Metaprogramming

From Rosetta Code
Task
Metaprogramming
You are encouraged to solve this task according to the task description, using any language you may know.

Name and briefly demonstrate any support your language has for metaprogramming. Your demonstration may take the form of cross-references to other tasks on Rosetta Code. When possible, provide links to relevant documentation.

For the purposes of this task, "support for metaprogramming" means any way the user can effectively modify the language's syntax that's built into the language (like Lisp macros) or that's conventionally used with the language (like the C preprocessor). Such facilities need not be very powerful: even user-defined infix operators count. On the other hand, in general, neither operator overloading nor eval count. The task author acknowledges that what qualifies as metaprogramming is largely a judgment call.

ALGOL 68

Works with: ALGOL 68G version Any - tested with release 2.8.win32

ALGOL 68 allows the definition of new unary and binary operators, as well at the overloading of existing operators. This sample adds a COBOL-like INSPECT "statement" by defining suitable operators.

# This example uses ALGOL 68 user defined operators to add a COBOL-style     #
# "INSPECT statement" to ALGOL 68                                            #
#                                                                            #
# The (partial) syntax of the COBOL INSPECT is:                              #
#    INSPECT string-variable REPLACING ALL     string BY string              #
# or INSPECT string-variable REPLACING LEADING string BY string              #
# or INSPECT string-variable REPLACING FIRST   string BY string              #
#                                                                            #
# Because "BY" is a reserved bold word in ALGOL 68, we use "WITH" instead    #
#                                                                            #
# We define unary  operators INSPECT, ALL, LEADING and FIRST                 #
#       and binary operators REPLACING and WITH                              #
# We choose the priorities of REPLACING and WITH so that parenthesis is not  #
# needed to ensure the correct interpretation of the "statement"             #
#                                                                            #
# We also provide a unary DISPLAY operator for a partial COBOL DISPLAY       #
# statement                                                                  #

# INSPECTEE is returned by the INSPECT unary operator                        #
MODE INSPECTEE   = STRUCT( REF STRING item, INT option );

# INSPECTTOREPLACE is returned by the binary REPLACING operator              #
MODE INSPECTTOREPLACE
                 = STRUCT( REF STRING item, INT option, STRING to replace );
# REPLACEMENT is returned by the unary ALL, LEADING and FIRST operators      #
MODE REPLACEMENT = STRUCT( INT option, STRING replace );

# REPLACING option codes, these are the option values for a REPLACEMENT      #
INT  replace all     = 1;
INT  replace leading = 2;
INT  replace first   = 3;

OP   INSPECT   = ( REF STRING s )INSPECTEE: ( s, 0 );
OP   ALL       = ( STRING replace )REPLACEMENT: ( replace all,     replace );
OP   LEADING   = ( STRING replace )REPLACEMENT: ( replace leading, replace );
OP   FIRST     = ( STRING replace )REPLACEMENT: ( replace first,   replace );
OP   ALL       = ( CHAR   replace )REPLACEMENT: ( replace all,     replace );
OP   LEADING   = ( CHAR   replace )REPLACEMENT: ( replace leading, replace );
OP   FIRST     = ( CHAR   replace )REPLACEMENT: ( replace first,   replace );

OP   REPLACING = ( INSPECTEE inspected, REPLACEMENT replace )INSPECTTOREPLACE:
                     ( item    OF inspected
                     , option  OF replace
                     , replace OF replace
                     );

OP   WITH      = ( INSPECTTOREPLACE inspected, CHAR   replace with )REF STRING:
    BEGIN
        STRING with := replace with;
        inspected WITH with
    END; # WITH #

OP   WITH      = ( INSPECTTOREPLACE inspected, STRING replace with )REF STRING:
    BEGIN

        STRING    to replace  = to replace OF inspected;
        INT       pos        := 0;
        STRING    rest       := item OF inspected;
        STRING    result     := "";

        IF   option OF inspected = replace all
        THEN
            # replace all occurances of "to replace" with "replace with"     #
            WHILE string in string( to replace, pos, rest )
            DO
                result +:= rest[ 1 : pos - 1 ] + replace with;
                rest    := rest[ pos + UPB to replace : ]
            OD

        ELIF option OF inspected = replace leading
        THEN
            # replace leading occurances of "to replace" with "replace with" #
            WHILE IF string in string( to replace, pos, rest )
                  THEN
                      pos = 1
                  ELSE
                      FALSE
                  FI
            DO
                result +:= replace with;
                rest    := rest[ 1 + UPB to replace : ]
            OD

        ELIF option OF inspected = replace first
        THEN
            # replace first occurance of "to replace" with "replace with"    #
            IF string in string( to replace, pos, rest )
            THEN
                result +:= rest[ 1 : pos - 1 ] + replace with;
                rest    := rest[ pos + UPB to replace : ]
            FI

        ELSE
            # unsupported replace option #
            write( ( newline, "*** unsupported INSPECT REPLACING...", newline ) );
            stop
        FI;

        result +:= rest;
        item OF inspected := result
    END; # WITH #

OP   DISPLAY = ( STRING s )VOID: write( ( s, newline ) );


PRIO REPLACING = 2, WITH = 1;




main: (

    # test the INSPECT and DISPLAY "verbs" #

    STRING  text := "some text";
    DISPLAY text;

    INSPECT text REPLACING FIRST   "e"    WITH "bbc";
    DISPLAY text;

    INSPECT text REPLACING ALL     "b"    WITH "X";
    DISPLAY text;

    INSPECT text REPLACING ALL     "text" WITH "some";
    DISPLAY text;

    INSPECT text REPLACING LEADING "som"  WITH "k";
    DISPLAY text


)

Output:

some text
sombbc text
somXXc text
somXXc some
kXXc some

Arturo

Arturo has been designed with flexibility in mind (see: DSL creation) and as different languages of the same heritage (e.g. REBOL, Red, etc) has meta-programming capabilities as part of the language itself.

Let's see some examples:

Infix Operators

sumThemUp: function [x,y][
    x+y
]

alias.infix '--> 'sumThemUp

do [
    print 3 --> 4
]
Output:
7

Runtime Code Evaluation

code: "print 123"
do code
Output:
123

Symbol Creation & Access at Runtime

myvar: "iAmAVariable"

let myvar 2

print myvar         ; print the name of the variable

print var myvar     ; print the value of the variable
print iAmAVariable ; the same
Output:
iAmAVariable
2
2

Data as Code & Code as Data

block: [print]
block: block ++ to :integer "34"

print "Here is our code:"
print as.code block

print ""
print "And here's its result:"
do block
Output:
Here is our code:
[print 34]

And here's its result:
34

C

C preprocessor can be used to extend language to some extent.

It's possible to create static assertions

// http://stackoverflow.com/questions/3385515/static-assert-in-c
#define STATIC_ASSERT(COND,MSG) typedef char static_assertion_##MSG[(!!(COND))*2-1]
// token pasting madness:
#define COMPILE_TIME_ASSERT3(X,L) STATIC_ASSERT(X,static_assertion_at_line_##L)
#define COMPILE_TIME_ASSERT2(X,L) COMPILE_TIME_ASSERT3(X,L)
#define COMPILE_TIME_ASSERT(X)    COMPILE_TIME_ASSERT2(X,__LINE__)

COMPILE_TIME_ASSERT(sizeof(long)==8); 
int main()
{
    COMPILE_TIME_ASSERT(sizeof(int)==4); 
}

Another common usage is to create custom loops

//requires C99
#define ITERATE_LIST(n, list) \
 for(Node *n = (list)->head; n; n = n->next)

...
ITERATE_LIST(n, list)
{
    printf("node value: %s\n", n->value);
}

For examples in real world, look FreeCiv, and OpenTTD macros(city_map_iterate for FreeCiv, FOR_ALL_ENGINES for OpenTTD).

Also, C does not support functions overloading, but because macro calls do not require type it's possible to emulate overloading to some extent

#define my_min(x, y) ((x) < (y) ? (x) : (y))
...
printf("%f %d %ll\n", my_min(0.0f, 1.0f), my_min(1,2), my_min(1ll, 2ll));

The Order programming language is implemented entirely using the C preprocessor, providing a portable, high-level, functional programming language that can be used to metaprogram any C99 project, in a fashion loosely similar to Lisp's macro system.

Metalang99 is also a functional language aimed at full-blown preprocessor metaprogramming in pure C99. With the aid of Metalang99, such things as Datatype99 and Interface99 became possible.

Library: Gadget

Gadget is a basic library that combines the use of macros and functions to facilitate programming in pure C.

Some useful macros that you can find in this library are the following:

#define  Str_init(_V_)       char * _V_=NULL;

#define  Free_secure(_X_)    if(_X_) { free(_X_); _X_=NULL; }

#define  Let(_X_,_Y_)       \
    do{\
       if(_X_) free(_X_);\
       int len = strlen(_Y_);\
       _X_ = (char*)calloc( len + 1, 1);\
       if(_X_) { memcpy(_X_, _Y_, len); }\
       else { perror("\033[38;5;196mLet: No hay memoria para <"#_X_">(CALLOC)\n\033[0m"); }\
    }while(0);

/* inicia el trabajo con el stack */
#define  Stack       if( (PILA_GADGET = 1) )

/* finaliza el trabajo con el stack. La pila debe quedar en "0" */
#define  Stack_off   \
      PILA_GADGET = 0; \
      if(CONTADOR_PILA>=0){ Msg_red("Proceso termina con stack ocupado: borro sobrante\n");\
      CONTADOR_PILA=-1; }

/* 
   STORE almacena el valor en la variable indicada, obtenido desde el
   stack. */
#define Store(_X_,_Y_)   \
 do{\
   _Y_;\
   if(PILA_GADGET){\
       if( CONTADOR_PILA>=0 ){\
           Let(_X_, pila_de_trabajo[CONTADOR_PILA]);CONTADOR_PILA--;\
       }\
   }else{ Msg_amber("Store: No hay datos en la pila");}\
 }while(0);
...
#define Main  \
        int main(int argc, char* argv[]){\
            __TOKEN__=NULL;\
            Init_token();\
            Init_stack;

/* SALIDA NORMAL */
#define End              End_token(); \
                         Free_stack_str;\
                         return(0); }

With these macros it is possible to write programs like this:

#include <gadget/gadget.h>
LIB_GADGET_START

Main
   String w, v="María tenía un corderito";

   Stack{
       Store( v, Substr(v, Str_at("tenía",v),Str_len( Upper(v) )) );
       Store( v, Trim(Left( Upper(v), Str_at("CORDERITO",Upper(v))-1)));
   }Stack_off;

   Print "msg stack : [%s]\n\n", v;
   
   Let( v, "María tenía un corderito");

  /* Str_len() sirve sin stack, pero en este caso es mejor usar strlen() de C. */
   w = Substr(v, Str_at("tenía",v),Str_len(v));
   Print "msg normal: %s\n", w;

   Free secure w,v;
End

Note: the "Free_secure()" and "Str_init()" macros are preprocessed before entering the compile cycle.

Other interesting macros that can extend the C language are the "Assert" and "Exception" macros, which use the "hated GOTO":

#define Throw(_X_)       if( !Is_ok ) { goto _X_; }
#define Exception(_H_)   _H_: if( !Is_ok++ )
#define Assert(_X_,_Y_)  if( !(_X_) ) { Is_ok=0; goto _Y_; }

Example:

#include <gadget/gadget.h>
LIB_GADGET_START

Main
      int retVal=0;
      Assert( Arg_count == 2, fail_input );
      Get_arg_str( filename, 0 );
      Get_arg_float( number, 1 );

      Print "First argument (filename) = %s\n", filename;
      Print "Second argument (a number) = %f\n", number;

      Free secure filename;

   Exception( fail_input ){
      Msg_yellow("Use:\n  ./prog <number>\n");
      retVal=1;
   }
Return( retVal );

There are also macros that, in combination with functions, allow you to extend the C language and simplify its programming:

/* declara un array vacío */                     
#define New_mt_array(_X_)  \
    MT_CELL *_X_ = NULL;\
    Define_New_Array(_X_)\
    _X_##_data.type = MULTI_TYPE;
....
/* acceso a celdas sin puntero */
#define Cell(_X_, ...) CONCAT2(Cell, COUNT_ARGUMENTS(__VA_ARGS__))(_X_, ##__VA_ARGS__)

#define Cell1(_X_,ARG1)              _X_[ ( ARG1 ) ]
#define Cell2(_X_,ARG1,ARG2)         _X_[ ( ARG1 ) * ( _X_##_data.cols ) + ( ARG2 ) ]
#define Cell3(_X_,ARG1,ARG2,ARG3)    _X_[ ( ( ARG1 ) * ( _X_##_data.cols ) + ( ARG2 ) ) + ( ARG3 ) * ( (_X_##_data.cols) * (_X_##_data.rows) ) ]

#define Cell4(_X_,ARG1,ARG2,ARG3,ARG4) \
        _X_[ ( ( ARG1 ) * ( _X_##_data.cols ) + ( ARG2 ) ) + (( ARG3 ) * ( (_X_##_data.cols) * (_X_##_data.rows) )) + (( ARG4 ) * ( (_X_##_data.cols) * (_X_##_data.rows) * _X_##_data.pags )) ]
...
/* RANGOS para acceso iterado */
#define Range_for(_X_, ...)  CONCAT2(Range_for, COUNT_ARGUMENTS(__VA_ARGS__))(_X_, ##__VA_ARGS__)

/* para un array 1D */
#define Range_for3(_X_,A1,A2,A3)  \
        _X_##_data.rowi=A1;_X_##_data.rowinc=A2;_X_##_data.rowe=A3;

/* para un array 2D */
#define Range_for6(_X_,A1,A2,A3,B1,B2,B3) \
        _X_##_data.rowi=A1;_X_##_data.rowinc=A2;_X_##_data.rowe=A3; \
        _X_##_data.coli=B1;_X_##_data.colinc=B2;_X_##_data.cole=B3; 
....

Example:

#include <gadget/gadget.h>

LIB_GADGET_START

void Muestra_archivo_original();

Main
   Assert (Exist_file("load_matrix.txt"), file_not_found);

   /* recupero informacion del archivo para su apertura segura */
   F_STAT dataFile = Stat_file("load_matrix.txt");
   Assert (dataFile.is_matrix, file_not_matrixable)  // tiene forma de matriz???
   
   New multitype test;
            
   /* establezco los rangos a leer */
   Range for test [0:1:dataFile.total_lines-1, 0:1:dataFile.max_tokens_per_line-1];
            
   /* cargamos el array detectando números enteros como long */
   test = Load_matrix_mt( pSDS(test), "load_matrix.txt", dataFile, DET_LONG);
            
   /* modifica algunas cosas del archivo */
   Let( $s-test[0,1], "Columna 1");
   $l-test[2,1] = 1000;
   $l-test[2,2] = 2000;
            
   /* inserto filas */
   /* preparo la fila a insertar */
   New multitype nueva_fila;
   sAppend_mt(nueva_fila,"fila 3.1");  /* sAppend_mt() and Append_mt() are macros */
   Append_mt(nueva_fila,float,0.0);
   Append_mt(nueva_fila,int,0);
   Append_mt(nueva_fila,double,0.0);
   Append_mt(nueva_fila,long, 0L);
   
   /* insertamos la misma fila en el array, 3 veces */
   test = Insert_row_mt(pSDS(test),pSDS(nueva_fila), 4);
   test = Insert_row_mt(pSDS(test),pSDS(nueva_fila), 4);
   test = Insert_row_mt(pSDS(test),pSDS(nueva_fila), 4);
   Free multitype nueva_fila;
            
   Print "\nGuardando archivo en \"save_matrix.txt\"...\n";
   DEC_PREC = 20; /* establece precision decimal */
            
   All range for test;
   Save_matrix_mt(SDS(test), "save_matrix.txt" );

   Free multitype test;
            
   Print "\nArchivo original:\n";
   Muestra_archivo_original();
   
   Exception( file_not_found ){
      Msg_red("File not found\n");
   }
   Exception( file_not_matrixable ){
      Msg_red("File is not matrixable\n");
   }

End

void Muestra_archivo_original(){
    String csys;
    csys = `cat load_matrix.txt`;
    Print "\n%s\n", csys;
    Free secure csys;
}

Note: "Range_for()", "Cell()", "v=`...`, and "New_mt_array()" macros are preprocessed before entering the compile cycle.

C#

Metaprogramming in C# can be achieved using the Text Template Transformation Toolkit. It is a textual preprocessor embedded in Visual Studio (it can also be executed from the command-line, e.g. in build scripts). It is language-agnostic, and therefore can generate code for C#, Visual Basic or other languages. This also means that it has no features which help manipulating the underlying language: it is purely textual, and does not include a C# parser to transform existing C# files (so you will need to roll your own or use Roslyn), and does not include utilities which would help with combining pieces of code.

Clojure

See Clojure Macros Reference article.

Common Lisp

Built-In Fruits of Metaprogramming

Common Lisp is based on decades of metaprogramming, so programmers don't have to roll their own to benefit from it. For instance, the LOOP syntax is just a macro. Prior to becoming a standard language feature, it was just a library that users shared. The object system originated in the same way.

Calculate mean, and sample variance and sample standard deviation of some numbers:

(loop for count from 1
      for x in '(1 2 3 4 5)
      summing x into sum
      summing (* x x) into sum-of-squares
      finally
        (return
          (let* ((mean (/ sum count))
                 (spl-var (- (* count sum-of-squares) (* sum sum)))
                 (spl-dev (sqrt (/ spl-var (1- count))))) 
            (values mean spl-var spl-dev))))
=>
5/2 ;
105 ;
4.582576

Being a macro, if LOOP were removed from Lisp, it could be supplied by the application program. In fact, sometimes programs have included their own LOOP to work around bugs in some implementations.

Here is what CLISP makes of the above, by investigating the macro expansion using the ANSI standard macroexpand function:

[5]>    
  (macroexpand'
     (loop for count from 1
           for x in '(1 2 3 4 5)
           summing x into sum
           summing (* x x) into sum-of-squares
           finally
             (return
               (let* ((mean (/ sum count))
                      (spl-var (- (* count sum-of-squares) (* sum sum)))
                      (spl-dev (sqrt (/ spl-var (1- count)))))
                 (values mean spl-var spl-dev)))))
(MACROLET ((LOOP-FINISH NIL (SYSTEM::LOOP-FINISH-ERROR)))
 (BLOCK NIL
  (LET ((COUNT 1))
   (LET ((#:LIST-3047 '(1 2 3 4 5)))
    (PROGN
     (LET ((X NIL))
      (LET ((SUM-OF-SQUARES 0) (SUM 0))
       (MACROLET ((LOOP-FINISH NIL '(GO SYSTEM::END-LOOP)))
        (TAGBODY SYSTEM::BEGIN-LOOP (WHEN (ENDP #:LIST-3047) (LOOP-FINISH))
         (SETQ X (CAR #:LIST-3047))
         (PROGN (SETQ SUM (+ SUM X))
          (SETQ SUM-OF-SQUARES (+ SUM-OF-SQUARES (* X X))))
         (PSETQ COUNT (+ COUNT 1)) (PSETQ #:LIST-3047 (CDR #:LIST-3047))
         (GO SYSTEM::BEGIN-LOOP) SYSTEM::END-LOOP
         (MACROLET
          ((LOOP-FINISH NIL (SYSTEM::LOOP-FINISH-WARN) '(GO SYSTEM::END-LOOP)))
          (PROGN
           (RETURN
            (LET*
             ((MEAN (/ SUM COUNT))
              (SPL-VAR (- (* COUNT SUM-OF-SQUARES) (* SUM SUM)))
              (SPL-DEV (SQRT (/ SPL-VAR (1- COUNT)))))
             (VALUES MEAN SPL-VAR SPL-DEV)))))))))))))) ; T

Next, we can leave ANSI behind and call CLISP's internal code walker to expand the entire form, removing all traces of the definitions of local macros, leaving behind only pure code based on special forms and function calls:

(system::expand-form 
    '(loop for count from 1
           for x in '(1 2 3 4 5)
           summing x into sum
           summing (* x x) into sum-of-squares
           finally                            
             (return
               (let* ((mean (/ sum count))
                      (spl-var (- (* count sum-of-squares) (* sum sum)))
                      (spl-dev (sqrt (/ spl-var (1- count)))))
                 (values mean spl-var spl-dev))))))
(BLOCK NIL
 (LET ((COUNT 1))
  (LET ((#:LIST-3230 '(1 2 3 4 5)))
   (LET ((X NIL))
    (LET ((SUM-OF-SQUARES 0) (SUM 0))
     (TAGBODY SYSTEM::BEGIN-LOOP
      (WHEN (ENDP #:LIST-3230) (GO SYSTEM::END-LOOP))
      (SETQ X (CAR #:LIST-3230))
      (PROGN (SETQ SUM (+ SUM X))
       (SETQ SUM-OF-SQUARES (+ SUM-OF-SQUARES (* X X))))
      (PSETQ COUNT (+ COUNT 1)) (PSETQ #:LIST-3230 (CDR #:LIST-3230))
      (GO SYSTEM::BEGIN-LOOP) SYSTEM::END-LOOP
      (RETURN-FROM NIL
       (LET*
        ((MEAN (/ SUM COUNT))
         (SPL-VAR (- (* COUNT SUM-OF-SQUARES) (* SUM SUM)))
         (SPL-DEV (SQRT (/ SPL-VAR (1- COUNT)))))
        (VALUES MEAN SPL-VAR SPL-DEV)))))))))

Implement monadic comprehensions

We can use Lisp macros, and other features, to add support to Lisp for monads, which come from functional languages. The following module of code provides a new macro form called COMPREHEND which works with monads. If we use the LIST monad, we get list comprehensions. For instance:

;; The -> notation is not part of Lisp, it is used in examples indicate the output of a form.
;;
;;
(comprehend 'list-monad (cons x y) (x '(1 2 3)) (y '(A B C)))

     -> ((1 . A) (1 . B) (1 . C) 
         (2 . A) (2 . B) (2 . C) 
         (3 . A) (3 . B) (3 . C))

As you can see, the comprehension processes all combinations of X and Y from both sets, and collects the application of (CONS X Y) to these elements.

In other words {∀x∀y:(cons x y) | x ∈ { 1, 2 ,3 } ∧ y ∈ { A, B, C }}

Other monads are possible: idenitity, state transfomer, etc. Some of these are provided in the code below.

Furthermore, a form called DEFINE-MONAD is provided to define new kinds of monads. It is used to define the basic monads. DEFINE-MONAD also optionally generates a (trivial) short-hand comprehension macro for your monad type. So instead of (comprehend 'list ...) it is possible to write is also (list-comp ...).

Note how the state transformer monad uses the identity monad comprehension in its definition.

Also, a monad is a class, and there is a way in the DEFINE-MONAD syntax to declare what the base classes are (multiple inheritance) as well as any additional custom slots.

Another example, using the identity monad. With the identity monad, the comprehension becomes a sequence of successive variable bindings, and a form evaluated in the scope of those bindings. It is basically like a "Lispified" Haskell DO syntax:

(identity-comp (list x y z) (x 1) (y (* 3 x)) (z (+ x y)))
->  (1 3 4)

I.e. combine the values X, Y and Z into a triplet list, were X is 1, Y is 3X, and Z is X + Y.

To see the original version of this code with lengthy comments, have a look in the Lisp Pastebin. http://paste.lisp.org/display/71196

(defgeneric monadic-map (monad-class function))

(defgeneric monadic-join (monad-class container-of-containers &rest additional))

(defgeneric monadic-instance (monad-class-name))

(defmacro comprehend (monad-instance expr &rest clauses)
  (let ((monad-var (gensym "CLASS-")))
    (cond
      ((null clauses) `(multiple-value-call #'monadic-unit 
                         ,monad-instance ,expr))
      ((rest clauses) `(let ((,monad-var ,monad-instance))
                         (multiple-value-call #'monadic-join ,monad-var 
                           (comprehend ,monad-var
                             (comprehend ,monad-var ,expr ,@(rest clauses))
                             ,(first clauses)))))
      (t (destructuring-bind (var &rest container-exprs) (first clauses)
           (cond
             ((and var (symbolp var))
              `(funcall (monadic-map ,monad-instance (lambda (,var) ,expr)) 
                        ,(first container-exprs)))
             ((and (consp var) (every #'symbolp var))
              `(multiple-value-call (monadic-map ,monad-instance 
                                                 (lambda (,@var) ,expr)) 
                                     ,@container-exprs))
             (t (error "COMPREHEND: bad variable specification: ~s" vars))))))))

(defmacro define-monad (class-name 
                        &key comprehension
                             (monad-param (gensym "MONAD-"))
                             bases slots initargs
                              ((:map ((map-param) 
                                      &body map-body)))
                              ((:join ((join-param 
                                        &optional 
                                          (j-rest-kw '&rest)
                                          (j-rest (gensym "JOIN-REST-")))
                                        &body join-body)))
                              ((:unit ((unit-param 
                                        &optional 
                                          (u-rest-kw '&rest)
                                          (u-rest (gensym "UNIT-REST-")))
                                       &body unit-body))))
  `(progn
     (defclass ,class-name ,bases ,slots)
     (defmethod monadic-instance ((monad (eql ',class-name)))
       (load-time-value (make-instance ',class-name ,@initargs)))
     (defmethod monadic-map ((,monad-param ,class-name) ,map-param)
       (declare (ignorable ,monad-param))
       ,@map-body)
     (defmethod monadic-join ((,monad-param ,class-name) 
                              ,join-param &rest ,j-rest)
       (declare (ignorable ,monad-param ,j-rest))
       ,@join-body)
     (defmethod monadic-unit ((,monad-param ,class-name)
                              ,unit-param &rest ,u-rest)
       (declare (ignorable ,monad-param ,u-rest))
       ,@unit-body)
     ,@(if comprehension
         `((defmacro ,comprehension (expr &rest clauses)
             `(comprehend (monadic-instance ',',class-name) 
                          ,expr  ,@clauses))))))

(defmethod monadic-map ((monad symbol) function)
  (monadic-map (monadic-instance monad) function))

(defmethod monadic-join ((monad symbol) container-of-containers &rest rest)
  (apply #'monadic-join (monadic-instance monad) container-of-containers rest))

(defmethod monadic-unit ((monad symbol) element &rest rest)
  (apply #'monadic-unit (monadic-instance monad) element rest))

(define-monad list-monad
  :comprehension list-comp
  :map ((function) (lambda (container) (mapcar function container)))
  :join ((list-of-lists) (reduce #'append list-of-lists))
  :unit ((element) (list element)))

(define-monad identity-monad
  :comprehension identity-comp
  :map ((f) f)
  :join ((x &rest rest) (apply #'values x rest))
  :unit ((x &rest rest) (apply #'values x rest)))

(define-monad state-xform-monad
  :comprehension state-xform-comp
  :map ((f) 
          (lambda (xformer) 
            (lambda (s)
               (identity-comp (values (funcall f x) new-state) 
                              ((x new-state) (funcall xformer s))))))
  :join ((nested-xformer)
           (lambda (s)
             (identity-comp (values x new-state)
                            ((embedded-xformer intermediate-state) 
                             (funcall nested-xformer s))
                            ((x new-state) 
                             (funcall embedded-xformer intermediate-state)))))
  :unit ((x) (lambda (s) (values x s))))

Python in Lisp

The CLPython project (http://common-lisp.net/project/clpython) provides a Python implementation embedded in Common Lisp. Python modules can be included in Lisp programs and interoperate with Lisp code. There is even a mixed-mode interactive loop ("REPL") where one can use a dialect which mixes Python and Lisp:

From the project documentation:

CLPython is able to turn a regular Lisp listener (REPL) into a "mixed-mode" listener that supports both Lisp and Python source as input:

clpython(213): (clpython:enter-mixed-lisp-python-syntax)
; The mixed Lisp/Python syntax mode is now enabled;
; Lispy *readtable* is now set.
clpython(214): print 123 * 2
246
clpython(215): range(100)[98:2:-2]
#(98 96 94 92 90 88 86 84 82 80 ...)
clpython(216): (+ 1 2)
3

It supports multi-line Python statements as long as the next lines are properly indented:

clpython(70): for i in range(4):
  print i,
  print i*2
0 0
1 2
2 4
3 6

Unfortunately, further metaprogramming within the Python is evidently discouraged (see Python section below).

D

Mixins enable string constants to be compiled as regular D code and inserted into the program. Combining this with compile time manipulation of strings enables the creation of domain-specific languages.

enum GenStruct(string name, string fieldName) =
    "struct " ~ name ~ "{ int " ~ fieldName ~ "; }";

// Equivalent to:  struct Foo { int bar; }
mixin(GenStruct!("Foo", "bar"));

void main() {
    Foo f;
    f.bar = 10;
}

E

Forms of metaprogramming existant in E:

  • Quasi-literals provide convenient notation for data structures and values for which there is not literal syntax, as discussed in String#E.
  • E program fragments may be quoted, manipulated as an AST, and evaluated, similarly to Lisp; lexical environments are first-class objects (though static with respect to the evaluated code). Demonstrated in Runtime evaluation#E and Eval in environment#E.
  • Control structures may be defined, as demonstrated in Extend your language#E.

Erlang

Metaprogramming, as understood for this task, is done with parse transformations in Erlang. This is what the documentation says: "Programmers are strongly advised not to engage in parse transformations".

Forth

Perhaps the most obvious use of Metaprogramming in Forth is Forth itself. The Forth virtual machine traditionally has a primitive operation called BRANCH, which as you could guess, does an unconditional branch to somewhere in the program. There is also ?BRANCH (sometimes called 0BRANCH) which branches only if the top of the Forth DATA stack is zero. There are typically also primitive "DO" and "LOOP" operators. All of these primitives cannot be used on their own and must be compiled into program code by the Forth compiler which needs to compute where to branch or loop to in the context of the code. The compiler itself is implemented in Forth and defines the syntax for conditional branches and loops. Understanding this code is not "needed" for application programming but exists inside the Forth system. The application programmer is free to use these same tools to create new branching and looping syntax if there is a need to do so. (FOR/NEXT, CASE ENDCASE etc.)

\ BRANCH and LOOP COMPILERS

\ branch offset computation operators
: AHEAD    ( -- addr)  HERE   0 , ;
: BACK     ( addr -- ) HERE   - , ;
: RESOLVE  ( addr -- ) HERE OVER - SWAP ! ;

\ LEAVE stack is called L0. It is initialized by QUIT.
: >L        ( x -- ) ( L: -- x )  2 LP +!  LP @ ! ;
: L>        ( -- x )  ( L: x -- ) LP @ @   -2 LP +! ;

\ finite loop compilers
: DO        ( -- ) POSTPONE <DO>     HERE 0 >L   3 ;  IMMEDIATE
: ?DO       ( -- ) POSTPONE <?DO>    HERE 0 >L   3 ;  IMMEDIATE
: LEAVE     ( -- ) ( L: -- addr )
            POSTPONE UNLOOP   POSTPONE BRANCH AHEAD >L ; IMMEDIATE

: RAKE      ( -- ) ( L: 0 a1 a2 .. aN -- )
            BEGIN  L> ?DUP WHILE  RESOLVE  REPEAT ;   IMMEDIATE

: LOOP      ( -- )  3 ?PAIRS POSTPONE <LOOP>  BACK  RAKE ; IMMEDIATE
: +LOOP     ( -- )  3 ?PAIRS POSTPONE <+LOOP> BACK  RAKE ; IMMEDIATE

\ conditional branches
: IF          ( ? -- ) POSTPONE ?BRANCH AHEAD 2 ;        IMMEDIATE
: THEN        ( -- )  ?COMP  2 ?PAIRS RESOLVE ;          IMMEDIATE
: ELSE        ( -- )  2 ?PAIRS  POSTPONE BRANCH AHEAD SWAP 2
                      POSTPONE THEN 2 ;                  IMMEDIATE

\ infinite loop compilers
: BEGIN       ( -- addr n) ?COMP HERE  1  ;              IMMEDIATE
: AGAIN       ( -- )   1 ?PAIRS POSTPONE BRANCH BACK   ;  IMMEDIATE
: UNTIL       ( ? -- ) 1 ?PAIRS POSTPONE ?BRANCH BACK  ;  IMMEDIATE
: WHILE       ( ? -- ) POSTPONE IF  2+  ;                IMMEDIATE
: REPEAT      ( -- )   2>R POSTPONE AGAIN  2R> 2- POSTPONE THEN ; IMMEDIATE

Simple Usage Examples

 : CHARSET    [CHAR] ~  [CHAR] ! DO  I EMIT LOOP ;

: >DIGIT ( n -- c) DUP 9 > IF  7 +  THEN [CHAR] 0 + ;  

: -TRAILING  ( adr len -- adr len')  \ remove trailing blanks (spaces)
             BEGIN  
                2DUP + 1- C@ BL =    \ test if last char = blank
             WHILE  
                1-                   \ dec. length  
             REPEAT ;

FreeBASIC

Single line and multiple line macros can be used to modify or extend the language's syntax and are about as powerful as those found in the C language.

For example, we can create a 'forall' loop to iterate through the characters of a string rather than use a traditional 'for' loop:

' FB 1.05.0 Win64

#Macro ForAll(C, S)
For _i as integer = 0 To Len(s) 
#Define C (Chr(s[_i]))
#EndMacro
 
#Define In ,
 
Dim s As String = "Rosetta"
ForAll(c in s)
  Print c; " ";
Next
 
Print
Sleep
Output:
R o s e t t a 

Go

Although Go has a relatively small number of keywords (25), it also has 39 predeclared identifiers many of which would be considered to be keywords in other languages. The latter include:

1. The names of the basic types such as int, float64, bool and string.

2. Constants such as true, false and nil.

3. Functions such as append, copy, len, make, new and panic.

The predeclared identifiers have 'universal scope' and can therefore be redeclared to mean something different in all other scopes. To the extent that this can be considered to modify the normal operation of the language, Go has "support for metaprogramming" within the meaning of this task but has no other facilities of this nature.

Needless to say, redeclaring the predeclared identifiers in this way is potentially confusing and should not therefore be done without good reason.

In the following example the 'copy' function, which is normally used to copy slices, is redeclared to copy a 'person' struct instead:

package main

import "fmt"

type person struct{
    name string
    age int
}

func copy(p person) person {
    return person{p.name, p.age}
}

func main() {
    p := person{"Dave", 40}
    fmt.Println(p)
    q := copy(p)
    fmt.Println(q)
    /*
    is := []int{1, 2, 3}
    it := make([]int, 3)
    copy(it, is)
    */ 
}
Output:
{Dave 40}
{Dave 40}

If the commented-out code, which uses 'copy' in its normal sense, were uncommented then the program would fail to compile with the following message:

./metaprog.go:22:9: too many arguments in call to copy
	have ([]int, []int)
	want (person)

Haskell

Metaprogramming is implemented using Template Haskell.

J

J names have one of four different grammatic types: noun, verb, adverb, conjunction. Nouns and verbs are nothing special from a metaprogramming point of view. However, adverbs and conjunctions are evaluated at "parse time" and can be used to introduce expression variants. (The result of an adverb, or of a conjunction may be either a noun, a verb, an adverb or a conjunction.)

Additionally, J script blocks (a block of text terminated by a right parenthesis on a single line) can and are used with differing interpreters (which may be built into the language or user written).

The J implementation of the Y combinator could be considered an example of metaprogramming.

J's explicit definitions might also be considered an example of metaprogramming, since explicit definitions have data dependent syntactic types.

Julia

Julia's metaprogramming features are descibed in the online documentation at https://docs.julialang.org/en/v1/manual/metaprogramming/index.html

Here is an example of metaprogramming. Julia in base form does not have C's do { } while() statement.

Using metaprogramming, the do/while can be somewhat emulated:

macro dowhile(condition, block)
    quote
        while true
            $(esc(block))
            if !$(esc(condition))
                break
            end
        end
    end
end 

macro dountil(condition, block)
    quote
        while true
            $(esc(block))
            if $(esc(condition))
                break
            end
        end
    end
end 

using Primes

arr = [7, 31]

@dowhile (!isprime(arr[1]) && !isprime(arr[2])) begin
    println(arr)
    arr .+= 1
end
println("Done.")

@dountil (isprime(arr[1]) || isprime(arr[2])) begin
    println(arr)
    arr .+= 1
end
println("Done.")
Output:

[7, 31] [8, 32] [9, 33] [10, 34] Done. [11, 35] [12, 36] Done.

Kotlin

Although Kotlin doesn't support 'true' meta-programming, it does have facilities which make it possible to create something which closely resembles a language extension (see Extend_your_language#Kotlin).

It is also possible to define infix functions which look like user defined operators:

// version 1.0.6

infix fun Double.pwr(exp: Double) = Math.pow(this, exp)

fun main(args: Array<String>) {
   val d = 2.0 pwr 8.0
   println(d)
}
Output:
256.0

Lingo

Lingo allows to create (and pre-compile) arbitrary code at runtime. You can't really change the language's syntax, but you can overwrite (or extend) built-in commands. Here as example some code that overwrite's Lingo's halt command, which would normally exit the current program:

r = RETURN
str = "on halt"&r&"--do nothing"&r&"end"
new(#script).scripttext = str

Lua

Due to the way Lua's syntactic sugar is designed, metatables can make some Lua code look like a Domain-Specific Language, despite technically being (mostly) just specialized operator overloading.

For example:

class "foo" : inherits "bar"
{
  
}

is perfectly valid syntax. (Lua does not having a built in concept of classes or inheritance.)

M2000 Interpreter

Module Meta {
      FunName$="Alfa"
      Code1$=FunName$+"=lambda (X)->{"
      Code2$={
            =x**2
      }
      Code3$="}"
      
      Inline code1$+code2$+code3$
      
      Print Function(FunName$, 4)=16
      
}
Meta

Mathematica/Wolfram Language

Mathematica can overload all symbols, though sometimes Unprotect has to be invoked. You can also introduce your own infix operators:

CircleTimes[x_, y_] := Mod[x, 10] Mod[y, 10]
14\[CircleTimes]13
Output:
12

For more info see: Operators in Mathematica

Nemerle

Nemerle provides support for macros, which range from defining new infix operators (in fact many 'built-in' operators are macros) to new keywords or control structures.

See here, here, and here on the Nemerle wiki for more information.

Nim

Infix Operators

You can define your own infix operators:

proc `^`*[T: SomeInteger](base, exp: T): T =
  var (base, exp) = (base, exp)
  result = 1
 
  while exp != 0:
    if (exp and 1) != 0:
      result *= base
    exp = exp shr 1
    base *= base
 
echo 2 ^ 10 # 1024

Compile Time evaluation

when is a compile time if and can be used to prevent code from even being parsed at compile time, for example to write platform specific code:

when defined windows:
  echo "Call some Windows specific functions here"
elif defined linux:
  echo "Call some Linux specific functions here"
else:
  echo "Code for the other platforms"

Normal code can be executed at compile time if it is in a static block:

static:
  echo "Hello Compile time world: ", 2 ^ 10

As well as stored in compile time constants:

const x = 2 ^ 10

Templates

The expensive procedure has to be evaluated even when debug is false:

import os

const debug = false

proc expensive: string =
  sleep(milsecs = 100)
  result = "That was difficult"

proc log(msg: string) =
  if debug:
    echo msg

for i in 1..10:
  log expensive()

This can be prevented using templates, as template calls are replaced with the template body at compile time:

template log(msg: string) =
  if debug:
    echo msg

for i in 1..10:
  log expensive()

Templates can use block syntax with statement parameters:

template times(x, y: untyped): untyped =
  for i in 1..x:
    y

10.times: # or times 10:  or times(10):
  echo "hi"
  echo "bye"

Term Rewriting Templates

Term Rewriting Templates can be used to write your own optimizations:

template optLog1{a and a}(a): auto = a
template optLog2{a and (b or (not b))}(a,b): auto = a
template optLog3{a and not a}(a: int): auto = 0

var
  x = 12
  s = x and x
  # Hint: optLog1(x) --> ’x’ [Pattern]

  r = (x and x) and ((s or s) or (not (s or s)))
  # Hint: optLog2(x and x, s or s) --> ’x and x’ [Pattern]
  # Hint: optLog1(x) --> ’x’ [Pattern]

  q = (s and not x) and not (s and not x)
  # Hint: optLog3(s and not x) --> ’0’ [Pattern]

Macros

The most powerful metaprogramming capabilities are offered by macros. They can generate source code or even an AST directly.

dumpTree can be useful when creating an AST, as it show you the AST of any code:

import macros

dumpTree:
  if x:
    if y:
      p0
    else:
      p1
  else:
    if y:
      p2
    else:
      p3

This prints:

StmtList
  IfStmt
    ElifBranch
      Ident !"x"
      StmtList
        IfStmt
          ElifBranch
            Ident !"y"
            StmtList
              Ident !"p0"
          Else
            StmtList
              Ident !"p1"
    Else
      StmtList
        IfStmt
          ElifBranch
            Ident !"y"
            StmtList
              Ident !"p2"
          Else
            StmtList
              Ident !"p3"

Using this information we can create an if2 macro for two conditions, as is done in the "Extend your language" task.

Ol

Ol supports Scheme r7rs macro syntax. Moreover, a large part of Ol scheme implementation made with macros.

For example the "define" macro implementation based on 'setq' (assigns value to variable) and 'lambda' (creates function) keywords and provides uniform way to define variables and functions.

(define-syntax define
   (syntax-rules (lambda) 
      ((define ((name . args) . more) . body)
         (define (name . args) (lambda more . body)))
      ((define (name . args) . body)
         (setq name (letq (name) ((lambda args . body)) name)))
      ((define name (lambda (var ...) . body))
         (setq name (letq (name) ((lambda (var ...) . body)) name)))
      ((define name val)
         (setq name val))
      ((define name a b . c)
         (define name (begin a b . c)))))

Now we can use

(define (sum a b) (+ a b))
; instead of
(setq sum (lambda (a b) (+ a b)))

OxygenBasic

OxygenBasic supports metalanguage useable with various macro formats.

Unlike the C preprocessor, OxygenBasic metalanguage is resolved inline.

'EQUATES

% half 0.5
$ title "My Metaprogram"

'CONDITIONAL BLOCKS

 #ifdef ...
   ...
 #elseif ...
   ...
 #else
   ...
 #endif

'MACROS

'msdos-like
def sum
  %1 + %2
end def

'C-like
#define sum(a,b) a + b

'native
macro sum(a,b)
  a + b
end macro

'native macro functions
macro sum int(r,a,b)
  r = a + b
end macro

PARI/GP

The primary means of metaprogramming for GP is to extend it with PARI. As an example, defining an infix @ operator could work like

In src/functions/symbolic_operators/min0:

Function: _@_
Help: x@y: compute the lesser of x and y, or 0, whichever is larger.
Section: symbolic_operators
C-Name: gmin0
Prototype: GG
Description:
 (small, small):small	 smin0ss($1, $2)
 (mp, mp):mp            gmin0($1, $2)
 (gen, gen):gen     	 gmin0($1, $2)

In (e.g.) basemath/somefile.c:

long
smin0ss(long a, long b)
{
  long c = a < b ? a : b;
  return c > 0 ? c : 0;
}


GEN
gmin0(GEN a, GEN b)
{
  GEN c = gcmp(a, b) < 1 ? a : b; /* copy pointer */
  return signe(c) > 0 ? gcopy(c) : gen_0;
}

Perl

You can textually transform code with a source filter, a module that when used modifies the following lines of source. Filter::Util::Call provides a general means of writing source filters. Filter::Simple is an interface to Filter::Util::Call that lets you elide a lot of boilerplate code. More important, Filter::Simple can hide the contents of quoting constructs from your filter, obviating the biggest dangers of textual metaprogramming. For example, given the following module:

package UnicodeEllipsis;

use Filter::Simple;

FILTER_ONLY code => sub { s/…/../g };

this program:

use UnicodeEllipsis;

print join(' … ', 1  5), "\n";

prints:

 1 … 2 … 3 … 4 … 5

Devel::Declare lets you define a new keyword by setting up a hook to be run whenever the parser encounters a bareword of your choice. Devel::Declare is powerful, but it has a reputation for being difficult to understand. See this blog post for a relatively simple usage example.

Phix

Metaprogramming is frowned on in phix and generally considered a deliberate and unnecessary attempt to complicate matters.
Note however that I consider the necessity for metaprogramming in lisp-like languages to be a weakness, not a strength.
Some builtins can be overridden, but the language reserves the right to reject such wanton acts of intellectual terrorism.
One of the core principles of phix is that code should be utterly intuitive and easy to read.

compile-time assertions

The #isginfo{}, #isinit{}, and #istype{} directives instruct the compiler to perform various type-inference and legal value ranges checks. Primarily for compiler development use, rather than end user applications. No code is generated, but compilation will abort if they fail. Some static assertions can be performed with #isginfo{}, eg:

object x
#isginfo{x,0b0101,5,7,integer,3}
--    {var,type,min,max,etype,len}
    -- (0b0101 is dword sequence|integer)
    x = {1,2,3} -- sequence of integer, length 3
    x = 5       -- integer 5 (becomes min)
    x = 7       -- integer 7 (becomes max)

A compile-time error occurs if say either 7 is changed to 6 (but not both).
Note that you only get that error for "p -c test", not "p test".

symbol table hacking

See builtins\VM\prtnidN.e for details of how to locate and process the symbol table. Again I would not recommend it, but that would allow you to modify variables and invoke code fragments at will, in a metaprogrammy sort of way.

modifying the compiler

see Extend_your_language#Phix

PicoLisp

As in any Lisp, metaprogramming is an essential aspect of PicoLisp. In most cases normal functions are used to extend the language (see Extend your language#PicoLisp), read-macros operate on the source level, and also runtime macros are used occasionally.

PostScript

PostScript allows the reification of stack, scoping (dynamic scoping is default, but lexical scoping can be implemented using immediate loading), bindings using dictionaries, and even control flow. Here is an example of implementation of if statement

Library: initlib
/ift {
4 dict begin
    [/.if /.then] let*
    count array astore /.stack exch def
    /_restore {clear .stack aload pop}.
    .stack aload pop .if {
       _restore .then
    } {
       _restore
    } ifelse
end}.

The standard if expression in PostScript does not take a predicate. Instead it acts on the boolean value on top of the stack. This newly created word allows us to do

>| 2 {1 gt} {=} ift
2

Instead of

>| 2 dup 1 gt {=} ift
2

Note that even the let expression was implemented using meta programming

/let* {reverse {exch def} forall}.

Prolog

This example expands and prints a goal using clause/2:

:- initialization(main).
main :- clause(less_than(1,2),B),writeln(B).
less_than(A,B) :- A<B.

New goals can be created at runtime using assertz/1:

assertz((mother(Child, Mother) :-
                parent(Child, Mother),
                female(Mother))).

Python

Metaprogramming is frowned on in Python and considered un-pythonic. The only widely known example of metaprogramming in Python was an implementation of a goto (and comefrom) keyword done as an April-fools joke.

Another more recent library that shows it can be done in Python is MacroPy.

Works with: [MacroPy]

This is example is taken from MacroPy's GitHub page. It creates a macro called expand that, when invoked, generates the AST for a function in place of the original expression.

from macropy.core.macros import *
from macropy.core.quotes import macros, q, ast, u

macros = Macros()

@macros.expr
def expand(tree, **kw):
    addition = 10
    return q[lambda x: x * ast[tree] + u[addition]]

It is then invoked like this:

func = expand[1 + 2]
print func(5)

Quackery

The various forms of metaprogramming available in Quackery are discussed in The Book of Quackery. Here, two types are illustrated.

1: Extending the Quackery compiler by adding a compiler directive to skip over inline comments indicated by a semicolon. (Quackery has block comments delimited by "(" and ")" but not "comment to end of line".)

2: Adding a new control-flow structure (a switch statement) by using meta-control-flow words. (The ones with ]reverse-nested[ names, indicating they convey properties to the nest that invoked them.)

( +---------------------------------------------------+ )
( | add inline comments ";" to Quackery with "builds" | )
( +---------------------------------------------------+ )

[ dup $ "" = not while
  behead carriage =
  until ]            builds ;             ( [ $ --> [ $ )


; +---------------------------------------------------+
; |  add switch to Quackery with ]else[ ]'[ & ]done[  |
; +---------------------------------------------------+

[ stack ]                is switch.arg    (     --> s   )
protect switch.arg

[ switch.arg put ]       is switch        (   x -->     )

[ switch.arg release ]   is otherwise 

[ switch.arg share 
  != iff ]else[ done  
  otherwise
  ]'[ do ]done[ ]        is case          (   x -->     )


[ switch 
  1       case [ say "The number 1."     cr ]
  $ "two" case [ say 'The string "two".' cr ]
     otherwise [ say "Something else."   cr ] ] is test
                                          (   x -->     )


' tally test     ; output should be: Something else.
$ "two" test     ; output should be: The string "two".
      1 test     ; output should be: The number 1.
Output:
Something else.
The string "two".
The number 1.

R

R does not have much to offer in this regard. It has generic functions, but they're little more than the forbidden option of operator overloading. We equally cannot use any eval tricks, because the task has also forbidden those. As for macros, although R is inspired by Scheme, it has nothing of the sort. For example, see the admitted cheating in Extend your language#R.

As for the permitted things that R does have, it makes it very easy to define new infix operators. We have shown one such example at Matrix-exponentiation operator#Infix operator. To my knowledge, this is only documented in 'An Introduction to R', section 10.2. As for doing this ourselves, we will implement a version of the "nCk" syntax that some calculators use for "n choose k", i.e. the binomial coefficient:

'%C%' <- function(n, k) choose(n, k)
5 %C% 2 #Outputs 10.

Racket

Racket has an extremely rich set of metaprogramming tools, which scale from simple pattern-based macros to implementing entire new languages with their own syntax, such as Datalog and Algol 60. Many parts of Racket itself, including its class-based object system, are implemented as macros that expand to a much smaller set of core forms.

As a descendent of the Scheme tradition, Racket provides hygienic pattern-based macros, allows the use of the full Racket language (including programmer-defined extensions) in implementing macros, and supports locally-defined macros.

Racket adds many extensions to this tradition, such as syntax-parse, which simplifies writing robust macros with good error reporting. For more information on Racket's metaprogramming features, see the relevant chapters of The Racket Guide and The Racket Reference.

For a simple example, this is the definition and a use of the macro list-when:

#lang racket

(define-syntax-rule (list-when test body)
  (if test
      body
      '()))

(let ([not-a-string 42])
  (list-when (string? not-a-string)
    (string->list not-a-string)))

Unlike a plain function, which would eagerly evaluate its arguments, list-when only evaluates its body expression when its test expression passes: otherwise, it evaluates to the empty list. Therefore, the example above does not produce an error.

Alternatively, list-when could be defined using syntax-parse, which provides better error messages for syntax errors:

(require (for-syntax syntax/parse))

(define-syntax list-when
  (syntax-parser
    [(_ test:expr body:expr)
     #'(if test
           body
           null)]))

Raku

(formerly Perl 6)

Raku makes it very easy to do metaprogramming. It is a basic goal of the language.

It is trivial to add a new operator. Most Raku operators are written as normal multiple-dispatch functions in the setting (known as a "prelude" in other languages, but in Raku the setting is a lexical scope notionally surrounding your compilation unit).

There is no a built in factorial operator Raku. It was purposely left out to use as a demonstration of how easy it is to add it. :-)

sub postfix:<!> { [*] 1..$^n }
say 5!;  # prints 120

You may augment a base class with a new method, as long as you declare that you are going to cheat.

Here we add a new method to do natural sorting to the base class Any. (List and Array are both subclasses of Any)

use MONKEY-TYPING; # Needed to do runtime augmentation of a base class.

augment class List {
    method nsort { self.list.sort: {$^a.lc.subst(/(\d+)/, -> $/ {0 ~ $0.chars.chr ~ $0 }, :g) ~ "\x0" ~ $^a} }
};

say ~<a201st a32nd a3rd a144th a17th a2 a95>.nsort;
say ~<a201st a32nd a3rd a144th a17th a2 a95>.sort;

Prints

a2 a3rd a17th a32nd a95 a144th a201st
a144th a17th a2 a201st a32nd a3rd a95

Grammar mixins work in Raku because grammar rules are just methods in grammar classes, and Raku automatically writes a JIT lexer for you whenever you derive a new language. This functionality already works internally in the standard parser—what is not yet implemented is the augment slang hook to allow user code to do this mixin. Raku itself is already parsed using such grammar mixins to provide extensible quoting and regex syntax. For example, every time you pick your own quote characters, you're actually deriving a new Raku dialect that supports those start and stop characters. Likewise any switches to impose single or double-quote semantics, or heredoc semantics, is merely a grammar mixin on the basic Q language.

say "Foo = $foo\n";  # normal double quotes
say Q:qq 【Foo = $foo\n】; # a more explicit derivation, new quotes

Rascal

Rascal has been developed for metaprogramming. Rascal modules already have functionality to analyse Java code (see documentation).

Syntax Definition

In Rascal, grammars can be easily defined. The example below shows the syntax definition for the easy languages C and E1. ViewParseTree visualises the parse tree and lets the user interactively check whether sentences belong to the grammar. The greater than symbol in language E1 means that multiplication has a higher priority than addition.

extend ViewParseTree;

layout Whitespace = [\ \t\n]*;
syntax A = "a";
syntax B = "b";
start syntax C = "c" | A C B;

layout Whitespace = [\ \t\n]*;            
lexical Integer = [0-9]+;
start syntax E1 = Integer
               | E "*" E
	       > E "+" E
	       | "(" E ")"
               ;

An example of the parse viewer for E1

Syntax Tree Traversal

Furthermore, Rascal has built-in functions to traverse trees. This can be used to visit all the nodes in the abstract syntax trees that are automatically generated by Rascal. This provides a powerful tool to analyse code. The following example counts for each operator how many of these the programme contains.

map[str, int] operatorUsage(PROGRAM P) {
    m = ();
    visit(P){
    	case add(_,_): m["add"] ? 0 += 1;
    	case sub(_,_): m["sub"] ? 0 += 1;
    	case conc(_,_): m["conc"] ? 0 += 1;
    }
    return m;
}

Where the abstract syntax is defined as follows

public data TYPE =
	  natural() | string();
	  
public alias PicoId = str;
	  
public data PROGRAM =
  program(list[DECL] decls, list[STATEMENT] stats);

public data DECL =
  decl(PicoId name, TYPE tp);

public data EXP = 
      id(PicoId name)
    | natCon(int iVal)
    | strCon(str sVal)
    | add(EXP left, EXP right)
    | sub(EXP left, EXP right)
    | conc(EXP left, EXP right)
    ;
    
public data STATEMENT =
  asgStat(PicoId name, EXP exp)
| ifElseStat(EXP exp, list[STATEMENT] thenpart, list[STATEMENT] elsepart)
| ifThenStat(EXP exp, list[STATEMENT] thenpart)
| whileStat(EXP exp, list[STATEMENT] body)
| doUntilStat(EXP exp, list[STATEMENT] body)
| unlessStat(EXP exp, list[STATEMENT] body)
;

Pico in Rascal

This is part of the Pico syntax expressed in Rascal.

module lang::pico::Syntax

import Prelude;

lexical Id  = [a-z][a-z0-9]* !>> [a-z0-9];
lexical Natural = [0-9]+ ;
lexical String = "\"" ![\"]*  "\"";

layout Layout = WhitespaceAndComment* !>> [\ \t\n\r%];

lexical WhitespaceAndComment 
   = [\ \t\n\r]
   | @category="Comment" "%" ![%]+ "%"
   | @category="Comment" "%%" ![\n]* $
   ;

start syntax Program 
   = program: "begin" Declarations decls {Statement  ";"}* body "end" ;

syntax Declarations 
   = "declare" {Declaration ","}* decls ";" ;  
 
syntax Declaration = decl: Id id ":" Type tp;

syntax Type 
   = natural:"natural" 
   | string :"string" 
   ;

syntax Statement 
   = asgStat: Id var ":="  Expression val 
   | ifElseStat: "if" Expression cond "then" {Statement ";"}*  thenPart "else" {Statement ";"}* elsePart "fi"
   | ifThenStat: "if" Expression cond "then" {Statement ";"}*  thenPart "fi"
   | whileStat: "while" Expression cond "do" {Statement ";"}* body "od"
   | doUntilStat: "do" {Statement ";"}* body "until" Expression cond "od"
   | unlessStat: Statement "unless" Expression cond
  ;  
     
syntax Expression 
   = id: Id name
   | strCon: String string
   | natCon: Natural natcon
   | bracket "(" Expression e ")"
   > left conc: Expression lhs "||" Expression rhs
   > left ( add: Expression lhs "+" Expression rhs
          | sub: Expression lhs "-" Expression rhs
          )
  ;

public start[Program] program(str s) {
  return parse(#start[Program], s);
}

public start[Program] program(str s, loc l) {
  return parse(#start[Program], s, l);
}

REXX

/*┌───────────────────────────────────────────────────────────────────┐
  │ The REXX language doesn't allow for the changing or overriding of │
  │ syntax per se,  but any of the built-in-functions (BIFs) can be   │
  │ overridden by just specifying your own.                           │
  │                                                                   │
  │ To use the REXX's version of a built-in-function, you simply just │
  │ enclose the BIF in quotation marks (and uppercase the name).      │
  │                                                                   │
  │ The intent is two-fold:  the REXX language doesn't have any       │
  │ reserved words,  nor reserved  BIFs  (Built-In-Functions).        │
  │                                                                   │
  │ So, if you don't know that  VERIFY  is a BIF,  you can just code  │
  │ a subroutine (or function)  with that name (or any name), and not │
  │ worry about your subroutine being pre-empted.                     │
  │                                                                   │
  │ Second:  if you're not satisfied how a BIF works, you can code    │
  │ your own.   This also allows you to front-end a BIF for debugging │
  │ or modifying the BIF's behavior.                                  │
  └───────────────────────────────────────────────────────────────────┘ */
yyy='123456789abcdefghi'

rrr =  substr(yyy,5)                   /*returns  efghi                 */
mmm = 'SUBSTR'(yyy,5)                  /*returns  56789abcdefgji        */
sss = "SUBSTR"(yyy,5)                  /* (same as above)               */
exit                                   /*stick a fork in it, we're done.*/

/*──────────────────────────────────SUBSTR subroutine───────────────────*/
substr: return right(arg(1),arg(2))

/*┌───────────────────────────────────────────────────────────────────┐
  │ Also, some REXX interpreters treat whitespace(s) as blanks   when │
  │ performing comparisons.    Some of the whitespace characters are: │
  │                                                                   │
  │           NL  (newLine)                                           │
  │           FF  (formFeed)                                          │
  │           VT  (vertical tab)                                      │
  │           HT  (horizontal tab or TAB)                             │
  │           LF  (lineFeed)                                          │
  │           CR  (carriage return)                                   │
  │           EOF (end-of-file)                                       │
  │         and/or others.                                            │
  │                                                                   │
  │ Note that some of the above are ASCII or EBCDIC specific.         │
  │                                                                   │
  │ Some REXX interpreters use the   OPTIONS   statement to force     │
  │ REXX to only treat blanks as spaces.                              │
  │                                                                   │
  │ (Both the  verb  and  option  may be in lower/upper/mixed case.)  │
  │                                                                   │
  │ REXX interpreters which don't recognize any  option  won't treat  │
  │ the (below) statement as an error.                                │
  └───────────────────────────────────────────────────────────────────┘ */
options  strict_white_space_comparisons   /*can be in lower/upper/mixed.*/

Ring

The next program add new method to the object class during the runtime

o1 = new point { x=10 y=20 z=30 }
addmethod(o1,"print", func { see x + nl + y + nl + z + nl } )
o1.print()
Class point
        x y z

The next example presents how to create a class that defines two instructions The first instruction is : I want window The second instruction is : Window title = Expression Also keywords that can be ignored like the ‘the’ keyword

New App
{
        I want window
        The window title = "hello world"
}

Class App

        func geti
                if nIwantwindow = 0
                        nIwantwindow++
                ok

        func getwant
                if nIwantwindow = 1
                        nIwantwindow++
                ok

        func getwindow
                if nIwantwindow = 2
                        nIwantwindow= 0
                        see "Instruction : I want window" + nl
                ok
                if nWindowTitle = 0
                        nWindowTitle++
                ok

        func settitle cValue
                if nWindowTitle = 1
                        nWindowTitle=0
                        see "Instruction : Window Title = " + cValue + nl
                ok

        private

                # Attributes for the instruction I want window
                        i want window
                        nIwantwindow = 0
                # Attributes for the instruction Window title
                # Here we don't define the window attribute again
                        title
                        nWindowTitle = 0
                # Keywords to ignore, just give them any value
                        the=0

Ruby

An rudimentary example of metaprogramming is presented in this simple identification system template:

class IDVictim
  
  # Create elements of this man, woman, or child's identification.
  attr_accessor :name, :birthday, :gender, :hometown
  
  # Allows you to put in a space for anything which is not covered by the
  # preexisting elements.
  def self.new_element(element)
    attr_accessor element
  end
  
end

The "self.new_element" class method allows one to (later) specify a new attribute to be added to the defaults of "name", "birthday", "gender", and "hometown".

Run BASIC

(This is not really metaprogramming, at least not under any useful meaning...)

' ---------------------------------------------------
' create a file to be run
' RB can run the entire program
'  or execute a function withing the RUNNED program
' ---------------------------------------------------
open "runned.bas" for output as #f                      ' open runned.bas as output

print #f, "text$ = ""I'm rinning the complete program.  ' print this program to the output
Or you can run a function.
The program or function within the RUNNED program
can execute all Run BASIC commands."""

print #f, "
x = displayText(text$)"

print #f, "                                            ' besides RUNNING the entireprogram
Function displayText(text$)                            ' we will execute this function only
print text$                                            '
end function"

' ----------------------------------------
' Execute the entire RUNNED program
' ----------------------------------------
RUN "runned.bas",#handle          ' setup run command to execute runned.bas and give it a handle
render #handle                    ' render the handle will execute the program

' ----------------------------------------
' Execute a function in the RUNNED program
' ----------------------------------------
RUN "runned.bas",#handle          ' setup run command to execute runned.bas and give it a handle
#handle displayText("Welcome!")   ' only execute the function withing the runned program
render #handle                    ' render the handle will execute the program

Rust

Rust supports extensive metaprogramming via macros. Note that rust macros differ from, say, C preprocessor macros in that they are not mere text substitution (so operator precedence is preserved and name shadowing is not an issue). Here is an example from rustbyexample.com that implements and tests the +=, -=, and *= operators for Vectors.

// dry.rs
use std::ops::{Add, Mul, Sub};

macro_rules! assert_equal_len {
    // The `tt` (token tree) designator is used for
    // operators and tokens.
    ($a:ident, $b: ident, $func:ident, $op:tt) => (
        assert!($a.len() == $b.len(),
                "{:?}: dimension mismatch: {:?} {:?} {:?}",
                stringify!($func),
                ($a.len(),),
                stringify!($op),
                ($b.len(),));
    )
}

macro_rules! op {
    ($func:ident, $bound:ident, $op:tt, $method:ident) => (
        fn $func<T: $bound<T, Output=T> + Copy>(xs: &mut Vec<T>, ys: &Vec<T>) {
            assert_equal_len!(xs, ys, $func, $op);

            for (x, y) in xs.iter_mut().zip(ys.iter()) {
                *x = $bound::$method(*x, *y);
                // *x = x.$method(*y);
            }
        }
    )
}

// Implement `add_assign`, `mul_assign`, and `sub_assign` functions.
op!(add_assign, Add, +=, add);
op!(mul_assign, Mul, *=, mul);
op!(sub_assign, Sub, -=, sub);

mod test {
    use std::iter;
    macro_rules! test {
        ($func: ident, $x:expr, $y:expr, $z:expr) => {
            #[test]
            fn $func() {
                for size in 0usize..10 {
                    let mut x: Vec<_> = iter::repeat($x).take(size).collect();
                    let y: Vec<_> = iter::repeat($y).take(size).collect();
                    let z: Vec<_> = iter::repeat($z).take(size).collect();

                    super::$func(&mut x, &y);

                    assert_eq!(x, z);
                }
            }
        }
    }

    // Test `add_assign`, `mul_assign` and `sub_assign`
    test!(add_assign, 1u32, 2u32, 3u32);
    test!(mul_assign, 2u32, 3u32, 6u32);
    test!(sub_assign, 3u32, 2u32, 1u32);
}
Output:
$ rustc --test dry.rs && ./dry
running 3 tests
test test::mul_assign ... ok
test test::add_assign ... ok
test test::sub_assign ... ok

test result: ok. 3 passed; 0 failed; 0 ignored; 0 measured

Scala

import scala.language.experimental.macros
import scala.reflect.macros.Context

object Macros {
  def impl(c: Context) = {
    import c.universe._
    c.Expr[Unit](q"""println("Hello World")""")
  }

  def hello: Unit = macro impl
}

Shen

Being a Lisp, metaprogramming is easily achievable in Shen through macros. However, defining macros is only possible when the typechecker is off.

(define make-list
  [A|D] -> [cons (make-list A) (make-list D)]
  X -> X)

(defmacro info-macro
  [info Exp] -> [output "~A: ~A~%" (make-list Exp) Exp])

(info (* 5 6)) \\ outputs [* 5 6]: 30

Like most macro systems, defmacro looks like a function that takes a sexp and returns one. However, Shen's defmacro is special in that it allows arbitrary activation of sexps.

(0-) (defmacro +-macro
       [A + B] -> [+ A B])
macro
+-macro

(1-) (1 + (* 2 3))
7

It's important to be careful when using macros like this; this example would be bad because + is sometimes used as an argument to a function (e.g. (fold-left + 0) would compile to (+ fold-left 0)). However, the fact that a symbol can at once match a macro and denote a function can give the illusion of optional arguments or polyadicity. This is how many mathematical operators and functions like append work while retaining their type signature:

(2-) (tc +)
true

(3+) (+ 1 2 3)
6 : number

(4+) +
+ : (number --> (number --> number))

(5-) (tc -)
false

(6-) (macroexpand [+ 1 2 3])
[+ 1 [+ 2 3]]

Sidef

Sidef recognizes all mathematical operators in Unicode and allows the user to define methods that behave like infix operators, even for built-in types.

class Number {
    method (arg) {
        self + arg
    }
}

say (21  42)

Another example of metaprogramming, is the definition of methods at run-time:

var colors = Hash(
               'black'   => "000",
               'red'     => "f00",
               'green'   => "0f0",
               'yellow'  => "ff0",
               'blue'    => "00f",
               'magenta' => "f0f",
               'cyan'    => "0ff",
               'white'   => "fff",
             )

for color,code in colors {
    String.def_method("in_#{color}", func (self) {
        '<span style="color: #' + code + '">' + self + '</span>'
    })
}

say "blue".in_blue
say "red".in_red
say "white".in_white
Output:
<span style="color: #00f">blue</span>
<span style="color: #f00">red</span>
<span style="color: #fff">white</span>

Smalltalk

In Smalltalk, a class can redefine which compiler class is to be used when methods are compiled (aka "accepted in the class browser"). That compiler may be as simple as a subclass of the standard compiler with additional language features (such as additional literal types, extended string syntax etc.) or a completely different language build using one of the available compiler-compiler packages (tgen, petite - a peg parser, and others), or a hand written parser. There exists number of such packages to implement Scheme, Prolog, JavaScript, O-Meta, a number of domain specific language and data description languages (eg. for C data structures or ASN1 types).

As a simple example, here is how external library functions are handled by a pragma detector, to generate a callout to eg. C-functions:

apiSyslog:priority format:format message:message
    <cdecl: int 'syslog' (int char* char*) >
    ^ self primitiveFailed.

SNOBOL4

There are several features of SNOBOL4 which could be considered meta-programming. The first of these is the ability to define synonyms for existing operators or functions, a feature which can help in creating DSLs of sorts in SNOBOL4 programs. For example the following code will alias the built-in function IDENT to SAME and the unary operator * to $:

        OPSYN('SAME','IDENT')
        OPSYN('$','*',1)

This is a simplistic use of OPSYN, however. More interesting is the aliasing of a function to an operator:

        OPSYN('F','*',1)

In this setup, calling F(X) is the same as using the sequence *X which, in more complicated expressions, could result in better readability.

Other metaprogramming features supported would include the use of unevaluated expressions. If, in code, E is an expression it has a value as soon as it is defined and/or assigned to. *E, on the other hand, has a value only when it is evaluated either in the context of a pattern or in the context of an EVAL. The following example shows the motivation for unevaluated expressions in pattern matching contexts:

        &ANCHOR = 0 ; &TRIM = 1
        WORD = BREAK(' .,') . W SPAN(' .,')
        STRING1 = INPUT                       :F(ERROR)
        STRING2 = INPUT                       :F(ERROR)
LOOP    STRING1 WORD =                        :F(OUTPUT)
        STRING2 ' ' W ANY(' .,')              :F(LOOP)
        LIST = LIST W ', '                    :(LOOP)
OUTPUT  OUTPUT = LIST
END

In this code, two strings are input and a list of words appearing in both strings is generated. The problem with this code is that the pattern structure ' ' W ANY(' .,') is built on each iteration. Since pattern building is expensive, putting it in a loop like this is bad form. It cannot be moved outside of the loop, however, since the value of W changes for each iteration. The solution to this is to defer the evaluation of the variable W until it is needed while keeping the rest of the pattern intact:

        &ANCHOR = 0 ; &TRIM = 1
        WORD = BREAK(' .,') . W SPAN(' .,')
        FINDW = ' ' *W ANY(' .,')
        STRING1 = INPUT                       :F(ERROR)
        STRING2 = INPUT                       :F(ERROR)
LOOP    STRING1 WORD =                        :F(OUTPUT)
        STRING2 FINDW                         :F(LOOP)
        LIST = LIST W ', '                    :(LOOP)
OUTPUT  OUTPUT = LIST
END

In this code, the pattern is constructed only once in the line FINDW = ' ' *W ANY(' .,'). The value of the variable W, however, is only provided when FINDW is used in a pattern match. In this case it is given its value from the line before when STRING1 is matched against the pattern WORD. In this way the expense of building the pattern is paid only once, but the flexibility of matching a sequence of values is retained.

The final example of metaprogramming that's available lies in the idiosyncratic way that user-defined functions work in SNOBOL4. The fact that the DEFINE function can be recalled at any point to redefine any function is a powerful feature that can lead to very efficient code. (It can also lead to very unreadable code, of course, if not properly used.)

Consider this hand-waving example for the motivation:

* This example provides a bizarrely-expensive addition operation.
* It assumes the existence of an expensive procedure—say a database
* lookup—to extract the value to be added.  This version uses the
* typical initialize-on-definition approach to implementation.
         DEFINE('XADD(X)','XADD')
         ADDVALUE = CALL_SOME_EXPENSIVE_OPERATION()        :(XADD.END)
XADD     XADD = X + ADDVALUE                               :(RETURN)
XADD.END

In normal operation the interpreter will execute the DEFINE function and then execute the ADDVALUE = ... line, branching *past* the actual body of the function to the label XADD.END. If, however, there are many such functions, and especially if there's the possibility that these functions are never actually called, this could render program startup very slow. For purposes of amortizing initialization time, or for purposes of saving unnecessary initialization, the following code is better:

          DEFINE('XADD(X)','XADD.INIT')                    :(XADD.END)
XADD.INIT ADDVALUE = CALL_SOME_EXPENSIVE_OPERATION()
          DEFINE('XADD(X)','XADD')
XADD      XADD = X + ADDVALUE                              :(RETURN)
XADD.END

The code now defines the XADD function and immediately, without doing initialization, jumps to the XADD.END label, bypassing both initialization and the function body. The trick here is that it defines the entry point of the function to be the XADD.INIT label. Now the first time XADD is called, control is transferred to XADD.INIT, the expensive initialization is performed, then the XADD function is *redefined* to point to the XADD label as the entry point. From this point onward all calls to XADD only perform the calculation, not the expensive initialization while the expensive initialization isn't paid at all unless the function is used at least once.

There are, of course, many other uses for function redefinition in this style which are suitable for metaprogramming efforts. Indeed such features are used prominently in the debugging subsystems of SNOBOL4 implementations.

Standard ML

fun to (a, b) = List.tabulate (b-a,fn i => a+i ) ;
infix 5 to ;

example

- 2 to 9 ;
val it = [2,3,4,5,6,7,8] : int list

Tcl

Metaprogramming is considered to be normal in Tcl; the whole language was designed to support new operations that work with a similar level of integration to existing commands (and indeed, the standard commands are not syntactically special in any way), and the upvar and uplevel commands are specifically for this sort of use. Moreover, there are no language keywords that need to be worked around; words/tokens can be used to mean anything necessary. For example:

proc loopVar {var from lower to upper script} {
    if {$from ne "from" || $to ne "to"} {error "syntax error"}
    upvar 1 $var v
    if {$lower <= $upper} {
        for {set v $lower} {$v <= $upper} {incr v} {
            uplevel 1 $script
        }
    } else {
        # $upper and $lower really the other way round here
        for {set v $lower} {$v >= $upper} {incr v -1} {
            uplevel 1 $script
        }
    }
}

The above creates a new loopVar command that might be used like this:

loopVar x from 1 to 4 {
    loopVar y from $x to 6 {
        puts "pair is ($x,$y)"
        if {$y >= 4} break
    }
}

Which prints this:

pair is (1,1)
pair is (1,2)
pair is (1,3)
pair is (1,4)
pair is (2,2)
pair is (2,3)
pair is (2,4)
pair is (3,3)
pair is (3,4)
pair is (4,4)

As you can see, the new looping command is wholly integrated with the rest of the Tcl language.

Code generation is also possible. The easiest approach is to use the list command to generate substitution-free command, leaving all substitutions to places that are written by the programmer directly.

Finally, the total lack of keywords is exemplified by this classic fragment of Tcl:

set set set

In this, the first set is a command (that updates a variable), the second is the name of a variable in the current namespace, and the third is a string that will be placed in a variable.

TXR

TXR has a built-in Lisp dialect called TXR Lisp, which supports meta-programming, some of which is patterned after ANSI Common Lisp. TXR provides:

  • run-time access to its parser for Lisp expressions: (read "(+ a b c)");
  • a parser for regular exprssions: (regex-parse "a.*b") which produces abstract syntax;
  • a run-time compiler for converting regular expression abstract syntax to compiled regular expression object;
  • a eval function which expands and evaluates Lisp abstract syntax;
  • global as well as lexically scoped macros, for both compound forms (with automatically destructured parameter lists) and symbols (symbol macros): the operators defmacro, defsymacro, macrolet and symacrolet;
  • structural quasiquote for convenient macro writing.

Example define a while loop which supports break and continue. Two forms of break are supported break which causes the loop to terminate with the return value nil and (break <form>) which returns the specified value.

(defmacro whil ((condition : result) . body)
  (let ((cblk (gensym "cnt-blk-"))
        (bblk (gensym "brk-blk-")))
    ^(macrolet ((break (value) ^(return-from ,',bblk ,value)))
       (symacrolet ((break (return-from ,bblk))
                    (continue (return-from ,cblk)))
         (block ,bblk
           (for () (,condition ,result) ()
             (block ,cblk ,*body)))))))

(let ((i 0))
  (whil ((< i 100))
    (if (< (inc i) 20)
      continue)
    (if (> i 30)
      break)
    (prinl i)))

(prinl
  (sys:expand
    '(whil ((< i 100))
       (if (< (inc i) 20)
         continue)
       (if (> i 30)
         break)
       (prinl i))))
Output:
20
21
22
23
24
25
26
27
28
29
30
(block #:brk-blk-0062
  (for () ((< i 100) ())
    () (block #:cnt-blk-0061
         (if (< (sys:setq i (succ i))
                20) (return-from
                      #:cnt-blk-0061))
         (if (> i 30)
           (return-from
             #:brk-blk-0062))
         (prinl i))))

Wren

Analogous to D's mixins, Wren has an optional Meta module which enables strings to be compiled to regular Wren code or expressions and inserted into the script. The D example looks like this in Wren.

import "meta" for Meta

var genericClass = Fn.new { |cname, fname|
    var s1 = "class %(cname) {\n"
    var s2 = "construct new(%(fname)){\n_%(fname) = %(fname)\n}\n"
    var s3 = "%(fname) { _%(fname) }\n"
    var s4 = "}\nreturn %(cname)\n"
    return Meta.compile(s1 + s2 + s3 + s4).call() // returns the Class object
}

var CFoo = genericClass.call("Foo", "bar")
var foo = CFoo.new(10)
System.print([foo.bar, foo.type])
Output:
[10, Foo]

Z80 Assembly

Meta-programming is very limited, but macros can make parameter passing much more straightforward.

macro xchg,regpair1,regpair2
;swaps the contents of two registers.
push regpair1
push regpair2
pop regpair1
pop regpair2
endm

Then in your code you can do the following:

xchg bc,de ;exchanges BC with DE


zkl

Meta programming is quite limited unless you hack the compiler (which is written in zkl) or you write a DSL. Otherwise, there is a "kinda like" C pre-processor support.

#define name [0|1]
#if [0|1|name]
#else, #endif
//Full zkl functionality but limited access to the parse stream; only #defines
#ifdef name
#fcn name {code}
// Shove text into the parse stream
#text name text
#tokenize name, #tokenize f, #tokenize f(a)
#<<<#
text, any text, inside #<<<# pairs is ignored
#<<<#
string:=
#<<<
"here docs: 
all text in #<<< pairs is collected into one [long] line and passed 
verbatim to the tokenizer. Illustrated here as quoted (\") strings
can not span lines.";
#<<<
println(string);  // contains newlines

In addition, there is a concept of "parse space/time" - it is after parsing and before compiling where the full power of the language can be used to "so stuff". For example, enums can be implemented like so:

const{ var _n=-1; var [proxy] N=fcn{ _n+=1 } }
const X=N;     // → 0
println(_n);   // → 2 code time is after const time
const Y=N,Z=N; // → 1,2