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Re: letrec bug
Re: letrec bug
Tue, 28 Oct 2008 22:27:48 +0100
2008/10/28 Mikael Djurfeldt <address@hidden>:
> 2008/10/28 Mikael Djurfeldt <address@hidden>:
>> 2008/10/28 Bill Schottstaedt <address@hidden>:
>>> I believe this shows a bug in letrec:
>>> guile> (let ((x 1)) (let ((x 32) (y x)) y))
>>> guile> (let ((x 1)) (letrec ((x 32) (y x)) y))
>>> In standard input:
>>> 2: 0* (let* ((x 1)) (letrec ((x 32) (y x)) y))
>>> 2: 1 (letrec ((x 32) (y x)) y)
>>> standard input:2:14: In expression (letrec (# #) y):
>>> standard input:2:14: Variable used before given a value: x
>>> ABORT: (unbound-variable)
>> From R5RS:
>> "One restriction on `letrec' is very important: it must be possible
>> to evaluate each <init> without assigning or referring to the
>> value of any <variable>."
> Sorry. I missed the surrounding let. Yes, it is a bug.
Sorry again (should never easily throw out comments when its about
letrec :). The binding of x which letrec introduces covers the entire
letrec expression. This means that it *is* an error to refer to it in
another init. We should be grateful that Guile detects this, because
such code will likely behave in an implementation dependent, and not
standard conforming, manner.
"The <variable>s are bound to fresh locations holding
undefined values, the <init>s are evaluated in the resulting
environment (in some unspecified order)"