[r6rs-discuss] "Unspecified"

[Robby Findler]

Anton, you seem to be suggesting that the programmer is specifying  
the meaning of a phrase in the programming language, but that's  
*your* job! :)

Robby

On Sep 21, 2006, at 6:36 PM, Anton van Straaten wrote:

> ((Speaking for myself, not for the editors:))
>
> Jonathan A Rees wrote:
>> On Sep 20, 2006, at 6:12 PM, Jason Orendorff wrote:
>>> Does anyone else feel a little weird about the phrase "the   
>>> unspecified value"?
>> I do. This feature of the R6RS draft is an oxymoron and offensive  
>> to  common sense.
>> In order for something to be unspecified, someone or something  
>> (in  this case, the report or its authors) has to fail to specify  
>> what it  is.
>
> When you write (if b x) in a program, you haven't specified what  
> should be returned if b is false.  The value of the expression in  
> that situation is unspecified.  The perpetrator of the unspecified  
> behavior is not the R6RS draft, but rather the program(mer).  The  
> R6RS draft simply specifies that in such situations,  
> implementations must return a value which denotes that no value was  
> specified.
>
>> That is just what 'unspecified' means, and we don't have any   
>> choice about it. If you say that the value of an expression is  
>> 'the  unspecified value', you have specified what it is - pure  
>> double-talk.
>
> "The unspecified value" is shorthand for something longer, such as  
> "the value which denotes that no value was specified".
>
> I agree that there's potential for confusion with other uses of the  
> word "unspecified", and that should be clarified.
>
>> I think what is intended is that there is some particular value V   
>> (which needs a name so that we can talk about it) that will be   
>> delivered in a number of special situations.
>
> It's more specific than that, though - the value in question is  
> delivered in certain situations in which the value of an expression  
> is unspecified.
>
>> It's incorrect to tell people (page 20) that V is meaningless. As   
>> soon as you specify anything about it, such as that it is not a   
>> number or that you can get your hands on it in multiple  
>> particular  ways, it has meaning.
>
> I agree with that.
>
>> Whether you *want* to specify anything about the value delivered  
>> in  all these situations is not at all obvious. I can think of  
>> several  possible reasons (none of which I agree with) but I don't  
>> see any  rationale for this incompatible change (or any other, for  
>> that  matter) spelled out anywhere; what did I miss?
>> (And don't point to my own installation of #{unspecific} into  
>> Scheme  48 as contradicting what I say - the name is 'unspecific'  
>> instead of  'unspecified', it was intended to be undocumented  
>> (unspecified!), and  it was meant to be tongue-in-cheek, not  
>> binding.)
>
> A value named "unspecified" (or #<unspecified>, #!unspecified etc.)  
> is used by Chicken, Larceny, and SCM, at least.  Others (in  
> addition to Scheme 48) use some variant such as "unspecific".   
> Others, of course, use a variation on "void", apparently borrowed  
> from C.
>
> While there's a certain quirky charm in choosing not to specify the  
> value of expressions which have a value but don't explicitly  
> specify what that value is, in practice, the major Scheme  
> implementations all provide such a value, all with slightly  
> different names.  The question is really what the benefit is in not  
> specifying this value.  If Schemes were routinely using something  
> other than a mostly meaningless "unspecified" value, which had some  
> valid purpose, the situation would be different.
>
> Anton
>
>
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