GNU bug report logs - #65132
[PATCH] fix cond1 macro

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From: Timothy Sample <samplet <at> ngyro.com>
To: Ekaitz Zarraga <ekaitz <at> elenq.tech>
Cc: 65132 <at> debbugs.gnu.org
Subject: bug#65132: [PATCH] fix cond1 macro
Date: Mon, 07 Aug 2023 14:15:47 -0600

Hi all,

Ekaitz Zarraga <ekaitz <at> elenq.tech> writes:

> From ab6e4980ea301b5d1e14a98464e5de3e726984f1 Mon Sep 17 00:00:00 2001
> From: Ekaitz Zarraga <ekaitz <at> elenq.tech>
> Date: Mon, 7 Aug 2023 20:23:42 +0200
> Subject: [PATCH] fix cond1 macro

Just adding some context, as we talked about this on IRC.

If you take the docs example as is,

    (define-syntax cond1
      (syntax-rules (=> else)
        ((cond1 test => fun)
         (let ((exp test))
           (if exp (fun exp) #f)))
        ((cond1 test exp exp* ...)
         (if test (begin exp exp* ...)))
        ((cond1 else exp exp* ...)
         (begin exp exp* ...))))

and invoke it as

    (cond1 else #t)

you get

    Syntax error:
    unknown location: else: bad use of 'else' syntactic keyword in
    subform else of else

This is because ‘else’ matches the ‘(cond1 test exp exp* ...)’ pattern
and gets inserted as the test:

    (if else (begin #t))

This patch puts the more specific ‘else’ pattern before the more general
‘test’ pattern so it has a chance to be matched.


-- Tim

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