[racket] quote-syntax in macro-generator

From: Jon Rafkind (rafkind at cs.utah.edu)
Date: Fri Jun 1 14:16:30 EDT 2012

Today's macro PSA is about macro-generating-macro forms. If you use a macro to generate a transformer then you must be careful about nesting `syntax' forms otherwise you will interpolate templates too many times. Specifically #'#'x will interpolate x twice. Given the following bindings:

  chicken = (syntax (egg ...))
  pattern = (syntax chicken)

The problem is if an additional `syntax' is wrapped around `pattern'. After one interpolation of `syntax' we will have

  (syntax (egg ...))

At which point `egg' had better be bound as a pattern variable otherwise interpolation will fail.

To work around this problem the `quote-syntax' form should be used:

  #'(quote-syntax pattern)

which prevents the (syntax (egg ...)) layer from being interpolated because `quote-syntax' does not do interpolation.

The following example demonstrates the issue. On the line 'HERE' the inner syntax should be `quote-syntax' otherwise the following error is produced

  x.rkt:27:24: syntax: no pattern variables before ellipses in template at: ... in: (_ x ...)

To be fair this issue only comes up because I used `make-transformer' in the body of a phase 2 function so the output of `make-transformer' had to have two levels of syntax, because its a macro that should ultimately evaluate to a syntax form. A simpler solution would have been to do

  #'(define-syntax name (make-transformer pattern action))

So that `make-transformer' only had to return one level of syntax. But anyway I got into this situation for other reasons.

#lang racket

(require (for-meta 0 racket/splicing)
         (for-meta 1 syntax/parse)
         (for-meta 2 syntax/parse

  (define-syntax (make-transformer stx)
    (syntax-parse stx
      [(_ (pattern ...) action)
       #'#'(lambda (stx) ;; HERE change to #'(quote-syntax
             (syntax-parse stx
               [(_ pattern ...) action]))])))

(define-syntax (macro-generator stx)
  (syntax-parse stx
    [(_ name pattern action)
         ([make (lambda (stx)
                  (syntax-parse stx
                    [(_ name)
                     (with-syntax ([transformer (make-transformer pattern action)])
                       #'(define-syntax name transformer))]))])
         (make name))]))

(macro-generator foo (x ...) #'(list x ...))
(foo 1 2 3)

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