // // Unmatched (, ), or " are errors. // An empty or all whitespace input string is an error. // Left over text after the sexp is an error. // // An empty list is a valid sexp, but there is no nil, no cons, no dot. func parseSexp(s string) (sexp, error) {
s1, rem := ps2(s, -1)
if err, isErr := s1.i.(error); isErr {
return sexp{}, err
}
if rem > "" {
return s1, errors.New("Left over text: " + rem)
}
return s1, nil
}
// recursive. n = -1 means not parsing a list. n >= 0 means the number // of list elements parsed so far. string result is unparsed remainder // of the input string s0. func ps2(s0 string, n int) (x sexp, rem string) {
tok, s1 := gettok(s0)
switch t := tok.(type) {
case error:
return sexp{tok}, s1
case nil: // this is also an error
if n < 0 {
return sexp{errors.New("blank input string")}, s0
} else {
return sexp{errors.New("unmatched (")}, ""
}
case byte:
switch {
case t == '(':
x, s1 = ps2(s1, 0) // x is a list
if _, isErr := x.i.(error); isErr {
return x, s0
}
case n < 0:
return sexp{errors.New("unmatched )")}, ""
default:
// found end of list. allocate space for it.
return sexp{make(list, n)}, s1
}
default:
x = sexp{tok} // x is an atom
}
if n < 0 {
// not in a list, just return the s-expression x
return x, s1
}
// in a list. hold on to x while we parse the rest of the list.
l, s1 := ps2(s1, n+1)
// result l is either an error or the allocated list, not completely
// filled in yet.
if _, isErr := l.i.(error); !isErr {
// as long as no errors, drop x into its place in the list
l.i.(list)[n] = x
}
return l, s1
}
// gettok gets one token from string s. // return values are the token and the remainder of the string. // dynamic type of tok indicates result: // nil: no token. string was empty or all white space. // byte: one of '(' or ')' // otherwise string, qString, int, float64, or error. func gettok(s string) (tok interface{}, rem string) {
s = strings.TrimSpace(s)
if s == "" {
return nil, ""
}
switch s[0] {
case '(', ')':
return s[0], s[1:]
case '"':
if i := strings.Index(s[1:], `"`); i >= 0 {
return qString(s[1 : i+1]), s[i+2:]
}
return errors.New(`unmatched "`), s
}
i := 1
for i < len(s) && s[i] != '(' && s[i] != ')' && s[i] != '"' &&
!unicode.IsSpace(rune(s[i])) {
i++
}
if j, err := strconv.Atoi(s[:i]); err == nil {
return j, s[i:]
}
if f, err := strconv.ParseFloat(s[:i], 64); err == nil {
return f, s[i:]
}
return s[:i], s[i:]
}
func (s sexp) dump(i int) {
fmt.Printf("%*s%v: ", i*3, "", reflect.TypeOf(s.i))
if l, isList := s.i.(list); isList {
fmt.Println(len(l), "elements")
for _, e := range l {
e.dump(i + 1)
}
} else {
fmt.Println(s.i)
}
}</lang>
- Output:
input:
((data "quoted data" 123 4.5)
(data (!@# (4.5) "(more" "data)")))
parsed:
((data "quoted data" 123 4.5) (data (!@# (4.5) "(more" "data)")))
representation:
main.list: 2 elements
main.list: 4 elements
string: data
main.qString: "quoted data"
int: 123
float64: 4.5
main.list: 2 elements
string: data
main.list: 4 elements
string: !@#
main.list: 1 elements
float64: 4.5
main.qString: "(more"
main.qString: "data)"
Haskell
<lang haskell>import Data.Functor import Text.Parsec ((<|>), (<?>), many, many1, char, try, parse, sepBy, choice, between) import Text.Parsec.Char (noneOf) import Text.Parsec.Token (integer, float, whiteSpace, stringLiteral, makeTokenParser) import Text.Parsec.Language (haskell)
data Val = Int Integer
| Float Double
| String String
| Symbol String
| List [Val] deriving (Eq, Show)
tProg = many tExpr <?> "program"
where tExpr = between ws ws (tList <|> tAtom) <?> "expression"
ws = whiteSpace haskell
tAtom = (try (Float <$> float haskell) <?> "floating point number")
<|> (try (Int <$> integer haskell) <?> "integer")
<|> (String <$> stringLiteral haskell <?> "string")
<|> (Symbol <$> many1 (noneOf "()\"\t\n\r ") <?> "symbol")
<?> "atomic expression"
tList = List <$> between (char '(') (char ')') (many tExpr) <?> "list"
p = either print (putStrLn . unwords . map show) . parse tProg ""
main = do
let expr = "((data \"quoted data\" 123 4.5)\n (data (!@# (4.5) \"(more\" \"data)\")))"
putStrLn ("The input:\n" ++ expr ++ "\nParsed as:")
p expr</lang>
Icon and Unicon
The following should suffice as a demonstration.
String escaping and quoting could be handled more robustly.
The example takes single and double quotes.
Single quotes were used instead of doubles in the input.
<lang Icon>link ximage
procedure main()
in := "((data 'quoted data' 123 4.5) (data (!@# (4.5) '(more' 'data)')))"
# in := map(in,"'","\"") # uncomment to put back double quotes if desired
write("Input: ",image(in))
write("Structure: \n",ximage(S := string2sexp(in)))
write("Output: ",image(sexp2string(S)))
end
procedure sexp2string(S) #: return a string representing the s-expr
s := ""
every t := !S do {
if type(t) == "list" then
s ||:= "(" || trim(sexp2string(t)) || ")"
else
if upto('() \t\r\n',t) then
s ||:= "'" || t || "'"
else
s ||:= t
s ||:= " "
}
return trim(s)
end
procedure string2sexp(s) #: return a s-expression nested list
if s ? ( sexptokenize(T := []), pos(0) ) then
return sexpnest(T)
else
write("Malformed: ",s)
end
procedure sexpnest(T,L) #: transform s-expr token list to nested list
/L := []
while t := get(T) do
case t of {
"(" : {
put(L,[])
sexpnest(T,L[*L])
}
")" : return L
default : put(L, numeric(t) | t)
}
return L
end
procedure sexptokenize(T) #: return list of tokens parsed from an s-expr string static sym initial sym := &letters++&digits++'~`!@#$%^&*_-+|;:.,<>[]{}'
until pos(0) do
case &subject[&pos] of {
" " : tab(many(' \t\r\n')) # consume whitespace
"'"|"\"" :
(q := move(1)) & put(T,tab(find(q))) & move(1) # quotes
"(" : put(T,move(1)) & sexptokenize(T) # open
")" : put(T,move(1)) &return T # close
default : put(T, tab(many(sym))) # other symbols
}
return T
end</lang>
ximage.icn formats arbitrary structures into printable strings
- Output:
Input: "((data 'quoted data' 123 4.5) (data (!@# (4.5) '(more' 'data)')))"
Structure:
L2 := list(1)
L2[1] := L3 := list(2)
L3[1] := L4 := list(4)
L4[1] := "data"
L4[2] := "quoted data"
L4[3] := 123
L4[4] := 4.5
L3[2] := L5 := list(2)
L5[1] := "data"
L5[2] := L6 := list(4)
L6[1] := "!@#"
L6[2] := L7 := list(1)
L7[1] := 4.5
L6[3] := "(more"
L6[4] := "data)"
Output: "((data 'quoted data' 123 4.5) (data (!@# (4.5) '(more' 'data)')))"
J
Since J already has a way of expressing nested lists, this implementation is for illustration purposes only. No attempt is made to handle arrays which are not representable using sexpr syntax.
This implementation does not support escape characters. If escape characters were added, we would need additional support in the tokenizer (an extra character class, and in the state table an extra column and two extra rows, or almost double the number of state transitions: 35 instead of 20), and additional support in the data language (unfmt would need to strip out escape characters and fmt would need to insert escape characters -- so each of these routines would also perhaps double in size.) And that's a lot of bulk for serialize/deserialize mechanism which, by design, cannot represent frequently used data elements (such as matrices and gerunds).
<lang j>NB. character classes: 0: paren, 1: quote, 2: whitespace, 3: wordforming (default) chrMap=: '()';'"';' ',LF,TAB,CR
NB. state columns correspond to the above character classes NB. first digit chooses next state. NB. second digit is action 0: do nothing, 1: start token, 2: end token states=: 10 10#: ".;._2]0 :0
11 21 00 31 NB. state 0: initial state 12 22 02 32 NB. state 1: after () or after closing " 40 10 40 40 NB. state 2: after opening " 12 22 02 30 NB. state 3: after word forming character 40 10 40 40 NB. state 4: between opening " and closing "
)
tokenize=: (0;states;<chrMap)&;:
rdSexpr=:3 :0 :.wrSexpr
s=. r=. [ 'L R'=. ;:'()'
for_token. tokenize y do.
select. token
case. L do. r=. [ s=. s,<r
case. R do. s=. }:s [ r=. (_1{::s),<r
case. do. r=. r,token
end.
end.
>{.r
)
wrSexpr=: ('(' , ;:^:_1 , ')'"_)^:L.L:1^:L. :.rdSexpr
fmt=: 3 :0 :.unfmt
if. '"' e. {.y do. }.,}: y NB. quoted string
elseif. 0=#$n=.".y do. n NB. number or character
elseif. do. s:<y NB. symbol
end.
)
unfmt=: 3 :0 :.fmt
select. 3!:0 y
case. 1;4;8;16;128 do. ":!.20 y
case. 2;131072 do.
select. #$y
case. 0 do. ',y,'
case. 1 do. '"',y,'"'
end.
case. 64 do. (":y),'x'
case. 65536 do. >s:inv y
end.
)
readSexpr=: fmt L:0 @rdSexpr :.writeSexpr writeSexpr=: wrSexpr @(unfmt L:0) :.readSexpr</lang>
Example use:
<lang j> readSexpr '((data "quoted data" 123 4.5) (data (!@# (4.5) "(more" "data)")))' ┌───────────────────────────┬────────────────────────────────┐ │┌─────┬───────────┬───┬───┐│┌─────┬────────────────────────┐│ ││`data│quoted data│123│4.5│││`data│┌────┬─────┬─────┬─────┐││ │└─────┴───────────┴───┴───┘││ ││`!@#│┌───┐│(more│data)│││ │ ││ ││ ││4.5││ │ │││ │ ││ ││ │└───┘│ │ │││ │ ││ │└────┴─────┴─────┴─────┘││ │ │└─────┴────────────────────────┘│ └───────────────────────────┴────────────────────────────────┘
writeSexpr readSexpr '((data "quoted data" 123 4.5) (data (!@# (4.5) "(more" "data)")))'
((data "quoted data" 123 4.5) (data (!@# (4.5) "(more" "data)")))</lang>
Java
This code is based on [1] and [2] It is available under the GPL. The author is Joel F. Klein. He has graciously given permission to share the code under the FDL for the purpose of publishing it on RosettaCode.
The code as presented here is taken unmodified from an application being developed right now where it is used.
LispTokenizer.java
<lang java>package jfkbits;
import java.io.BufferedReader; import java.io.IOException; import java.io.Reader; import java.io.StreamTokenizer; import java.io.StringReader; import java.util.Iterator;
public class LispTokenizer implements Iterator<Token> {
// Instance variables have default access to allow unit tests access. StreamTokenizer m_tokenizer; IOException m_ioexn;
/** Constructs a tokenizer that scans input from the given string.
* @param src A string containing S-expressions.
*/
public LispTokenizer(String src)
{
this(new StringReader(src));
}
/** Constructs a tokenizer that scans input from the given Reader.
* @param r Reader for the character input source
*/
public LispTokenizer(Reader r)
{
if(r == null)
r = new StringReader("");
BufferedReader buffrdr = new BufferedReader(r);
m_tokenizer = new StreamTokenizer(buffrdr);
m_tokenizer.resetSyntax(); // We don't like the default settings
m_tokenizer.whitespaceChars(0, ' ');
m_tokenizer.wordChars(' '+1,255);
m_tokenizer.ordinaryChar('(');
m_tokenizer.ordinaryChar(')');
m_tokenizer.ordinaryChar('\);
m_tokenizer.commentChar(';');
m_tokenizer.quoteChar('"');
}
public Token peekToken()
{
if(m_ioexn != null)
return null;
try
{
m_tokenizer.nextToken();
}
catch(IOException e)
{
m_ioexn = e;
return null;
}
if(m_tokenizer.ttype == StreamTokenizer.TT_EOF)
return null;
Token token = new Token(m_tokenizer);
m_tokenizer.pushBack();
return token;
}
public boolean hasNext()
{
if(m_ioexn != null)
return false;
try
{
m_tokenizer.nextToken();
}
catch(IOException e)
{
m_ioexn = e;
return false;
}
if(m_tokenizer.ttype == StreamTokenizer.TT_EOF)
return false;
m_tokenizer.pushBack();
return true;
}
/** Return the most recently caught IOException, if any,
*
* @return
*/
public IOException getIOException()
{
return m_ioexn;
}
public Token next()
{
try
{
m_tokenizer.nextToken();
}
catch(IOException e)
{
m_ioexn = e;
return null;
}
Token token = new Token(m_tokenizer);
return token;
}
public void remove()
{
}
}</lang>
Token.java
<lang java>package jfkbits; import java.io.StreamTokenizer;
public class Token {
public static final int SYMBOL = StreamTokenizer.TT_WORD; public int type; public String text; public int line;
public Token(StreamTokenizer tzr)
{
this.type = tzr.ttype;
this.text = tzr.sval;
this.line = tzr.lineno();
}
public String toString()
{
switch(this.type)
{
case SYMBOL:
case '"':
return this.text;
default:
return String.valueOf((char)this.type);
}
}
}</lang>
Atom.java
<lang java>package jfkbits;
import jfkbits.LispParser.Expr;
public class Atom implements Expr {
String name;
public String toString()
{
return name;
}
public Atom(String text)
{
name = text;
}
}</lang>
StringAtom.java
<lang java>package jfkbits;
public class StringAtom extends Atom {
public String toString()
{
// StreamTokenizer hardcodes escaping with \, and doesn't allow \n inside words
String escaped = name.replace("\\", "\\\\").replace("\n", "\\n").replace("\r", "\\r").replace("\"", "\\\"");
return "\""+escaped+"\"";
}
public StringAtom(String text)
{
super(text);
}
public String getValue()
{
return name;
}
} </lang>
ExprList.java
<lang java>package jfkbits;
import java.util.AbstractCollection; import java.util.Arrays; import java.util.Iterator; import java.util.ArrayList;
import jfkbits.LispParser.Expr;
public class ExprList extends ArrayList<Expr> implements Expr {
ExprList parent = null; int indent =1;
public int getIndent()
{
if (parent != null)
{
return parent.getIndent()+indent;
}
else return 0;
}
public void setIndent(int indent)
{
this.indent = indent;
}
public void setParent(ExprList parent)
{
this.parent = parent;
}
public String toString()
{
String indent = "";
if (parent != null && parent.get(0) != this)
{
indent = "\n";
char[] chars = new char[getIndent()];
Arrays.fill(chars, ' ');
indent += new String(chars);
}
String output = indent+"(";
for(Iterator<Expr> it=this.iterator(); it.hasNext(); )
{
Expr expr = it.next();
output += expr.toString();
if (it.hasNext())
output += " ";
}
output += ")";
return output;
}
@Override
public synchronized boolean add(Expr e)
{
if (e instanceof ExprList)
{
((ExprList) e).setParent(this);
if (size() != 0 && get(0) instanceof Atom)
((ExprList) e).setIndent(2);
}
return super.add(e);
}
}</lang>
LispParser.java
<lang java>package jfkbits;
public class LispParser
{
LispTokenizer tokenizer;
public LispParser(LispTokenizer input)
{
tokenizer=input;
}
public class ParseException extends Exception
{
}
public interface Expr
{
// abstract parent for Atom and ExprList
}
public Expr parseExpr() throws ParseException
{
Token token = tokenizer.next();
switch(token.type)
{
case '(': return parseExprList(token);
case '"': return new StringAtom(token.text);
default: return new Atom(token.text);
}
}
protected ExprList parseExprList(Token openParen) throws ParseException
{
ExprList acc = new ExprList();
while(tokenizer.peekToken().type != ')')
{
Expr element = parseExpr();
acc.add(element);
}
Token closeParen = tokenizer.next();
return acc;
}
} </lang>
LispParserDemo.java
<lang java>import jfkbits.ExprList; import jfkbits.LispParser; import jfkbits.LispParser.ParseException; import jfkbits.LispTokenizer;
public class LispParserDemo {
public static void main(String args[])
{
LispTokenizer tzr = new LispTokenizer(
"((data \"quoted data\" 123 4.5)\n (data (!@# (4.5) \"(more\" \"data)\")))");
LispParser parser = new LispParser(tzr);
try
{
Expr result = parser.parseExpr();
System.out.println(result);
}
catch (ParseException e1)
{
// TODO Auto-generated catch block
e1.printStackTrace();
}
}
}</lang>
JavaScript
<lang JavaScript>String.prototype.parseSexpr = function() { var t = this.match(/\s*("[^"]*"|\(|\)|"|[^\s()"]+)/g) for (var o, c=0, i=t.length-1; i>=0; i--) { var n, ti = t[i].trim() if (ti == '"') return else if (ti == '(') t[i]='[', c+=1 else if (ti == ')') t[i]=']', c-=1 else if ((n=+ti) == ti) t[i]=n else t[i] = '\ + ti.replace('\, '\\\) + '\ if (i>0 && ti!=']' && t[i-1].trim()!='(' ) t.splice(i,0, ',') if (!c) if (!o) o=true; else return } return c ? undefined : eval(t.join()) }
Array.prototype.toString = function() { var s=; for (var i=0, e=this.length; i<e; i++) s+=(s?' ':)+this[i] return '('+s+')' }
Array.prototype.toPretty = function(s) {
if (!s) s =
var r = s + '(
'
var s2 = s + Array(6).join(' ')
for (var i=0, e=this.length; i<e; i+=1) {
var ai = this[i]
r += ai.constructor != Array ? s2+ai+'
' : ai.toPretty(s2)
}
return r + s + ')
'
}
var str = '((data "quoted data" 123 4.5)\n (data (!@# (4.5) "(more" "data)")))'
document.write('text:
', str.replace(/\n/g,'
').replace(/ /g,' '), '
')
var sexpr = str.parseSexpr()
if (sexpr === undefined)
document.write('Invalid s-expr!', '
')
else
document.write('s-expr:
', sexpr, '
', sexpr.constructor != Array ? : 'pretty print:
' + sexpr.toPretty())</lang>
- Output:
text:
((data "quoted data" 123 4.5)
(data (!@# (4.5) "(more" "data)")))
s-expr:
((data "quoted data" 123 4.5) (data (!@# (4.5) "(more" "data)")))
pretty print:
(
(
data
"quoted data"
123
4.5
)
(
data
(
!@#
(
4.5
)
"(more"
"data)"
)
)
)
Julia
<lang julia> function rewritequotedparen(s)
segments = split(s, "\"")
for i in 1:length(segments)
if i & 1 == 0 # even i
ret = replace(segments[i], r"\(", s"_O_PAREN")
segments[i] = replace(ret, r"\)", s"_C_PAREN")
end
end
join(segments, "\"")
end
function reconsdata(n, s)
if n > 1
print(" ")
end
if s isa String && ismatch(r"[\$\%\!\$\#]", s) == false
print("\"$s\"")
else
print(s)
end
end
function printAny(anyarr)
print("(")
for (i, el) in enumerate(anyarr)
if el isa Array
print("(")
for (j, el2) in enumerate(el)
if el2 isa Array
print("(")
for(k, el3) in enumerate(el2)
if el3 isa Array
print(" (")
for(n, el4) in enumerate(el3)
reconsdata(n, el4)
end
print(")")
else
reconsdata(k, el3)
end
end
print(")")
else
reconsdata(j, el2)
end
end
if i == 1
print(")\n ")
else
print(")")
end
end
end
println(")")
end
removewhitespace(s) = replace(replace(s, r"\n", " "), r"^\s*(\S.*\S)\s*$", s"\1") quote3op(s) = replace(s, r"([\$\!\@\#\%]{3})", s"\"\1\"") paren2bracket(s) = replace(replace(s, r"\(", s"["), r"\)", s"]") data2symbol(s) = replace(s, "[data", "[:data") unrewriteparens(s) = replace(replace(s, "_C_PAREN", ")"), "_O_PAREN", "(") addcommas(s) = replace(replace(s, r"\]\s*\[", "],["), r" (?![a-z])", ",")
inputstring = """ ((data "quoted data" 123 4.5)
(data (!@# (4.5) "(more" "data)"))) """
println("The input string is:\n", inputstring) processed = (inputstring |> removewhitespace |> rewritequotedparen |> quote3op
|> paren2bracket |> data2symbol |> unrewriteparens |> addcommas)
nat = eval(parse("""$processed""")) println("The processed native structure is:\n", nat) println("The reconstructed string is:\n"), printAny(nat) </lang>
- Output:
The input string is:
((data "quoted data" 123 4.5) (data (!@# (4.5) "(more" "data)")))The processed native structure is: Array{Any,1}[Any[:data, "quoted data", 123, 4.5], Any[:data, Any["!@#", [4.5], "(more", "data)"]]]
The reconstructed string is:
((data "quoted data" 123 4.5) (data(!@# (4.5) "(more" "data)")))
Kotlin
<lang groovy>// version 1.2.31
const val INDENT = 2
fun String.parseSExpr(): List<String>? {
val r = Regex("""\s*("[^"]*"|\(|\)|"|[^\s()"]+)""")
val t = r.findAll(this).map { it.value }.toMutableList()
if (t.size == 0) return null
var o = false
var c = 0
for (i in t.size - 1 downTo 0) {
val ti = t[i].trim()
val nd = ti.toDoubleOrNull()
if (ti == "\"") return null
if (ti == "(") {
t[i] = "["
c++
}
else if (ti == ")") {
t[i] = "]"
c--
}
else if (nd != null) {
val ni = ti.toIntOrNull()
if (ni != null) t[i] = ni.toString()
else t[i] = nd.toString()
}
else if (ti.startsWith("\"")) { // escape embedded double quotes
var temp = ti.drop(1).dropLast(1)
t[i] = "\"" + temp.replace("\"", "\\\"") + "\""
}
if (i > 0 && t[i] != "]" && t[i - 1].trim() != "(") t.add(i, ", ")
if (c == 0) {
if (!o) o = true else return null
}
}
return if (c != 0) null else t
}
fun MutableList<String>.toSExpr(): String {
for (i in 0 until this.size) {
this[i] = when (this[i]) {
"[" -> "("
"]" -> ")"
", " -> " "
else -> {
if (this[i].startsWith("\"")) { // unescape embedded quotes
var temp = this[i].drop(1).dropLast(1)
"\"" + temp.replace("\\\"", "\"") + "\""
}
else this[i]
}
}
}
return this.joinToString("")
}
fun List<String>.prettyPrint() {
var level = 0
loop@for (t in this) {
var n: Int
when(t) {
", ", " " -> continue@loop
"[", "(" -> {
n = level * INDENT + 1
level++
}
"]", ")" -> {
level--
n = level * INDENT + 1
}
else -> {
n = level * INDENT + t.length
}
}
println("%${n}s".format(t))
}
}
fun main(args: Array<String>) {
val str = """((data "quoted data" 123 4.5)""" + "\n" +
""" (data (!@# (4.5) "(more" "data)")))"""
val tokens = str.parseSExpr()
if (tokens == null)
println("Invalid s-expr!")
else {
println("Native data structure:")
println(tokens.joinToString(""))
println("\nNative data structure (pretty print):")
tokens.prettyPrint()
val tokens2 = tokens.toMutableList()
println("\nRecovered S-Expression:")
println(tokens2.toSExpr())
println("\nRecovered S-Expression (pretty print):")
tokens2.prettyPrint()
}
}</lang>
- Output:
Native data structure:
[[data, "quoted data", 123, 4.5], [data, [!@#, [4.5], "(more", "data)"]]]
Native data structure (pretty print):
[
[
data
"quoted data"
123
4.5
]
[
data
[
!@#
[
4.5
]
"(more"
"data)"
]
]
]
Recovered S-Expression:
((data "quoted data" 123 4.5) (data (!@# (4.5) "(more" "data)")))
Recovered S-Expression (pretty print):
(
(
data
"quoted data"
123
4.5
)
(
data
(
!@#
(
4.5
)
"(more"
"data)"
)
)
)
Nim
<lang nim>import strutils
const Input = """ ((data "quoted data" 123 4.5)
(data (!@# (4.5) "(more" "data)")))
"""
type
TokenKind = enum tokInt, tokFloat, tokString, tokIdent tokLPar, tokRPar tokEnd Token = object case kind: TokenKind of tokString: stringVal: string of tokInt: intVal: int of tokFloat: floatVal: float of tokIdent: ident: string else: discard
proc lex(input: string): seq[Token] =
var pos = 0
template current: char = if pos < input.len: input[pos] else: '\x00'
while pos < input.len:
case current
of ';':
inc(pos)
while current notin {'\r', '\n'}:
inc(pos)
if current == '\r': inc(pos)
if current == '\n': inc(pos)
of '(': inc(pos); result.add(Token(kind: tokLPar))
of ')': inc(pos); result.add(Token(kind: tokRPar))
of '0'..'9':
var
num = ""
isFloat = false
while current in Digits:
num.add(current)
inc(pos)
if current == '.':
num.add(current)
isFloat = true
inc(pos)
while current in Digits:
num.add(current)
inc(pos)
result.add(if isFloat: Token(kind: tokFloat, floatVal: parseFloat(num))
else: Token(kind: tokInt, intVal: parseInt(num)))
of ' ', '\t', '\n', '\r': inc(pos)
of '"':
var str = ""
inc(pos)
while current != '"':
str.add(current)
inc(pos)
inc(pos)
result.add(Token(kind: tokString, stringVal: str))
else:
const BannedChars = {' ', '\t', '"', '(', ')', ';'}
var ident = ""
while current notin BannedChars:
ident.add(current)
inc(pos)
result.add(Token(kind: tokIdent, ident: ident))
result.add(Token(kind: tokEnd))
type
SExprKind = enum seInt, seFloat, seString, seIdent, seList SExpr = ref object case kind: SExprKind of seInt: intVal: int of seFloat: floatVal: float of seString: stringVal: string of seIdent: ident: string of seList: children: seq[SExpr] ParseError = object of CatchableError
proc `$`*(se: SExpr): string =
case se.kind
of seInt: result = $se.intVal
of seFloat: result = $se.floatVal
of seString: result = '"' & se.stringVal & '"'
of seIdent: result = se.ident
of seList:
result = "("
for i, ex in se.children:
if ex.kind == seList and ex.children.len > 1:
result.add("\n")
result.add(indent($ex, 2))
else:
if i > 0:
result.add(" ")
result.add($ex)
result.add(")")
var
tokens = lex(Input) pos = 0
template current: Token =
if pos < tokens.len: tokens[pos] else: Token(kind: tokEnd)
proc parseInt(token: Token): SExpr =
result = SExpr(kind: seInt, intVal: token.intVal)
proc parseFloat(token: Token): SExpr =
result = SExpr(kind: seFloat, floatVal: token.floatVal)
proc parseString(token: Token): SExpr =
result = SExpr(kind: seString, stringVal: token.stringVal)
proc parseIdent(token: Token): SExpr =
result = SExpr(kind: seIdent, ident: token.ident)
proc parse(): SExpr
proc parseList(): SExpr =
result = SExpr(kind: seList)
while current.kind notin {tokRPar, tokEnd}:
result.children.add(parse())
if current.kind == tokEnd:
raise newException(ParseError, "Missing right paren ')'")
else:
inc(pos)
proc parse(): SExpr =
var token = current inc(pos) result = case token.kind of tokInt: parseInt(token) of tokFloat: parseFloat(token) of tokString: parseString(token) of tokIdent: parseIdent(token) of tokLPar: parseList() else: nil
echo parse()</lang>
- Output:
(
(data "quoted data" 123 4.5)
(data
(!@# (4.5) "(more" "data)")))
OCaml
You may be interested by this chapter of the book Real World Ocaml
The file SExpr.mli containing the interface:
<lang ocaml>(** This module is a very simple parsing library for S-expressions. *) (* Copyright (C) 2009 Florent Monnier, released under MIT license. *)
type sexpr = Atom of string | Expr of sexpr list (** the type of S-expressions *)
val parse_string : string -> sexpr list (** parse from a string *)
val parse_ic : in_channel -> sexpr list (** parse from an input channel *)
val parse_file : string -> sexpr list (** parse from a file *)
val parse : (unit -> char option) -> sexpr list (** parse from a custom function, [None] indicates the end of the flux *)
val print_sexpr : sexpr list -> unit (** a dump function for the type [sexpr] *)
val print_sexpr_indent : sexpr list -> unit (** same than [print_sexpr] but with indentation *)
val string_of_sexpr : sexpr list -> string (** convert an expression of type [sexpr] into a string *)
val string_of_sexpr_indent : sexpr list -> string (** same than [string_of_sexpr] but with indentation *)</lang>
The file SExpr.ml containing the implementation:
<lang ocaml>(** This module is a very simple parsing library for S-expressions. *) (* Copyright (C) 2009 Florent Monnier, released under MIT license. *) (* modified to match the task description *)
type sexpr = Atom of string | Expr of sexpr list
type state =
| Parse_root of sexpr list | Parse_content of sexpr list | Parse_word of Buffer.t * sexpr list | Parse_string of bool * Buffer.t * sexpr list
let parse pop_char =
let rec aux st =
match pop_char() with
| None ->
begin match st with
| Parse_root sl -> (List.rev sl)
| Parse_content _
| Parse_word _
| Parse_string _ ->
failwith "Parsing error: content not closed by parenthesis"
end
| Some c ->
match c with
| '(' ->
begin match st with
| Parse_root sl ->
let this = aux(Parse_content []) in
aux(Parse_root((Expr this)::sl))
| Parse_content sl ->
let this = aux(Parse_content []) in
aux(Parse_content((Expr this)::sl))
| Parse_word(w, sl) ->
let this = aux(Parse_content []) in
aux(Parse_content((Expr this)::Atom(Buffer.contents w)::sl))
| Parse_string(_, s, sl) ->
Buffer.add_char s c;
aux(Parse_string(false, s, sl))
end
| ')' ->
begin match st with
| Parse_root sl ->
failwith "Parsing error: closing parenthesis without openning"
| Parse_content sl -> (List.rev sl)
| Parse_word(w, sl) -> List.rev(Atom(Buffer.contents w)::sl)
| Parse_string(_, s, sl) ->
Buffer.add_char s c;
aux(Parse_string(false, s, sl))
end
| ' ' | '\n' | '\r' | '\t' ->
begin match st with
| Parse_root sl -> aux(Parse_root sl)
| Parse_content sl -> aux(Parse_content sl)
| Parse_word(w, sl) -> aux(Parse_content(Atom(Buffer.contents w)::sl))
| Parse_string(_, s, sl) ->
Buffer.add_char s c;
aux(Parse_string(false, s, sl))
end
| '"' ->
(* '"' *)
begin match st with
| Parse_root _ -> failwith "Parse error: double quote at root level"
| Parse_content sl ->
let s = Buffer.create 74 in
aux(Parse_string(false, s, sl))
| Parse_word(w, sl) ->
let s = Buffer.create 74 in
aux(Parse_string(false, s, Atom(Buffer.contents w)::sl))
| Parse_string(true, s, sl) ->
Buffer.add_char s c;
aux(Parse_string(false, s, sl))
| Parse_string(false, s, sl) ->
aux(Parse_content(Atom(Buffer.contents s)::sl))
end
| '\\' ->
begin match st with
| Parse_string(true, s, sl) ->
Buffer.add_char s c;
aux(Parse_string(false, s, sl))
| Parse_string(false, s, sl) ->
aux(Parse_string(true, s, sl))
| _ ->
failwith "Parsing error: escape character in wrong place"
end
| _ ->
begin match st with
| Parse_root _ ->
failwith(Printf.sprintf "Parsing error: char '%c' at root level" c)
| Parse_content sl ->
let w = Buffer.create 16 in
Buffer.add_char w c;
aux(Parse_word(w, sl))
| Parse_word(w, sl) ->
Buffer.add_char w c;
aux(Parse_word(w, sl))
| Parse_string(_, s, sl) ->
Buffer.add_char s c;
aux(Parse_string(false, s, sl))
end
in
aux (Parse_root [])
let string_pop_char str =
let len = String.length str in let i = ref(-1) in (function () -> incr i; if !i >= len then None else Some(str.[!i]))
let parse_string str =
parse (string_pop_char str)
let ic_pop_char ic =
(function () ->
try Some(input_char ic)
with End_of_file -> (None))
let parse_ic ic =
parse (ic_pop_char ic)
let parse_file filename =
let ic = open_in filename in let res = parse_ic ic in close_in ic; (res)
let quote s =
"\"" ^ s ^ "\""
let needs_quote s =
List.exists (String.contains s) [' '; '\n'; '\r'; '\t'; '('; ')']
let protect s =
let s = String.escaped s in if needs_quote s then quote s else s
let string_of_sexpr s =
let rec aux acc = function
| (Atom tag)::tl -> aux ((protect tag)::acc) tl
| (Expr e)::tl ->
let s =
"(" ^
(String.concat " " (aux [] e))
^ ")"
in
aux (s::acc) tl
| [] -> (List.rev acc)
in
String.concat " " (aux [] s)
let print_sexpr s =
print_endline (string_of_sexpr s)
let string_of_sexpr_indent s =
let rec aux i acc = function
| (Atom tag)::tl -> aux i ((protect tag)::acc) tl
| (Expr e)::tl ->
let s =
"\n" ^ (String.make i ' ') ^ "(" ^
(String.concat " " (aux (succ i) [] e))
^ ")"
in
aux i (s::acc) tl
| [] -> (List.rev acc)
in
String.concat "\n" (aux 0 [] s)
let print_sexpr_indent s =
print_endline (string_of_sexpr_indent s)</lang>
Then we compile this small module and test it in the interactive loop:
$ ocamlc -c SExpr.mli
$ ocamlc -c SExpr.ml
$ ocaml SExpr.cmo
Objective Caml version 3.11.2
# open SExpr ;;
# let s = read_line () ;;
((data "quoted data" 123 4.5) (data (!@# (4.5) "(more" "data)")))
val s : string =
"((data \"quoted data\" 123 4.5) (data (!@# (4.5) \"(more\" \"data)\")))"
# let se = SExpr.parse_string s ;;
val se : SExpr.sexpr list =
[Expr
[Expr [Atom "data"; Atom "quoted data"; Atom "123"; Atom "4.5"];
Expr
[Atom "data";
Expr [Atom "!@#"; Expr [Atom "4.5"]; Atom "(more"; Atom "data)"]]]]
# SExpr.print_sexpr se ;;
((data "quoted data" 123 4.5) (data (!@# (4.5) "(more" "data)")))
- : unit = ()
# SExpr.print_sexpr_indent se ;;
(
(data "quoted data" 123 4.5)
(data
(!@#
(4.5) "(more" "data)")))
- : unit = ()
Perl
<lang perl>#!/usr/bin/perl -w use strict; use warnings;
sub sexpr { my @stack = ([]); local $_ = $_[0];
while (m{ \G # start match right at the end of the previous one \s*+ # skip whitespaces # now try to match any of possible tokens in THIS order: (?<lparen>\() | (?<rparen>\)) | (?<FLOAT>[0-9]*+\.[0-9]*+) | (?<INT>[0-9]++) | (?:"(?<STRING>([^\"\\]|\\.)*+)") | (?<IDENTIFIER>[^\s()]++) # Flags: # g = match the same string repeatedly # m = ^ and $ match at \n # s = dot and \s matches \n # x = allow comments within regex }gmsx) { die "match error" if 0+(keys %+) != 1;
my $token = (keys %+)[0]; my $val = $+{$token};
if ($token eq 'lparen') { my $a = []; push @{$stack[$#stack]}, $a; push @stack, $a; } elsif ($token eq 'rparen') { pop @stack; } else { push @{$stack[$#stack]}, bless \$val, $token; } } return $stack[0]->[0]; }
sub quote { (local $_ = $_[0]) =~ /[\s\"\(\)]/s ? do{s/\"/\\\"/gs; qq{"$_"}} : $_; }
sub sexpr2txt { qq{(@{[ map { ref($_) eq ? quote($_) : ref($_) eq 'STRING' ? quote($$_) : ref($_) eq 'ARRAY' ? sexpr2txt($_) : $$_ } @{$_[0]} ]})} }</lang> Check: <lang perl>my $s = sexpr(q{
((data "quoted data" 123 4.5)
(data (!@# (4.5) "(more" "data)")))
});
- Dump structure
use Data::Dumper; print Dumper $s;
- Convert back
print sexpr2txt($s)."\n";</lang> Output:
$VAR1 = [
[
bless( do{\(my $o = 'data')}, 'IDENTIFIER' ),
bless( do{\(my $o = 'quoted data')}, 'STRING' ),
bless( do{\(my $o = '123')}, 'INT' ),
bless( do{\(my $o = '4.5')}, 'FLOAT' )
],
[
bless( do{\(my $o = 'data')}, 'IDENTIFIER' ),
[
bless( do{\(my $o = '!@#')}, 'IDENTIFIER' ),
[
bless( do{\(my $o = '4.5')}, 'FLOAT' )
],
bless( do{\(my $o = '(more')}, 'STRING' ),
bless( do{\(my $o = 'data)')}, 'STRING' )
]
]
];
((data "quoted data" 123 4.5) (data (!@# (4.5) "(more" "data)")))
Perl 6
This parses the task, but it isn't really a good lisp parser, because it always wants whitespace between lists, so (()()) will fail ( (() ()) wont)
<lang perl6>grammar S-Exp {
rule TOP {^ <s-list> $};
token s-list { '(' ~ ')' [ <in_list>+ % [\s+] | ] }
token in_list { <s-token> | <s-list> }
proto token s-token {*}
token s-token:sym<Num> {\d*\.?\d+}
token s-token:sym<String> {'"' ['\"' |<-[\\"]>]*? '"'} #'
token s-token:sym<Atom> {<-[()\s]>+}
}
- The Actions class, for each syntactic rule there is a method
- that stores some data in the abstract syntax tree with make
class S-Exp::ACTIONS {
method TOP ($/) {make $<s-list>.ast}
method s-list ($/) {make [$<in_list>».ast]}
method in_list ($/) {make $/.values[0].ast}
method s-token:sym<Num> ($/){make +$/}
method s-token:sym<String> ($/){make ~$/.substr(1,*-1)}
method s-token:sym<Atom> ($/){make ~$/}
}
multi s-exp_writer (Positional $ary) {'(' ~ $ary.map(&s-exp_writer).join(' ') ~ ')'} multi s-exp_writer (Numeric $num) {~$num} multi s-exp_writer (Str $str) {
return $str unless $str ~~ /<[(")]>|\s/;
return '()' if $str eq '()';
'"' ~ $str.subst('"', '\"' ) ~ '"';
}
my $s-exp = '((data "quoted data" 123 4.5)
(data (!@# (4.5) "(more" "data)")))';
my $actions = S-Exp::ACTIONS.new(); my $perl_array = (S-Exp.parse($s-exp, :$actions)).ast;
say "the expression:\n$s-exp\n"; say "the perl6-expression:\n{$perl_array.perl}\n"; say "and back:\n{s-exp_writer($perl_array)}";</lang>
- Output:
the expression: ((data "quoted data" 123 4.5) (data (!@# (4.5) "(more" "data)"))) the perl6-expression: [["data", "quoted data", "123", 9/2], ["data", ["!\@#", [9/2], "(more", "data)"]]] and back: ((data "quoted data" 123 4.5) (data (!@# (4.5) "(more" "data)")))
Phix
The distinction between a symbol data and a quoted string "data" is simple: both are represented as strings, with the symbol being held as "data" and the quoted string being held as "\"data\"", and you can test for the latter by seeing if the first character is a double quote. Internally, it is easy to differentiate between a symbol (held as a string) and a number, but that may not be clear on the display: 4e-5 and 4-e5 may appear similar but the latter is probably a parse failure. It may be more sensible for get_term() to raise an error if the scanf fails, than assume it is a symbol like it does now. Also, I added pp_StrFmt -3 (a combination of existing -1 and -2 behaviour) specifically for this task. <lang Phix>constant s_expr_str = """ ((data "quoted data" 123 4.5)
(data (!@# (4.5) "(more" "data)")))"""
function skip_spaces(string s, integer sidx)
while sidx<=length(s) and find(s[sidx]," \t\r\n") do sidx += 1 end while return sidx
end function
function get_term(string s, integer sidx) -- get a single quoted string, symbol, or number. integer ch = s[sidx] string res = ""
if ch='\"' then
res &= ch
while 1 do
sidx += 1
ch = s[sidx]
res &= ch
if ch='\\' then
sidx += 1
ch = s[sidx]
res &= ch
elsif ch='\"' then
sidx += 1
exit
end if
end while
else
integer asnumber = (ch>='0' and ch<='9')
while not find(ch,") \t\r\n") do
res &= ch
sidx += 1
if sidx>length(s) then exit end if
ch = s[sidx]
end while
if asnumber then
sequence scanres = scanf(res,"%f")
if length(scanres)=1 then return {scanres[1][1],sidx} end if
-- error? (failed to parse number)
end if
end if
return {res,sidx}
end function
function parse_s_expr(string s, integer sidx) integer ch = s[sidx] sequence res = {} object element
if ch!='(' then ?9/0 end if
sidx += 1
while 1 do
sidx = skip_spaces(s,sidx)
-- error? (if past end of string/missing ')')
ch = s[sidx]
if ch=')' then exit end if
if ch='(' then
{element,sidx} = parse_s_expr(s,sidx)
else
{element,sidx} = get_term(s,sidx)
end if
res = append(res,element)
end while
sidx = skip_spaces(s,sidx+1)
return {res,sidx}
end function
sequence s_expr integer sidx {s_expr,sidx} = parse_s_expr(s_expr_str,1) if sidx<=length(s_expr_str) then
printf(1,"incomplete parse(\"%s\")\n",{s_expr_str[sidx..$]})
end if
puts(1,"\nThe string:\n") ?s_expr_str
puts(1,"\nDefault pretty printing:\n") --?s_expr pp(s_expr)
puts(1,"\nBespoke pretty printing:\n") --ppEx(s_expr,{pp_Nest,1,pp_StrFmt,-3,pp_Brkt,"()"}) ppEx(s_expr,{pp_Nest,4,pp_StrFmt,-3,pp_Brkt,"()"})</lang>
- Output:
The string:
"((data \"quoted data\" 123 4.5)\n (data (!@# (4.5) \"(more\" \"data)\")))"
Default pretty printing:
{{"data", "\"quoted data\"", 123'{',4.5},
{"data", {"!@#", {4.5}, "\"(more\"", "\"data)\""}}}
Bespoke pretty printing:
((data,
"quoted data",
123,
4.5),
(data,
(!@#,
(4.5),
"(more",
"data)")))
PicoLisp
The 'any' function parses an s-expression from a string (indentical to the way 'read' does this from an input stream). <lang PicoLisp>: (any "((data \"quoted data\" 123 4.5) (data (!@# (4.5) \"(more\" \"data)\")))") -> ((data "quoted data" 123 5) (data (!@# (5) "(more" "data)")))
- (view @)
+---+-- data | | | +-- "quoted data" | | | +-- 123 | | | +-- 5 | +---+-- data
|
+---+-- !@#
|
+---+-- 5
|
+-- "(more"
|
+-- "data)"</lang>
Implementing a subset of 'any' explicitly: <lang PicoLisp>(de readSexpr ()
(case (skip)
("(" (char) (readList))
("\"" (char) (readString))
(T (readAtom)) ) ) )
(de readList ()
(make
(loop
(NIL (skip))
(T (= @ ")") (char))
(link (readSexpr)) ) ) )
(de readString ()
(pack
(make
(until (= "\"" (or (peek) (quit "Unterminated string")))
(link (char)) )
(char) ) ) )
(de readAtom ()
(let X
(make
(until (or (sp? (peek)) (member (peek) '("(" ")")))
(link (char)) ) )
(or (format X) (intern (pack X))) ) )</lang>
It can be used in a pipe to read from a string: <lang PicoLisp>: (pipe (prin "((data \"quoted data\" 123 4.5) (data (!@# (4.5) \"(more\" \"data)\")))") (readSexpr)) -> ((data "quoted data" 123 5) (data (!@# (5) "(more" "data)")))</lang> 'sym' does the reverse (i.e. builds a symbol (string) from an expression). <lang PicoLisp>: (sym @@) -> "((data \"quoted data\" 123 5) (data (!@# (5) \"(more\" \"data)\")))"</lang> Implementing a subset of the built-in printer: <lang PicoLisp>(de printSexpr (Expr Fun)
(cond
((pair Expr)
(Fun "(")
(printSexpr (car Expr) Fun)
(for X (cdr Expr)
(Fun " ")
(printSexpr X Fun) )
(Fun ")") )
((str? Expr)
(Fun "\"")
(mapc Fun (chop Expr))
(Fun "\"") )
(T (mapc Fun (chop Expr))) ) )</lang>
This can be used for plain printing <lang PicoLisp>: (printSexpr
'((data "quoted data" 123 4.5) (data (!@# (4.5) "(more" "data)"))) prin )
((data "quoted data" 123 5) (data (!@# (5) "(more" "data)")))</lang> or to collect the characters into a string: <lang PicoLisp>: (pack
(make
(printSexpr
'((data "quoted data" 123 4.5) (data (!@# (4.5) "(more" "data)")))
link ) ) )
-> "((data \"quoted data\" 123 5) (data (!@# (5) \"(more\" \"data)\")))"</lang>
Pike
<lang pike>class Symbol(string name) {
string _sprintf(int type)
{
switch(type)
{
case 's': return name;
case 'O': return sprintf("(Symbol: %s)", name||"");
case 'q': return name;
case 't': return "Symbol";
default: return sprintf("%"+int2char(type), name);
}
}
mixed cast(string type)
{
switch(type)
{
case "string": return name;
default: throw(sprintf("can not cast 'Symbol' to '%s'", type));
}
}
}
mixed value(string token) {
if ((string)(int)token==token)
return (int)token;
array result = array_sscanf(token, "%f%s");
if (sizeof(result) && floatp(result[0]) && ! sizeof(result[1]))
return result[0];
else
return Symbol(token);
}
array tokenizer(string input) {
array output = ({});
for(int i=0; i<sizeof(input); i++)
{
switch(input[i])
{
case '(': output+= ({"("}); break;
case ')': output += ({")"}); break;
case '"': //"
output+=array_sscanf(input[++i..], "%s\"%[ \t\n]")[0..0];
i+=sizeof(output[-1]);
break;
case ' ':
case '\t':
case '\n': break;
default: string token = array_sscanf(input[i..], "%s%[) \t\n]")[0];
output+=({ value(token) });
i+=sizeof(token)-1;
break;
}
}
return output;
}
// this function is based on the logic in Parser.C.group() in the pike library; array group(array tokens) {
ADT.Stack stack=ADT.Stack();
array ret =({});
foreach(tokens;; string token)
{
switch(token)
{
case "(": stack->push(ret); ret=({}); break;
case ")":
if (!sizeof(ret) || !stack->ptr)
{
// Mismatch
werror ("unmatched close parenthesis\n");
return ret;
}
ret=stack->pop()+({ ret });
break;
default: ret+=({token}); break;
}
}
return ret;
}
string sexp(array input) {
array output = ({});
foreach(input;; mixed item)
{
if (arrayp(item))
output += ({ sexp(item) });
else if (intp(item))
output += ({ sprintf("%d", item) });
else if (floatp(item))
output += ({ sprintf("%f", item) });
else
output += ({ sprintf("%q", item) });
}
return "("+output*" "+")";
}
string input = "((data \"quoted data\" 123 4.5)\n (data (!@# (4.5) \"(more\" \"data)\")))"; array data = group(tokenizer(input))[0]; string output = sexp(data);</lang>
Output:
((data "quoted data" 123 4.5) (data (!@# (4.5) "(more" "data)")))
({ ({ (Symbol: data), "quoted data", 123, 4.5 }), ({ (Symbol: data), ({ (Symbol: !@#), ({ 4.5 }), "(more", "data)" }) }) })
((data "quoted data" 123 4.5) (data (!@# (45) "(more" "data)")))
Potion
How values are stored: Tuples for list, integers for integers, floats for floats, strings for symbols, quoted strings for strings. This implementation is not the most elegant/succinct or practical (it's trusty and has no real error handling). <lang potion>isdigit = (c): 47 < c ord and c ord < 58. iswhitespace = (c): c ord == 10 or c ord == 13 or c == " ".
- str: a string of the form "...<nondigit>[{<symb>}]..."
- i: index to start at (must be the index of <nondigit>)
- => returns (<the symbol as a string>, <index after the last char>)
parsesymbol = (str, i) :
datum = ()
while (str(i) != "(" and str(i) != ")" and not iswhitespace(str(i)) and str(i) != "\"") :
datum append(str(i++))
.
(datum join, i)
.
- str: a string of the form "...[<minus>]{<digit>}[<dot>{<digit>}]..."
- i: index to start at (must be the index of the first token)
- => returns (<float or int>, <index after the last digit>)
parsenumber = (str, i) :
datum = ()
dot = false
while (str(i) != "(" and str(i) != ")" and not iswhitespace(str(i)) and str(i) != "\"") :
if (str(i) == "."): dot = true.
datum append(str(i++))
.
if (dot): (datum join number, i).
else: (datum join number integer, i).
.
- str: a string of the form "...\"....\"..."
- i: index to start at (must be the index of the first quote)
- => returns (<the string>, <index after the last quote>)
parsestring = (str, i) :
datum = ("\"")
while (str(++i) != "\"") :
datum append(str(i))
.
datum append("\"")
(datum join, ++i)
.
- str: a string of the form "...(...)..."
- i: index to start at
- => returns (<tuple/list>, <index after the last paren>)
parselist = (str, i) :
lst = ()
data = ()
while (str(i) != "("): i++.
i++
while (str(i) != ")") :
if (not iswhitespace(str(i))) :
if (isdigit(str(i)) or (str(i) == "-" and isdigit(str(i + 1)))): data = parsenumber(str, i).
elsif (str(i) == "\""): data = parsestring(str, i).
elsif (str(i) == "("): data = parselist(str, i).
else: data = parsesymbol(str, i).
lst append(data(0))
i = data(1)
. else :
++i
.
.
(lst, ++i)
.
parsesexpr = (str) :
parselist(str, 0)(0)
.
parsesexpr("(define (factorial x) \"compute factorial\" (version 2.0) (apply * (range 1 x)))") string print "\n" print parsesexpr("((data \"quoted data\" 123 4.5)
(data (!@# (4.5) \"(more\" \"data)\")))") string print
"\n" print</lang>
Python
<lang python>import re
dbg = False
term_regex = r(?mx)
\s*(?:
(?P<brackl>\()|
(?P<brackr>\))|
(?P<num>\-?\d+\.\d+|\-?\d+)|
(?P<sq>"[^"]*")|
(?P[^(^)\s]+)
)
def parse_sexp(sexp):
stack = []
out = []
if dbg: print("%-6s %-14s %-44s %-s" % tuple("term value out stack".split()))
for termtypes in re.finditer(term_regex, sexp):
term, value = [(t,v) for t,v in termtypes.groupdict().items() if v][0]
if dbg: print("%-7s %-14s %-44r %-r" % (term, value, out, stack))
if term == 'brackl':
stack.append(out)
out = []
elif term == 'brackr':
assert stack, "Trouble with nesting of brackets"
tmpout, out = out, stack.pop(-1)
out.append(tmpout)
elif term == 'num':
v = float(value)
if v.is_integer(): v = int(v)
out.append(v)
elif term == 'sq':
out.append(value[1:-1])
elif term == 's':
out.append(value)
else:
raise NotImplementedError("Error: %r" % (term, value))
assert not stack, "Trouble with nesting of brackets"
return out[0]
def print_sexp(exp):
out =
if type(exp) == type([]):
out += '(' + ' '.join(print_sexp(x) for x in exp) + ')'
elif type(exp) == type() and re.search(r'[\s()]', exp):
out += '"%s"' % repr(exp)[1:-1].replace('"', '\"')
else:
out += '%s' % exp
return out
if __name__ == '__main__':
sexp = ( ( data "quoted data" 123 4.5)
(data (123 (4.5) "(more" "data)")))
print('Input S-expression: %r' % (sexp, ))
parsed = parse_sexp(sexp)
print("\nParsed to Python:", parsed)
print("\nThen back to: '%s'" % print_sexp(parsed))</lang>
- Output
Input S-expression: '((data "quoted data" 123 4.5)\n (data (123 (4.5) "(more" "data)")))' Parsed to Python: [['data', 'quoted data', 123, 4.5], ['data', [123, [4.5], '(more', 'data)']]] Then back to: '((data "quoted data" 123 4.5) (data (123 (4.5) "(more" "data)")))'
- Simpler parser
Note that in the example above the parser also recognises and changes the type of some tokens as well as generating a nested list. If that functionality is not needed, or better done elsewhere, then the parse function can be achieved more simply by just applying the regexp: <lang python>>>> from pprint import pprint as pp >>> x = [[(t,v) for t,v in termtypes.groupdict().items() if v][0] for termtypes in re.finditer(term_regex, sexp)] >>> pp(x) [('brackl', '('),
('brackl', '('),
('s', 'data'),
('sq', '"quoted data"'),
('num', '123'),
('num', '4.5'),
('brackr', ')'),
('brackl', '('),
('s', 'data'),
('brackl', '('),
('num', '123'),
('brackl', '('),
('num', '4.5'),
('brackr', ')'),
('sq', '"(more"'),
('sq', '"data)"'),
('brackr', ')'),
('brackr', ')'),
('brackr', ')')]
>>> </lang>
Racket
Racket has builtin support for S-expressions in the form of the read function. <lang racket>
- lang racket
(define input
- <<---
((data "quoted data" 123 4.5)
(data (!@# (4.5) "(more" "data)")))
---
)
(read (open-input-string input)) </lang> Output:
'((data "quoted data" 123 4.5) (data (!@# (4.5) "(more" "data)")))
REXX
The checking of errors has been minimized (issuing of error message is very rudimentary, as is the error recovery).
More grouping symbols have been added (brackets [ ], braces { }, and guillemets « »), as well as another types of literals.
Also added were two more separators (a comma and semicolon).
Separators that could be added are more whitespace characters (vertical/horizontal tabs, line feed, form feed, tab char, etc).
It would normally be considered improper, but the literal string delimiters were left intact; making it much easier to understand what is/was being parsed.
<lang rexx>/*REXX program parses an S-expression and displays the results to the terminal. */
input= '((data "quoted data" 123 4.5) (data (!@# (4.5) "(more" "data)")))'
say center('input', length(input), "═") /*display the header title to terminal.*/
say input /* " " input data " " */
say copies('═', length(input) ) /* " " header sep " " */
grpO.=; grpO.1 = '{' ; grpC.1 = "}" /*pair of grouping symbol: braces */
grpO.2 = '[' ; grpC.2 = "]" /* " " " " brackets */
grpO.3 = '(' ; grpC.3 = ")" /* " " " " parentheses */
grpO.4 = '«' ; grpC.4 = "»" /* " " " " guillemets */
q.=; q.1 = "'" ; q.2 = '"' /*1st and 2nd literal string delimiter.*/
- = 0 /*the number of tokens found (so far). */
tabs = 10 /*used for the indenting of the levels.*/ seps = ',;' /*characters used for separation. */ atoms = ' 'seps /* " " to separate atoms. */ level = 0 /*the current level being processed. */ quoted = 0 /*quotation level (for nested quotes).*/ grpU = /*used to go up an expression level.*/ grpD = /* " " " down " " " */ @.=; do n=1 while grpO.n\==
atoms = atoms || grpO.n || grpC.n /*add Open and Closed groups to ATOMS.*/
grpU = grpU || grpO.n /*add Open groups to GRPU, */
grpD = grpD || grpC.n /*add Closed groups to GRPD, */
end /*n*/ /* [↑] handle a bunch of grouping syms*/
literals=
do k=1 while q.k\==; literals=literals || q.k /*add literal delimiters*/
end /*k*/
!=; literalStart=
do j=1 to length(input); $=substr(input, j, 1) /* ◄■■■■■text parsing*/
/* ◄■■■■■text parsing*/
if quoted then do; !=! || $; if $==literalStart then quoted=0 /* ◄■■■■■text parsing*/
iterate /* ◄■■■■■text parsing*/
end /* [↑] handle running quoted str.*/ /* ◄■■■■■text parsing*/
/* ◄■■■■■text parsing*/
if pos($, literals)\==0 then do; literalStart=$; !=! || $; quoted=1 /* ◄■■■■■text parsing*/
iterate /* ◄■■■■■text parsing*/
end /* [↑] handle start of quoted str.*/ /* ◄■■■■■text parsing*/
/* ◄■■■■■text parsing*/
if pos($, atoms)==0 then do; !=! || $ ; iterate; end /*is an atom?*/ /* ◄■■■■■text parsing*/
else do; call add!; !=$; end /*isn't an atam?*/ /* ◄■■■■■text parsing*/
/* ◄■■■■■text parsing*/
if pos($, literals)==0 then do; if pos($, grpU)\==0 then level=level + 1 /* ◄■■■■■text parsing*/
call add! /* ◄■■■■■text parsing*/
if pos($, grpD)\==0 then level=level - 1 /* ◄■■■■■text parsing*/
if level<0 then say 'error, mismatched' $ /* ◄■■■■■text parsing*/
end /* ◄■■■■■text parsing*/
end /*j*/ /* ◄■■■■■text parsing*/
/* ◄■■■■■text parsing*/
call add! /*process any residual tokens.*/ /* ◄■■■■■text parsing*/ if level\==0 then say 'error, mismatched grouping symbol' /* ◄■■■■■text parsing*/ if quoted then say 'error, no end of quoted literal' literalStart /* ◄■■■■■text parsing*/
do m=1 for #; say @.m; end /*m*/ /*display the tokens to the terminal. */
exit /*stick a fork in it, we're all done. */ /*──────────────────────────────────────────────────────────────────────────────────────*/ add!: if != then return; #=#+1; @.#=left("", max(0, tabs*(level-1)))!; !=; return</lang>
- output when using the default input:
══════════════════════════════input══════════════════════════════
((data "quoted data" 123 4.5) (data (!@# (4.5) "(more" "data)")))
═════════════════════════════════════════════════════════════════
(
(
data
"quoted data"
123
4.5
)
(
data
(
!@#
(
4.5
)
"(more"
"data)"
)
)
)
Ruby
<lang ruby>class SExpr
def initialize(str)
@original = str
@data = parse_sexpr(str)
end
attr_reader :data, :original
def to_sexpr
@data.to_sexpr
end
private
def parse_sexpr(str)
state = :token_start
tokens = []
word = ""
str.each_char do |char|
case state
when :token_start
case char
when "("
tokens << :lbr
when ")"
tokens << :rbr
when /\s/
# do nothing, just consume the whitespace
when '"'
state = :read_quoted_string
word = ""
else
state = :read_string_or_number
word = char
end
when :read_quoted_string
case char
when '"'
tokens << word
state = :token_start
else
word << char
end
when :read_string_or_number
case char
when /\s/
tokens << symbol_or_number(word)
state = :token_start
when ')'
tokens << symbol_or_number(word)
tokens << :rbr
state = :token_start
else
word << char
end
end
end
sexpr_tokens_to_array(tokens)
end
def symbol_or_number(word)
Integer(word)
rescue ArgumentError
begin
Float(word)
rescue ArgumentError
word.to_sym
end
end
def sexpr_tokens_to_array(tokens, idx = 0)
result = []
while idx < tokens.length
case tokens[idx]
when :lbr
tmp, idx = sexpr_tokens_to_array(tokens, idx + 1)
result << tmp
when :rbr
return [result, idx]
else
result << tokens[idx]
end
idx += 1
end
result[0]
end
end
class Object
def to_sexpr self end
end
class String
def to_sexpr self.match(/[\s()]/) ? self.inspect : self end
end
class Symbol
alias :to_sexpr :to_s
end
class Array
def to_sexpr
"(%s)" % inject([]) {|a, elem| a << elem.to_sexpr}.join(" ")
end
end
sexpr = SExpr.new <<END
((data "quoted data" 123 4.5)
(data (!@# (4.5) "(more" "data)")))
END
puts "original sexpr:\n#{sexpr.original}" puts "\nruby data structure:\n#{sexpr.data}" puts "\nand back to S-Expr:\n#{sexpr.to_sexpr}"</lang>
- Output:
original sexpr: ((data "quoted data" 123 4.5) (data (!@# (4.5) "(more" "data)"))) ruby data structure: [[:data, "quoted data", 123, 4.5], [:data, [:"!@#", [4.5], "(more", "data)"]]] and back to S-Expr: ((data "quoted data" 123 4.5) (data (!@# (4.5) "(more" "data)")))
Scheme
Because Scheme, like all serious lisp implementations, has a native function called read for parsing s-expressions, this code will never be used. It serves more as an example of how to write simple parsers in Scheme. It also forgos turning things into their native scheme representation and uses strings for all atoms of data.
Note that this example includes erroneous closing quotes when checking for #\" because syntax highlighting sucks and no one should have to wade through blocks of red.
Using guile scheme 2.0.11
<lang scheme>(define (sexpr-read port)
(define (help port)
(let ((char (read-char port)))
(cond
((or (eof-object? char) (eq? char #\) )) '())
((eq? char #\( ) (cons (help port) (help port)))
((char-whitespace? char) (help port))
((eq? char #\"") (cons (quote-read port) (help port)))
(#t (unread-char char port) (cons (string-read port) (help port))))))
; This is needed because the function conses all parsed sexprs onto something,
; so the top expression is one level too deep.
(car (help port)))
(define (quote-read port)
(define (help port)
(let ((char (read-char port)))
(if
(or (eof-object? char) (eq? char #\""))
'()
(cons char (help port)))))
(list->string (help port)))
(define (string-read port)
(define (help port)
(let ((char (read-char port)))
(cond
((or (eof-object? char) (char-whitespace? char)) '())
((eq? char #\) ) (unread-char char port) '())
(#t (cons char (help port))))))
(list->string (help port)))
(define (format-sexpr expr)
(define (help expr pad)
(if
(list? expr)
(begin
(format #t "~a(~%" (make-string pad #\tab))
(for-each (lambda (x) (help x (1+ pad))) expr)
(format #t "~a)~%" (make-string pad #\tab)))
(format #t "~a~a~%" (make-string pad #\tab) expr)))
(help expr 0))
(format-sexpr (sexpr-read
(open-input-string "((data \"quoted data\" 123 4.5) (data (!@# (4.5) \"(more\" \"data)\")))")))</lang>
Output:
( ( data quoted data 123 4.5 ) ( data ( !@# ( 4.5 ) (more data) ) ) )
Sidef
<lang ruby>var t = frequire('Text::Balanced');
func sexpr(txt) {
txt.trim!;
var m = txt.match(/^\((.*)\)$/s) || die "Invalid: <<#{txt}>>";
txt = m[0];
var w;
var ret = [];
while (!txt.is_empty) {
given (txt.first) {
when('(') {
(w, txt) = t.extract_bracketed(txt, '()');
w = sexpr(w);
}
when ('"') {
(w, txt) = t.extract_delimited(txt, '"')
w.sub!(/^"(.*)"/, {|s1| s1 });
}
default {
txt.sub!(/^(\S+)/, {|s1| w = s1; });
}
}
ret << w;
txt.trim_beg!;
}
return ret;
}
func sexpr2txt(String e) {
e ~~ /[\s"\(\)]/ ? do { e.gsub!('"', '\\"'); %Q("#{e}") } : e;
}
func sexpr2txt(expr) {
'(' + expr.map {|e| sexpr2txt(e) }.join(' ') + ')';
}
var s = sexpr(%q{
((data "quoted data" 123 4.5)
(data (!@# (4.5) "(more" "data)")))
});
say s; # dump structure say sexpr2txt(s); # convert back</lang>
- Output:
[["data", "quoted data", "123", "4.5"], ["data", ["!\@#", ["4.5"], "(more", "data)"]]] ((data "quoted data" 123 4.5) (data (!@# (4.5) "(more" "data)")))
Tcl
Note that because Tcl doesn't expose a type system (well, not in a conventional sense) the parts of the parsed out data structure are tagged lists; the first element is one of “string”, “int”, “real” and “atom” to indicate a leaf token, or “list” to indicate a sublist. A “native” data structure could also be generated, but then that would turn things into lists that are not in the original. <lang tcl>package require Tcl 8.5
proc fromSexp {str} {
set tokenizer {[()]|"(?:[^\\""]|\\.)*"|(?:[^()""\s\\]|\\.)+|[""]}
set stack {}
set accum {}
foreach token [regexp -inline -all $tokenizer $str] {
if {$token eq "("} { lappend stack $accum set accum {} } elseif {$token eq ")"} { if {![llength $stack]} {error "unbalanced"} set accum [list {*}[lindex $stack end] [list list {*}$accum]] set stack [lrange $stack 0 end-1] } elseif {$token eq "\""} { error "bad quote" } elseif {[string match {"*"} $token]} { set token [string range $token 1 end-1] lappend accum [list string [regsub -all {\\(.)} $token {\1}]] } else { if {[string is integer -strict $token]} { set type int } elseif {[string is double -strict $token]} { set type real } else { set type atom } lappend accum [list $type [regsub -all {\\(.)} $token {\1}]] }
}
if {[llength $stack]} {error "unbalanced"}
return [lindex $accum 0]
} proc toSexp {tokList} {
set content [lassign $tokList type]
if {$type eq "list"} {
set s "(" set sep "" foreach bit $content { append s $sep [toSexp $bit] set sep " " } return [append s ")"]
} elseif {$type eq "string"} {
return "\"[regsub -all {[\\""]} [lindex $content 0] {\\\0}]\""
} else {
return [lindex $content 0]
}
}</lang> Demonstrating with the sample data: <lang tcl>set sample {((data "quoted data" 123 4.5)
(data (!@# (4.5) "(more" "data)")))}
set parsed [fromSexp $sample] puts "sample: $sample" puts "parsed: $parsed" puts "regen: [toSexp $parsed]"</lang> Output:
sample: ((data "quoted data" 123 4.5)
(data (!@# (4.5) "(more" "data)")))
parsed: list {list {atom data} {string {quoted data}} {int 123} {real 4.5}} {list {atom data} {list {atom !@#} {list {real 4.5}} {string (more} {string data)}}}
regen: ((data "quoted data" 123 4.5) (data (!@# (4.5) "(more" "data)")))
As you can see, whitespace is not preserved in non-terminal locations.
TXR
TXR is in the Lisp family, and uses S-Expressions. So right from the system prompt we can do:
$ txr -p '(read)' ((data "quoted data" 123 4.5) (data (!@# (4.5) "(more" "data)"))) [Ctrl-D][Enter] ((data "quoted data" 123 4.5) (data (! (sys:var #(4.5)) "(more" "data)")))
However, note that the @ character has a special meaning: @obj turns into (sys:var obj). The purpose of this notation is to support Lisp code that requires meta-variables and meta-expressions. This can be used, for instance, in pattern matching to distinguish binding variables and matching operations from literal syntax.
The following solution avoids "cheating" in this way with the built-in parser; it implements a from-scratch S-exp parser which treats !@# as just a symbol.
The grammar is roughly as follows:
expr := ws? atom
| ws? ( ws? expr* ws? )
atom := float | int | sym | str
float := sign? digit+ . digit* exponent?
| sign? digit* . digit+ exponent?
| sign? digit+ exponent
int := sign? digit+
str := " (\" | anychar )* "
sym := sym-char +
sym-char := /* non-whitespace, but not ( and not ) */Code:
<lang txr>@(define float (f))@\
@(local (tok))@\
@(cases)@\
@{tok /[+\-]?\d+\.\d*([Ee][+\-]?\d+)?/}@\
@(or)@\
@{tok /[+\-]?\d*\.\d+([Ee][+\-]?\d+)?/}@\
@(or)@\
@{tok /[+\-]?\d+[Ee][+\-]?\d+/}@\
@(end)@\
@(bind f @(flo-str tok))@\
@(end) @(define int (i))@\
@(local (tok))@\
@{tok /[+\-]?\d+/}@\
@(bind i @(int-str tok))@\
@(end) @(define sym (s))@\
@(local (tok))@\
@{tok /[^\s()]+/}@\
@(bind s @(intern tok))@\
@(end) @(define str (s))@\
@(local (tok))@\
@{tok /"(\\"|[^"])*"/}@\
@(bind s @[tok 1..-1])@\
@(end) @(define atom (a))@\
@(cases)@\ @(float a)@(or)@(int a)@(or)@(str a)@(or)@(sym a)@\ @(end)@\
@(end) @(define expr (e))@\
@(cases)@\ @/\s*/@(atom e)@\ @(or)@\ @/\s*\(\s*/@(coll :vars (e))@(expr e)@/\s*/@(last))@(end)@\ @(end)@\
@(end) @(freeform) @(expr e)@junk @(output) expr: @(format nil "~s" e) junk: @junk @(end)</lang>
Run:
$ txr s-expressions.txr - () expr: nil junk: $ txr s-expressions.txr - 3e3 expr: 3000.0 junk: $ txr s-expressions.txr - +3 expr: 3 junk: $ txr s-expressions.txr - abc* expr: abc* junk: $ txr s-expressions.txr - abc*) expr: abc* junk: ) $ txr s-expressions.txr - ((data "quoted data" 123 4.5) (data (!@# (4.5) "(more" "data)"))) expr: ((data "quoted data" 123 4.5) (data (!@# (4.5) "(more" "data)"))) junk:
TODO: Note that the recognizer for string literals does not actually process the interior escape sequences \"; these remain as part of the string data. The only processing is the stripping of the outer quotes from the lexeme.
Explanation of most confusing line:
<lang txr> @/\s*\(\s*/@(coll :vars (e))@(expr e)@/\s*/@(last))@(end)</lang>
First, we match an open parenthesis that can be embedded in whitespace. Then we have a @(coll) construct which terminates with @(end). This is a repetition construct for collecting zero or more items. The :vars (e) argument makes the collect strict: each repetition must bind the variable e. More importantly, in this case, if nothing is
collected, then e gets bound to nil (the empty list). The collect construct does not look at context beyond itself. To terminate the collect at the closing parenthesis we use @(last)). The second closing parenthesis here is literal text to be matched, not TXR syntax. This special clause establishes the terminating context without which the collect will munge all input. When the last clause matches, whatever it matches is consumed and the collect ends. (There is a related @(until) clause which terminates the collect, but leaves its own match unconsumed.)
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