<html> <head> <meta content="text/html; charset=ISO-8859-1" http-equiv="Content-Type"> </head> <body bgcolor="#FFFFFF" text="#000000"> Another thing about this, if the definition produced by the macro is instead generated by the `make-transformer' macro then it will have the wrong number of marks on it when everything is finally expanded. `syntax-local-introduce' fixes this by essentially removing that extra mark. <meta http-equiv="content-type" content="text/html; charset=ISO-8859-1"> <pre style="color: rgb(0, 0, 0); font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-align: -webkit-auto; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; word-wrap: break-word; white-space: pre-wrap; ">#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 [(_ name (pattern ...) action) #'(quote-syntax (define-syntax name (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 [(_) (syntax-local-introduce (make-transformer name pattern action))]))]) (make))])) (macro-generator foo (x ...) #'(list x ...)) (foo 1 2 3)</pre> The `name' passed as an argument to `make-transformer' will not have the mark applied to it by the transformer in the splicing-let-syntax, so the resulting define-syntax form will use a different 'foo' from the one passed to `macro-generator'. Using `syntax-local-introduce' applies the mark that was used when `(make)' was expanded thus removing all marks from the final expansion. On 06/01/2012 12:16 PM, Jon Rafkind wrote: <blockquote cite="mid:4FC906FE.4040406@cs.utah.edu" type="cite"> <meta http-equiv="content-type" content="text/html; charset=ISO-8859-1"> <meta http-equiv="content-type" content="text/html; charset=ISO-8859-1"> <pre style="color: rgb(0, 0, 0); font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-align: -webkit-auto; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; word-wrap: break-word; white-space: pre-wrap; ">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)</pre> <fieldset class="mimeAttachmentHeader"></fieldset> <pre wrap="">____________________ Racket Users list: <a class="moz-txt-link-freetext" href="http://lists.racket-lang.org/users">http://lists.racket-lang.org/users</a> </pre> </blockquote> </body> </html>