Subject: bug#14922: guard expression doesn't catch everything

From: Göran Weinholt <[email-masked]>
Date: Sun, 21 Jul 2013 11:29:03 +0200

Hello schemers,

the guard expression from (rnrs) would be a lot more useful if it
managed to catch all exceptions. As it is now, some errors will bypass
the guard:

scheme@(guile-user)> (import (rnrs))
scheme@(guile-user)> (guard (exn (else #f)) (fx+ #f #f))
$1 = #f
scheme@(guile-user)> (guard (exn (else #f)) (fx+))
;;; <stdin>:3:0: warning: possibly wrong number of arguments to `fx+'
rnrs/arithmetic/fixnums.scm:153:2: In procedure fx+:
rnrs/arithmetic/fixnums.scm:153:2: Wrong number of arguments to #<procedure fx+ (fx1 fx2)>

The background is that I'm working on a program that intentionally calls
procedures with bad arguments, and it needs to determine if the
procedure accepted the arguments or not. Ideally the object raised would
be a proper and correct R6RS condition object, but I suspect that most
of the existing Guile code doesn't raise conditions like that. I think
it would be a step forward if guard at least caught the exception, even
if the condition object might not be very useful.

Tested with Guile 2.0.9.40-824b-dirty.

Regards,

-- 
Göran Weinholt <[email-masked]>
"Bring me back // to a story left untold // so we can write the ending."
 -- Aly & Fila feat. Jwaydan - We Control The Sunlight