Re: A value for "nothing"

[John Cowan]

On Tue, Aug 28, 2018 at 11:40 AM Mark H Weaver <address@hidden> wrote:


> That's the phrase used in R7RS-small, which fails to define it, as you
> noted, but that shortcoming is limited to R7RS.
>

The relevant sentences in R5RS and R7RS are identical:  " If <test> yields
a false value and no <alternate> is specified, then the result of the
expression is unspecified."  Likewise, the paragraph from 1.3.2 you quote
below is identical in both standards.  So either they both define it or
they both don't.

In R6RS, section 11.4.3 (Conditionals) provides this example:
>

Unlike Wil Clinger, and apparently you, I don't believe that examples in
specs are normative.  But setting that aside for the moment:


> I take the use of the singular form of "value" here to imply that it
> returns only one value.
>

In R6RS 11.13, vector-set! is said to return unspecified values (note
plural), but in the examples appears "⇒ unspecified", showing that this
notation can be used where multiple unspecified values (or zero values) are
allowed.

In practice, I know of no Scheme implementation that returns other than one
value in any of these "unspecified values" situations, which IMO is a Good
Thing.

-- 
John Cowan          http://vrici.lojban.org/~cowan        address@hidden
With techies, I've generally found
If your arguments lose the first round
Make it rhyme, make it scan / Then you generally can
Make the same stupid point seem profound!           --Jonathan Robie