Recursive descent parser generator

From Rosetta Code
Recursive descent parser generator is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.

Write a recursive descent parser generator that takes a description of a grammar as input and outputs the source code for a parser in the same language as the generator. (So a generator written in C++ would output C++ source code for the parser.) You can assume that all of the rules have been preprocessed into a form suitable for the construction of a recursive descent parser. Check the following links for more details.

Use the parser generator and a grammar file to build a parser that takes an arithmetic expression and turns it in to three address code. The resulting parser should take this (or something similar) as input:

(one + two) * three - four * five

And generate this (or something similar) as output:

_0001 = one + two
_0002 = _0001 * three
_0003 = four * five
_0004 = _0002 - _0003

C++[edit]

Works with: C++11

This program translates an annotated LL(1) grammar into a C++ lexer plus parser. Each rule is required to return a string of some kind and the return values of the already matched nonterminals (and matched text of terminals) can be accessed with $1, $2, etc. which are replaced by the appropriate string variable.

It can't handle newlines as part of the grammar, the error checking is fairly limited and the error reporting is basically non-existent, but the parser it generates (not shown below) is human readable.

 
#include <iostream>
#include <fstream>
#include <string>
#include <sstream>
#include <map>
#include <set>
#include <regex>
using namespace std;
 
map<string, string> terminals;
map<string, vector<vector<string>>> nonterminalRules;
map<string, set<string>> nonterminalFirst;
map<string, vector<string>> nonterminalCode;
 
int main(int argc, char **argv) {
if (argc < 3) {
cout << "Usage: <input file> <output file>" << endl;
return 1;
}
 
ifstream inFile(argv[1]);
ofstream outFile(argv[2]);
 
regex blankLine(R"(^\s*$)");
regex terminalPattern(R"((\w+)\s+(.+))");
regex rulePattern(R"(^!!\s*(\w+)\s*->\s*((?:\w+\s*)*)$)");
regex argPattern(R"(\$(\d+))");
smatch results;
 
// Read terminal patterns
string line;
while (true) {
getline(inFile, line);
 
// Terminals section ends with a blank line
if (regex_match(line, blankLine))
break;
 
regex_match(line, results, terminalPattern);
terminals[results[1]] = results[2];
}
 
outFile << "#include <iostream>" << endl;
outFile << "#include <fstream>" << endl;
outFile << "#include <string>" << endl;
outFile << "#include <regex>" << endl;
outFile << "using namespace std;" << endl << endl;
 
// Generate the token processing functions
outFile << "string input, nextToken, nextTokenValue;" << endl;
outFile << "string prevToken, prevTokenValue;" << endl << endl;
 
outFile << "void advanceToken() {" << endl;
outFile << " static smatch results;" << endl << endl;
 
outFile << " prevToken = nextToken;" << endl;
outFile << " prevTokenValue = nextTokenValue;" << endl << endl;
 
for (auto i = terminals.begin(); i != terminals.end(); ++i) {
string name = i->first + "_pattern";
string pattern = i->second;
 
outFile << " static regex " << name << "(R\"(^\\s*(" << pattern << "))\");" << endl;
outFile << " if (regex_search(input, results, " << name << ", regex_constants::match_continuous)) {" << endl;
outFile << " nextToken = \"" << i->first << "\";" << endl;
outFile << " nextTokenValue = results[1];" << endl;
outFile << " input = regex_replace(input, " << name << ", \"\");" << endl;
outFile << " return;" << endl;
outFile << " }" << endl << endl;
}
 
outFile << " static regex eof(R\"(\\s*)\");" << endl;
outFile << " if (regex_match(input, results, eof, regex_constants::match_continuous)) {" << endl;
outFile << " nextToken = \"\";" << endl;
outFile << " nextTokenValue = \"\";" << endl;
outFile << " return;" << endl;
outFile << " }" << endl << endl;
 
outFile << " throw \"Unknown token\";" << endl;
outFile << "}" << endl << endl;
 
outFile << "bool same(string symbol) {" << endl;
outFile << " if (symbol == nextToken) {" << endl;
outFile << " advanceToken();" << endl;
outFile << " return true;" << endl;
outFile << " }" << endl;
outFile << " return false;" << endl;
outFile << "}" << endl << endl;
 
// Copy the header code to the output
while (true) {
getline(inFile, line);
 
// Copy lines until we reach the first rule
if (regex_match(line, results, rulePattern))
break;
 
outFile << line << endl;
}
 
// Build the nonterminal table
while (true) {
// results already contains the last matched rule
string name = results[1];
stringstream ss(results[2]);
 
string tempString;
vector<string> tempVector;
while (ss >> tempString)
tempVector.push_back(tempString);
nonterminalRules[name].push_back(tempVector);
 
// Read code until another rule is found
string code = "";
while (true) {
getline(inFile, line);
 
if (!inFile || regex_match(line, results, rulePattern))
break;
 
// Replace $1 with results[1], etc.
line = regex_replace(line, argPattern, "results[$1]");
 
code += line + "\n";
}
nonterminalCode[name].push_back(code);
 
// Stop when we reach the end of the file
if (!inFile)
break;
}
 
// Generate the first sets, inefficiently
bool done = false;
while (!done)
for (auto i = nonterminalRules.begin(); i != nonterminalRules.end(); ++i) {
string name = i->first;
done = true;
 
if (nonterminalFirst.find(i->first) == nonterminalFirst.end())
nonterminalFirst[i->first] = set<string>();
 
for (int j = 0; j < i->second.size(); ++j) {
if (i->second[j].size() == 0)
nonterminalFirst[i->first].insert("");
else {
string first = i->second[j][0];
if (nonterminalFirst.find(first) != nonterminalFirst.end()) {
for (auto k = nonterminalFirst[first].begin(); k != nonterminalFirst[first].end(); ++k) {
if (nonterminalFirst[name].find(*k) == nonterminalFirst[name].end()) {
nonterminalFirst[name].insert(*k);
done = false;
}
}
} else if (nonterminalFirst[name].find(first) == nonterminalFirst[name].end()) {
nonterminalFirst[name].insert(first);
done = false;
}
}
}
}
 
// Generate function signatures for the nonterminals
for (auto i = nonterminalRules.begin(); i != nonterminalRules.end(); ++i) {
string name = i->first + "_rule";
outFile << "string " << name << "();" << endl;
}
outFile << endl;
 
// Generate the nonterminal functions
for (auto i = nonterminalRules.begin(); i != nonterminalRules.end(); ++i) {
string name = i->first + "_rule";
outFile << "string " << name << "() {" << endl;
outFile << " vector<string> results;" << endl;
outFile << " results.push_back(\"\");" << endl << endl;
 
// Check if this rule can match an empty string
int epsilon = -1;
for (int j = 0; epsilon == -1 && j < i->second.size(); ++j)
if (i->second[j].size() == 0)
epsilon = j;
 
// Generate each production
for (int j = 0; j < i->second.size(); ++j) {
// Nothing to generate for an empty rule
if (j == epsilon)
continue;
 
string token = i->second[j][0];
if (terminals.find(token) != terminals.end())
outFile << " if (nextToken == \"" << i->second[j][0] << "\") {" << endl;
else {
outFile << " if (";
bool first = true;
for (auto k = nonterminalFirst[token].begin(); k != nonterminalFirst[token].end(); ++k, first = false) {
if (!first)
outFile << " || ";
outFile << "nextToken == \"" << (*k) << "\"";
}
outFile << ") {" << endl;
}
 
for (int k = 0; k < i->second[j].size(); ++k) {
if (terminals.find(i->second[j][k]) != terminals.end()) {
outFile << " if(same(\"" << i->second[j][k] << "\"))" << endl;
outFile << " results.push_back(prevTokenValue);" << endl;
outFile << " else" << endl;
outFile << " throw \"Syntax error - mismatched token\";" << endl;
} else
outFile << " results.push_back(" << i->second[j][k] << "_rule());" << endl;
}
 
// Copy rule code to output
outFile << nonterminalCode[i->first][j];
 
outFile << " }" << endl << endl;
}
 
if (epsilon == -1)
outFile << " throw \"Syntax error - unmatched token\";" << endl;
else
outFile << nonterminalCode[i->first][epsilon];
 
outFile << "}" << endl << endl;
}
 
// Generate the main function
outFile << "int main(int argc, char **argv) {" << endl;
outFile << " if(argc < 2) {" << endl;
outFile << " cout << \"Usage: <input file>\" << endl;" << endl;
outFile << " return 1;" << endl;
outFile << " }" << endl << endl;
 
outFile << " ifstream file(argv[1]);" << endl;
outFile << " string line;" << endl;
outFile << " input = \"\";" << endl << endl;
 
outFile << " while(true) {" << endl;
outFile << " getline(file, line);" << endl;
outFile << " if(!file) break;" << endl;
outFile << " input += line + \"\\n\";" << endl;
outFile << " }" << endl << endl;
 
outFile << " advanceToken();" << endl << endl;
 
outFile << " start_rule();" << endl;
outFile << "}" << endl;
}
 

Example grammar:

plus	\+
minus	-
times	\*
div	/
open	\(
close	\)
num	[0-9]+
var	[a-z]+

string nextLabel() {
	static string label = "0000";
	for(int i = label.length() - 1; i >= 0; --i) {
		if(label[i] == '9')
			label[i] = '0';
		else {
			++label[i];
			break;
		}
	}
	return "_" + label;
}

!! start -> expr start2
if($2 == "")
	return $1;
else {
	string label = nextLabel();
	cout << label << " = " << $1 << " " << $2 << endl;
	return label;
}

!! start2 -> plus start
return "+ " + $2;

!! start2 -> minus start
return "- " + $2;

!! start2 -> 
return "";

!! expr -> term expr2
if($2 == "")
	return $1;
else {
	string label = nextLabel();
	cout << label << " = " << $1 << " " << $2 << endl;
	return label;
}

!! expr2 -> times expr
return "* " + $2;

!! expr2 -> div expr
return "/ " + $2;

!! expr2 ->
return "";

!! term -> var
return $1;

!! term -> num
return $1;

!! term -> open start close
return $2;

Example input to parser (filename passed through command line):

(one + two) * three + four * five

Output (to standard out):

_0001 = one + two
_0002 = _0001 * three
_0003 = four * five
_0004 = _0002 + _0003

J[edit]

J's native recursive descent parser is adequate for this task, if we map names appropriately.

Implementation:

cocurrent 'base'
 
inlocale=: 4 :0 L:0
x,'_',y,'_'
)
 
parse=: 3 :0
sentence=. ;:y
opinds=. (;:'+*-')i.sentence
opfuns=. (;:'plus times minus') inlocale 'base'
scratch=. cocreate''
coinsert__scratch 'base'
names=. ~.sentence#~_1<:nc sentence
(names inlocale scratch)=: names
r=. do__scratch ;:inv opinds}((#sentence)#"0 opfuns),sentence
codestroy__scratch''
r
)
 
term=: 1 :0
2 :('m''',m,'''expr n')
)
 
expr=:1 :0
:
r=. genname''
emit r,'=:',x,m,y
r
)
 
plus=: '+' expr
times=: '*' term
minus=: '-' expr
 
N=: 10000
genname=: 3 :0
'z',}.":N=: N+1
)
 
emit=: smoutput
 

Task example:

   parse '(one + two) * three - four * five'
z0001=:four*five
z0002=:one+two
z0003=:z0002*three
z0004=:z0003-z0001
z0004

See also http://www.jsoftware.com/svn/base/trunk/packages/misc/trace.ijs for a model of the underlying parser being employed here.