[gambit-list] strange results using values

Arthur Smyles atsmyles at earthlink.net
Mon Sep 22 11:29:34 EDT 2008


My use case for using 'values' is to pass a primary value and secondary values to a caller. If the caller can only use one value then I want it to use the primary value and discard the secondary values. So what I would like to see is:

(if (values #f #f) #t #f) => #f 

since the primary value is false and the 'if' function can only use the primary value.

As it stands now, as a caller of an api that uses 'values', I'd have to deal with it as a specialized data structure, in which case, the utility of it is very limited since I could just as well pass a vector or some other structure. 

I think the above semantics could be implemented and still be R5 compliant.

Arthur

 


----- Original Message ----
From: Marc Feeley <feeley at iro.umontreal.ca>
To: Arthur Smyles <atsmyles at earthlink.net>
Cc: gambit-list at iro.umontreal.ca
Sent: Sunday, September 21, 2008 7:35:59 PM
Subject: Re: [gambit-list] strange results using values


On 21-Sep-08, at 3:25 PM, Arthur Smyles wrote:

> (if (values #f) #t #f)
>
> => #f
>
> (if (values #f #f) #t #f)
>
> => #t
>
> I was surprised to discover this. Although technically not a bug 
> since the scheme spec considers this situation unspecified, it makes 
> using multiple-values unreliable.

In what way is it "unreliable"?  The situation is defined as an error 
by the Scheme standard, which means a user cannot expect any specific 
behavior.

Gambit handles multiple values by treating the "values" procedure as a 
data structure constructor (just like "vector" except with a different 
type tag).  The exception is that when it is given a single argument 
it behaves like the identity function, so (values 123) = 123 .  This 
explains why (if (values #f #f) 11 22) gives 11, just like (if (vector 
#f #f) 11 22) gives 11, but (if (values #f) 11 22) = (if #f 11 22) = 
22 .

> This affects pp as well
>
> (pp (values #f #f))
>
> => #<unknown>

What would you expect this to give?  I'm not sure this is related to 
what you want to do, but these operations are available:

> (##values? (values 11 22))
#t
> (##vector->list (values 11 22))
(11 22)
> (for-each pp (##vector->list (values 11 22)))
11
22

Marc



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