This was mind-boggling to debug!
In syntax-rules, the pattern (x ...) will match #nil, since it
matches the empty list. This can have surprising consequences.
Consider:
(define-syntax test
(syntax-rules ()
((test (x ...))
(x ...))
((test x)
x)))
(test (+ 1 2)) ; => 3
(test 123) ; => 123
(test #f) ; => #f
;; However...
(test #nil) ; error
Anyway, attached is a patch to fix the issue in match.
- Taylan
From b6d0b715a8bf0cc39b9fc3d46efeaf010f0d4351 Mon Sep 17 00:00:00 2001
From: Taylan Kammer <[email protected]>
Date: Wed, 12 May 2021 21:46:54 +0200
Subject: [PATCH] Fix match when used directly on the #nil constant.
* module/ice-9/match.upstream.scm (match): Make sure we don't match #nil
where we don't intend to.
Fixes <https://bugs.gnu.org/48315>.
---
module/ice-9/match.upstream.scm | 9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)
diff --git a/module/ice-9/match.upstream.scm b/module/ice-9/match.upstream.scm
index b1fc371b8..3b181b75b 100644
--- a/module/ice-9/match.upstream.scm
+++ b/module/ice-9/match.upstream.scm
@@ -269,9 +269,12 @@
(match-syntax-error "missing match expression"))
((match atom)
(match-syntax-error "no match clauses"))
- ((match (app ...) (pat . body) ...)
- (let ((v (app ...)))
- (match-next v ((app ...) (set! (app ...))) (pat . body) ...)))
+ ;; The original implementation uses (app ...) not (op arg ...) here,
+ ;; but in Guile this would match #nil when it shouldn't. Failing to
+ ;; match () doesn't matter since it leads to an error anyway.
+ ((match (op arg ...) (pat . body) ...)
+ (let ((v (op arg ...)))
+ (match-next v ((op arg ...) (set! (op arg ...))) (pat . body) ...)))
((match #(vec ...) (pat . body) ...)
(let ((v #(vec ...)))
(match-next v (v (set! v)) (pat . body) ...)))
--
2.30.2
