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
racket/base))
(begin-for-syntax
(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)
#'(splicing-let-syntax
([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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