[racket] syntax-case and multiple ellipsis

[Spencer Florence]

Hey all,

I'm trying to write a macro with syntax-case that looks something like this:

 (syntax-case stx (in def)

    [(_ name

        (def a ...) ...

        (in clause do ...) ...)

     stuff])

But, this gives me the error "syntax-case: misplaced ellipsis in pattern (follows other ellipsis)" on the last ellipsis. Does anyone know a way around this?

--spencer

[Stephen Chang]

syntax-case won't let you do that, but syntax-parse will:

(syntax-parse #'(test A (def 1 2 3) (in 4 5 6)) #:datum-literals (in def)

[(_ name
(def a ...) ...
(in clause do ...) ...)

#'(a ... ... do ... ...)])

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[Jens Axel Søgaard]

Hi Spencer,

The problem is that syntax-case pattern doesn't allow allow
pattern of the form (x ... y ...) that is with two ellipsis on the
same level.

Luckily syntax-parse allow these:

(require syntax/parse)

(syntax-parse #'(a a a a 41 42 43)
[(c ... d ...)
#''((c ...) (d ...))])

(syntax-parse #'(a a a a 41 42 43)
[(c:id ... d ...)
#''((c ...) (d ...))])

/Jens Axel

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Jens Axel Søgaard

[Spencer Florence]

Unfortunately I'm working in typed racket, which (as far as I can tell) doesn't play nice with syntax-parse. Any other ideas?

[Stephen Chang]

Hmm, you might need to provide some more details about what you are
trying to define then? (This particular example seems to work with
syntax-parse in TR)

#lang typed/racket
(require (for-syntax syntax/parse))

(define-syntax (test stx)
(syntax-parse stx #:datum-literals (in def)

[(_ name
(def a ...) ...
(in clause do ...) ...)

#'(list a ... ... do ... ...)]))

(test A (def 1 2 3) (in 4 5 6))

[Jens Axel Søgaard]

Here is a syntax-case only solution:

(define-syntax (foo stx)
(syntax-case stx (in def)
[(_ name . more)
#'(collect-defs () name . more)]))

(define-syntax (collect-defs stx)
(syntax-case stx (in def)
[(_ (defs-seen ...) name (def a ...) . more)
#'(collect-defs (defs-seen ... (def a ...)) name . more)]
[(_ defs name . more)
#'(collect-ins defs () name . more)]))

(define-syntax (collect-ins stx)
(syntax-case stx (in def)
[(_ defs (ins-seen ...) name (in clause ...) . more)
#'(collect-ins defs (ins-seen ... (in clause ...)) name . more)]
[(_ defs ins name . more)
#'(finish defs ins name . more)]))

(define-syntax (finish stx)
(syntax-case stx ()
[(_ defs ins name . more)
#''((definitions: defs)
(ins: ins)
(name: name)
(more: more))]))

(foo "a-name"
(def x 41)
(def y 42)
(in a)
(in b)
"yada")

/Jens Axel

[Spencer Florence]

Silly me, I was trying (require syntax/parse). Thanks!

[Greg Hendershott]

Although it may not be the same issue, Alex Knauth posted about a
problem using syntax-parse with Typed Racket on May 27 (I didn't see
anyone reply):

http://lists.racket-lang.org/users/archive/2014-May/062730.html

On Tue, Jun 17, 2014 at 2:26 PM, Stephen Chang <stc...@ccs.neu.edu> wrote:
> Hmm, you might need to provide some more details about what you are
> trying to define then? (This particular example seems to work with
> syntax-parse in TR)
>
> #lang typed/racket
> (require (for-syntax syntax/parse))
>
> (define-syntax (test stx)
> (syntax-parse stx #:datum-literals (in def)
> [(_ name
> (def a ...) ...
> (in clause do ...) ...)
> #'(list a ... ... do ... ...)]))
>
> (test A (def 1 2 3) (in 4 5 6))
>
> On Tue, Jun 17, 2014 at 2:08 PM, Spencer Florence <spe...@florence.io> wrote:
>> Unfortunately I'm working in typed racket, which (as far as I can tell)
>> doesn't play nice with syntax-parse. Any other ideas?

[Greg Hendershott]

> Silly me, I was trying (require syntax/parse). Thanks!

Yeah, if I had a nickel for every time I've forgotten to (require
(for-syntax syntax/parse)) I would be... well, not rich, but able to
buy a fancy coffee. Which if I'd already drank, I probably wouldn't
have made the mistake.

If someone were to inflict a Microsoft Office Clippy on us, one of its
features should probably be, "Hi! It looks like you're writing a
syntax-parse macro!" and offer to fix the require.