Re: GIT version: values writing

[Hans Aberg <haberg-1@telia.com>]

On 20 Jan 2011, at 22:18, Ludovic Courtès wrote:

>>>> There seems to be a bug in GIT version when writing the 'values'  
>>>> data
>>>> type.
>>>> (list 2 (values 3 4) 5)
>>>> $1 = (2 3 5)
>>>
>>> This behavior is correct: in Guile 1.9, multiple-value returns are
>>> truncated any time the continuation expects fewer values.
>>>
>>> Here, the ‘values’ call returns 3 values, but the location where  
>>> it’s
>>> called expects only 1, hence the automatic truncation.
>>
>> After the post, I saw that R5RS says the behavior is undefined.
>>
>> However, quietly loosing values is inviting bugs.
>
> I may seem sloppy, but I find it convenient.

In view of that it may break code that relies on the old behavior, I  
think it would be better to throw an exception in the case of multiple  
values.

> In the c.l.s thread you
> mentioned, Joe Marshall writes:
>
>  With the current system, if you wish to add an extra return value,  
> you
>  have to adjust the code at *every* return site (provided you can
>  enumerate all the return sites!).  In Common Lisp, unexpected return
>  values are silently discarded, so modifying a function to return  
> extra
>  information is a local change.  Discarding the extra return values is
>  somewhat sloppy semantically, but far more convenient from a coding
>  standpoint.


It also says:

> With this approach,
>
> > [E]very procedure takes exactly one argument and returns exactly one
> > result.  The argument and the result will always be a sequence.  
> [...]
> > An expression in tail-recursive position returns a one-element
> > sequence.  Continuations accept sequences containing an arbitrary
> > number of elements as results.

This last correspond in infix syntax to combining objects f(x), where  
(x) may be a tuple (x_1, ..., x_k). When f return multiple values, it  
just returns a tuple, so if sent to another function accepting that  
number f arguments, all would work out.

I am implementing this in a syntax on top of Guile, thinking of using  
the old #<values ...> for my tuples.

But as it is not in R5RS, it does not matter. I'm making my own smob.

>> It is discussed here, along with some other ideas.
>>
>> https://groups.google.com/group/comp.lang.scheme/browse_thread/thread/b72d987aa6626cd2/e2f7cfa55fb51d55?hl=en
>
> Interesting thread.

Yes, I am hitting some of those problems rather quickly.