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

From: Hans Aberg
Subject: Re: (+ (values 1 2)) should be 1
Date: Tue, 24 May 2011 18:13:48 +0200

On 24 May 2011, at 17:07, Andy Wingo wrote:

>>>> The Guile manual, sec., 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.
> That is not what the standard says.  It says that the effect of
> returning an unexpected number of values is unspecified, not that the
> *value* is unspecified -- which wouldn't make sense anyway, as they are
> multiple values in the first place.

Right, but as the result is unspecified according to the standard, the Guile 
manual suggests that the value SCM_UNSPECIFIED as an interpretation of that. I 
merely say that I think it would be a good idea.

> See the R5RS, the R6RS, and the NEWS please.
>    ** Returning multiple values to compiled code will silently truncate the
>       values to the expected number
>    For example, the interpreter would raise an error evaluating the form,
>    `(+ (values 1 2) (values 3 4))', because it would see the operands as
>    being two compound "values" objects, to which `+' does not apply.
>    The compiler, on the other hand, receives multiple values on the stack,
>    not as a compound object. Given that it must check the number of values
>    anyway, if too many values are provided for a continuation, it chooses
>    to truncate those values, effectively evaluating `(+ 1 3)' instead.
>    The idea is that the semantics that the compiler implements is more
>    intuitive, and the use of the interpreter will fade out with time.
>    This behavior is allowed both by the R5RS and the R6RS.

There are various interpretations, as they can change at any time, they are 
rather useless for any regular programming.


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