Re: [Scheme-reports] when and unless

[Pierpaolo Bernardi <olopierpa@x>]

On Tue, May 24, 2011 at 16:44, John Cowan <cowan@x> wrote:
> leppie scripsit:
>
>> Take for example:
>>
>> (define (foo bar)
>>   (unless (list? bar) (error 'foo "not a list" foo))
>>   (map values bar))
>>
>> Now applying the above with say (foo 1) will raise an error. If I set
>> my debugger to break inside ERROR, I get a nice complete stacktrace,
>> including the caller, IOW FOO.
>>
>> Now take what is suggested:
>>
>> (define (foo bar)
>>   (if (not (list? bar)) (error 'foo "not a list" foo)
>>                            (map values bar)))
>>
>> Now inside ERROR, you do not have 'lost' the caller (FOO) as the
>> application of ERROR was a tail call. Given the context of FOO's
>> application that may also be a tail call, and so on.
>
> I would certainly want the last form in WHEN/UNTIL to be a tail position.
> The obvious implementation ((when x . y) (if x (begin . y))) would certainly
> make it so.

The point leppie made is subtler.

In the UNLESS case the whole UNLESS form is not in tail position,
while in the IF case it is.

P.

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