Re: (+ (values 1 2)) should be 1

[Hans Aberg]

On 24 May 2011, at 15:11, Andy Wingo wrote:

>>>>> But, it is 1 and 2, currently.  (+ FOO) inlines just to FOO, too
>>>>> optimistically.
>>>> 
>>>> It is unspecified according to rsr5.
>>> 
>>> I know.  I'm talking about Guile here.
>> 
>> The Guile manual, sec. 10.2.5.2, says that SCM_UNSPECIFIED is to be used 
>> when the Scheme standard says the return is an unspecified value.
>> 
>> So this Lisp extension breaks off from that. If one wants it, perhaps, there 
>> should be some way to invoke it.
> 
> Hans, you are misreading.  (+ 1) is 1 according to the R5RS.  (+ "foo")
> is an error.  (+ (values 1 2)) is unspecified, as an instance of
> returning an unexpected number of values to a continuation, but it is
> not an instance of the unspecified value.

Andy, I think (values 1 2) should here return SCM_UNSPECIFIED first argument to 
'+', so that people will know that the standard does leave the value 
unspecified.

> When a continuation that expects one value receives more values than it
> is expecting -- e.g., the <> in (+ <>) expects one value -- Guile
> truncates those values to the first one.  If such a continuation
> receives 0 values, Guile signals an error.

So here I think one should enable some kind of Lisp-extension to get that.

There is no guarantee that Guile will not change again unless doing something 
like that. So it is useless for regular programming.

Hans