GNU bug report logs -
#17940
Nitpick: (lambda () (begin)) errors
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bug#17940; Package
guile.
(Fri, 04 Jul 2014 22:40:02 GMT)
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Taylan Ulrich Bayirli/Kammer <taylanbayirli <at> gmail.com>:
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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):
Disclaimer: this is probably just a nitpick.
On guile-user someone brought up that (lambda () (begin)) errors,
arguably counter-intuitively:
http://lists.gnu.org/archive/html/guile-user/2014-07/msg00005.html
After much investigation over this trivial matter, I found that while
it's compliant with the RnRS (it's unspecified), it means Guile
extends the RnRS grammar in an "unexpected" way. According to R5 and
7RS:
<lambda expression> -> (lambda <formals> <body>)
<body> -> <definition>* <sequence>
<sequence> -> <command>* <expression>
<expression> -> (other stuff) | <derived expression>
<derived expression> -> (other stuff) | (begin <sequence>)
meaning that the "(begin)" in that lambda body can be matched up to
<derived expression>, where it then almost matches the `begin' form
but fails only because in the RnRS grammar it must have at least one
expression (see <sequence> again; this is also reflected in section
4.2.3 where this `begin' form is explained; its template clearly shows
it must have at least one expression operand).
Guile on the other hand *generally* allows the expression-begin to
have zero operands, having it return *unspecified*. So one would
expect it to work here as well, since one expects that this is a small
incremental extension to the RnRS grammar, but that's not the case.
If I'm not mistaken, `expand-body' in psyntax would be the place to
tackle this, and from a quick glance I'm guessing the benefit/effort
ratio for changing this is not very high. :-)
Taylan
Severity set to 'minor' from 'normal'
Request was from
Mark H Weaver <mhw <at> netris.org>
to
control <at> debbugs.gnu.org.
(Wed, 01 Oct 2014 23:31:01 GMT)
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(Fri, 13 Mar 2020 13:09:02 GMT)
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Message #10 received at 17940 <at> debbugs.gnu.org (full text, mbox):
I have run into this also, in writing macros.
My workaround is to always add (if #f #f)
in my forms that allow no expressions in the body.
See second-to-last line below.
(define-syntax sx-match-1
(syntax-rules ()
((_ v (pat exp ...) c1 ...)
(let ((kf (lambda () (sx-match-1 v c1 ...))))
(sxm-sexp v pat (begin (if #f #f) exp ...) (kf))))
((_ v) (error "sx-match: nothing matches"))))
This bug report was last modified 5 years and 94 days ago.
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