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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Received on Thu Sep 21 2006 - 21:50:05 UTC