Re: A value for "nothing"

[Mark H Weaver <mhw@netris.org>]

John Cowan <cowan@ccil.org> writes:

> On Tue, Aug 28, 2018 at 3:01 AM Mark H Weaver <mhw@netris.org> wrote:
>  
>  In RnRS, (define a (if #f #f)) is allowed and guaranteed to assign
>  *some* object to 'a' without signalling an error.  
>
> Actually, the phrase used is "the result is unspecified", which unfortunately
> is not defined in any RnRS.

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

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

  (if #f #f) ===> unspecified

whose meaning is defined in section 6.6 (Evaluation examples), which
states:

  Moreover, the "===>" symbol is also used to explicitly say that the
  value of an expression is unspecified.  For example:

    (eqv? "" "") ===> unspecified

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

R5RS is even more clear.  It states "If <test> yields a false value and
no <alternate> is specified, then the result of the expression is
unspecified."

Section 1.3.2 of R5RS makes it crystal clear what that means:

  If the value of an expression is said to be "unspecified," then the
  expression must evaluate to some object without signalling an error,
  but the value depends on the implementation; this report explicitly
  does not say what value should be returned.

> Racket produces a syntax error in this situation at least in its
> default language.

Racket has diverged from Scheme quite a bit, to the point that they
don't even call the language "Scheme" anymore, but rather "Racket".

       Mark