Explicit Renaming Macros; Implicitly

It's been one too many times that I hear respectable Schemers talk about how they like explicit renaming macros — not because they're more powerful, but because using them is close to using simple defmacros. In this post I'll show how you can easily write ER-like macros in PLT, just so I won't need to explain the same thing once again. Disclaimers:

• If you're not interested in ER-macros, then you shouldn't read this.
• I'm not claiming that ER macros are not respectable, I'm just surprised at the knee jerk reaction to syntax-case.
• This is not an attempt at providing some portable library or even a PLT library. The intention is to show that syntax-case-style macros are "as convenient" as ER macros, if you really want to get down to that level.
• This is also not an attempt at any kind of formal claim of equivalence in any direction, only a demonstration that you can get the same kind of convenience.
• The bottom line here should be just the convenience point, addressed at people who like ER macros for that, and who think that syntax-case macros are somehow magical or that you lose the ability to play with S-expressions.

The important fact here is that while PLT's syntax-case macro system does not give you raw S-expressions, what you get is a simple wrapper holding them. A macro is a syntax transformer: a function that consumes a syntax value and returns one. For example:

  (define-syntax (foo stx)
    #'123)

is a macro that always expands to 123 (with #'123 being the usual shorthand for (syntax 123)). A syntax object in PLT Scheme (the input to macro functions) is an S-expression, with some lexical information added — this includes the lexical context (in an opaque form), source location, and a few more things. To be more precise, a syntax value is a nested structure of wrappers holding lists and pairs, holding more wrappers, with identifiers at the leaves, where an identifier is a wrapper holding a symbol. It's easy to strip off all wrappers using syntax->datum if you like to work with S-expressions, but you don't want to strip it off of identifiers, since that will lose the important gravy. (In fact, the defmacro library works by stripping off all information, even from identifiers, then reconstructing it by trying to match names in the output form with the original input.) Instead of throwing away all information, what we want to do is preserve identifiers. We can use syntax->list for this: this is a function that takes a syntax value that contains a list, and strips off the top-level extra information leaving you with a list of syntaxes for the sub-expressions (returning #f if the input syntax does not hold a list). Once we have such a list, we can do the usual kind of processing with it, and when we're done turn the result back into a syntax using datum->syntax (which "borrows" the original input expression's information). For example,

  (define-syntax (add1 stx)
    (let ([+ #'+])
      (datum->syntax stx `(,+ 1 ,@(cdr (syntax->list stx))) stx)))

That's a very simple example though; if you try something a little more complicated, you quickly find out that all this unwrapping is inconvenient:

  (define-syntax (mylet stx)
    (let ([*lambda #'lambda])
      (datum->syntax
       stx
       `((,*lambda ,(map (lambda (x) (car (syntax->list x)))
                         (syntax->list (cadr (syntax->list stx))))
                   ,@(cddr (syntax->list stx)))
         ,@(map (lambda (x) (cadr (syntax->list x)))
                (syntax->list (cadr (syntax->list stx)))))
       stx)))

(Note also the *lambda that is used to avoid the lambda expressions used in the meta-code.) What can help here is some helper function that receive a syntax value as its input, and turn all wrapped lists into real lists recursively, but will leave identifiers intact:

  (begin-for-syntax
    (define (strip stx)
      (let ([maybe-list (syntax->list stx)])
        ;; syntax->list returns #f if the syntax is not a list
        (if maybe-list (map strip maybe-list) stx))))

But as long as we're writing a syntax utility, we can make it do a litte more work: feed the resulting tree to your transformer, grab its result, and do the necessary datum->syntax voodoo on it:

  (begin-for-syntax
    (define (er-like-transformer transformer)
      (define (strip stx)
        (let ([maybe-list (syntax->list stx)])
          ;; syntax->list returns #f if the syntax is not a list
          (if maybe-list (map strip maybe-list) stx)))
      (lambda (stx)
        (datum->syntax stx (transformer (strip stx)) stx))))

With this utility defined, the above macro becomes much easier to deal with:

  (define-syntax mylet
    (er-like-transformer
     (lambda (exp)
       (let ((vars  (map car (cadr exp)))
             (inits (map cadr (cadr exp)))
             (body  (cddr exp)))
         `((,(syntax lambda) ,vars ,@body)
           ,@inits)))))

...and this is almost identical to the explicit renaming version of the macro; for example, compare it with the sample code in the MIT-Scheme manual. The only change is that (rename 'lambda) is replaced with (syntax lambda). Obviously, this is very close, but doesn't show intentional captures. So I just grabbed the loop example from the same page, and did the same change — only this time I used #'foo instead of (syntax foo) (and I also changed the one-sided if to a when). The resulting macro works fine:

  (define-syntax loop
    (er-like-transformer
     (lambda (x)
       (let ((body (cdr x)))
         `(,#'call-with-current-continuation
           (,#'lambda (exit)
            (,#'let ,#'f () ,@body (,#'f))))))))
  
  (define-syntax while
    (syntax-rules ()
      ((while test body ...)
       (loop (when (not test) (exit #f))
             body ...))))
  
  (let ((x 10))
    (while (> x 0)
      (printf "~s\n" x)
      (set! x (- x 1))))

This is pretty close to a library, and indeed, as I was writing this text I found a post by Andre van Tonder on the Larceny mailing list that uses a very similar approach and does make a library out of it. This is done by adding two arguments to the expected ER-transformation function — one is a rename function (since the above method uses syntax it is limited to immediate identifiers), and the other is always passed as free-identifier=?. Such a compatibility library is, however, not the purpose of this post. Finally, there is still a minor issue with this — PLT has an implicit #%app which is used wherever there are parentheses that stand for a function application — and in this code they are used unhygienically. This is usually not a noticeable problem, and if it is, you can add explicit #%apps. It might also be possible to find a more proper solution (e.g., use a hash table to keep track of lists that were disassembled by the client transformer), but at this point it might be better to just use the more natural syntax-case anyway.