[racket-users] macro guard question (ellipsis)

[Kevin Forchione]

Hi guys,
I’ve been trying to figure out the syntax for a guard when the pattern has ellipsis and you want to guard one of the variables. Here’s an uninteresting macro but it shows what I mean.

#lang racket

(define-syntax (foo stx)
(syntax-case stx ()
[(_ (key val ...) ...)
(and (identifier? #'key) (identifier? #'val))
#'(quote (list ((quote key) (quote val) ...) ...))]))

(foo (a b) (c d e))

Of course it complains about the ellipsis being missing in the pattern. Is this something I can handle with a guard in this manner or do I need a helper macro of some sort?

Thanks!
Kevin

[Sorawee Porncharoenwase]

I’d write this. Not sure if there’s a better way.

(define-syntax (foo stx)
  (syntax-case stx ()
    [(_ (key val ...) ...)
     (andmap identifier? (syntax->list #'(key ... {~@ val ...} ...)))
     #'(quote (list ((quote key) (quote val) ...) ...))]))

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[Sorawee Porncharoenwase]

Also note that with syntax/parse, it becomes much easier:

(require (for-syntax syntax/parse))

(define-syntax (foo stx)
  (syntax-parse stx
    [(_ (key:id val:id ...) ...)
     #'(quote (list ((quote key) (quote val) ...) ...))]))

[Kevin Forchione]

On Jun 18, 2019, at 9:03 PM, Sorawee Porncharoenwase <sorawe...@gmail.com> wrote:

Also note that with syntax/parse, it becomes much easier:

(require (for-syntax syntax/parse))

(define-syntax (foo stx)
  (syntax-parse stx
    [(_ (key:id val:id ...) ...)
     #'(quote (list ((quote key) (quote val) ...) ...))]))

Thanks! Looks like it’s finally time for me to do a deep dive into syntax-parse. 

Kevin