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Accessing Procedure Source Code

Problem

JavaScript? programmers are familiar with accessing the source code of a procedure at runtime. Can we do this in Scheme?

We would also like to store other meta-information about a procedure, say, to provide a documentation string for the procedure, similar to Emacs Lisp doc-strings.

Solution

The answer is a qualified "yes," provided that the procedure is created with a special form described here.

(module doc-lambda mzscheme
  (define-values (struct:doc-lambda make-doc-lambda doc-lambda? doc-lambda-ref doc-lambda-set!)
    (make-struct-type 'doc-lambda #f 3 0 #f null #f 0))

  (define doc-lambda-proc (make-struct-field-accessor doc-lambda-ref 0 'proc))
  (define doc-lambda-doc (make-struct-field-accessor doc-lambda-ref 1 'doc))
  (define doc-lambda-src (make-struct-field-accessor doc-lambda-ref 2 'src))

  (define (procedure-doc x)
    (and (doc-lambda? x)
         (doc-lambda-doc x)))

  (define (procedure-source x)
    (and (doc-lambda? x)
         (doc-lambda-src x)))

  (define-syntax doc-lambda
    (syntax-rules ()
      [(_ doc-string args body0 body1 ...)
       (let ([doc-string-v doc-string])
         (make-doc-lambda
          (lambda args body0 body1 ...)
          doc-string-v
          `(doc-lambda ,doc-string-v args body0 body1 ...)))]))

  (provide doc-lambda procedure-doc procedure-source))

Discussion

The doc-lambda macro turns its source code into an S-expression by quoting it and storing it in a struct. It creates the actual procedure and stores that in the struct as well, along with the provided documentation string.

The struct is executable as a procedure thanks to MzScheme's "structs as procedures" (see chapter 4.6 in the MzScheme language manual): the final argument to make-struct-type tells MzScheme to treat the 0th item in the struct as the procedure to invoke when the struct is applied as a procedure.

Comments about this recipe

Contributors

-- DaveHerman - 16 May 2006