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bug#17940: Nitpick: (lambda () (begin)) errors
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From: |
Taylan Ulrich Bayirli/Kammer |
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Subject: |
bug#17940: Nitpick: (lambda () (begin)) errors |
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Date: |
Sat, 05 Jul 2014 01:39:04 +0300 |
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
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