[plt-scheme] Macro-expansion-time structs

That's clear now.
Thanks.
Jos

----- Original Message ----- 
From: "Matthew Flatt" <mflatt at cs.utah.edu>
To: "Jos Koot" <jos.koot at telefonica.net>
Cc: "Carl Eastlund" <cce at ccs.neu.edu>; "Henk Boom" <lunarc.lists at gmail.com>; 
"Eli Barzilay" <eli at barzilay.org>; <plt-scheme at list.cs.brown.edu>
Sent: Sunday, October 12, 2008 1:11 PM
Subject: Re: [plt-scheme] Macro-expansion-time structs


> At Sun, 12 Oct 2008 10:06:22 +0200, "Jos Koot" wrote:
>> Would (begin-for-syntax (define-struct etc ())) not work as well?
>
> The problem with
>
> #lang scheme/base
> (require (for-syntax scheme/base))
> (begin-for-syntax
>  (define-struct etc ()))
>
> is that `define-struct' expands to both `define' and `define-syntax',
> and `define-syntax' forms are not currently supported within
> `begin-for-syntax' (i.e., there's not `define-syntax-for-syntax').
>
>
> You can write
>
> #lang scheme/base
> (require (for-syntax scheme/base))
>
> (begin-for-syntax
>  (define-struct etc () #:omit-define-syntaxes))
>
> which doesn't define `etc', so the structure type doesn't work with
> `match' or other forms that need expand-time information about the
> structure type. But it does define `make-etc', `etc?', and so on, which
> is enough for many purposes.
>
>
> Matthew
>
>
>> ----- Original Message ----- 
>> From: "Carl Eastlund" <cce at ccs.neu.edu>
>> To: "Henk Boom" <lunarc.lists at gmail.com>
>> Cc: "Eli Barzilay" <eli at barzilay.org>; <plt-scheme at list.cs.brown.edu>
>> Sent: Sunday, October 12, 2008 6:37 AM
>> Subject: Re: [plt-scheme] Macro-expansion-time structs
>>
>>
>> > If you want the my-box struct bound at the module level, you can
>> > create another module, use define-struct normally, then import it for
>> > syntax.  Or you can use define-values-for-syntax like this (which
>> > won't get the struct's syntax binding, but will get anything else you
>> > need):
>> >
>> > #lang scheme/base
>> >
>> > (require (for-syntax scheme/base))
>> >
>> > (provide test)
>> >
>> > (define-values-for-syntax (make-my-box my-box-val my-box?)
>> > (let ()
>> >   (define-struct my-box (val))
>> >   (values make-my-box my-box-val my-box?)))
>> >
>> > (define-syntax test
>> >  (lambda (stx)
>> >    #`#,(my-box-val (make-my-box 2))))
>> >
>> > On Sat, Oct 11, 2008 at 11:22 PM, Henk Boom <lunarc.lists at gmail.com>
>> > wrote:
>> >> 2008/10/11 Carl Eastlund <cce at ccs.neu.edu>:
>> >>> You've defined the struct in the runtime environment.  You need to
>> >>> define it in the transformer environment if you want transformers to
>> >>> use it.  You might accomplish that like this (though this makes a
>> >>> local definition other transformers won't be able to use):
>> >>>
>> >>> #lang scheme/base
>> >>>
>> >>> (require (for-syntax scheme/base))
>> >>>
>> >>> (provide test)
>> >>>
>> >>> (define-syntax test
>> >>>  (let ()
>> >>>   (define-struct my-box (val))
>> >>>   (lambda (stx)
>> >>>     #`#,(my-box-val (make-my-box 2)))))
>> >>
>> >> I see. It's a pity that (begin-for-syntax (define-struct . . .)) 
>> >> doesn't
>> >> work.
>> >>
>> >> Thanks for the info!
>> >>    Henk
>> > _________________________________________________
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>>
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