This is such a common pattern that there is support for it with a feature
called syntax templates, in syntax/parse/experimental/template. You would use
(?? super) to add super only if it's given, and nothing otherwise.
-Ian
----- Original Message -----
From: Laurent <laurent.ors...@gmail.com>
To: Konrad Hinsen <konrad.hin...@fastmail.net>
Cc: users <users@racket-lang.org>
Sent: Wed, 25 Sep 2013 06:17:30 -0400 (EDT)
Subject: Re: [racket] Macros: dealing with optional elements
On Wed, Sep 25, 2013 at 11:31 AM, Konrad Hinsen
<konrad.hin...@fastmail.net>wrote:
>
> (define-syntax (foo stx)
> (syntax-parse stx
> [(_ id:id (~optional super:id) (field:id ...))
> (if (attribute super)
> #'(struct id super (field ...))
> #'(struct id (field ...)))]))
Unsyntax-splicing to the rescue!
#lang racket
(require (for-syntax syntax/parse))
(define-syntax (foo stx)
(syntax-parse stx
[(_ id:id (~optional super:id) (field:id ...))
#`(struct id
#,@(if (attribute super)
(list #'super)
'())
(field ...))
]))
(struct A (a))
(foo B (b))
(foo C A (c))
(A 1)
(B 2)
(C 3 4)
Laurent
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