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Re: [Scheme-reports] multiple values module
9 hours ago, John Cowan wrote:
> Andy Wingo scripsit:
> > What's the rationale for breaking call-with-values and values out to
> > their own module?
> Primarily because they aren't in IEEE/R4RS.
> > Suggestion: since it is trivial to implement `call-with-values'
> > with a define-record-type <values>, simply require it in the base
> > Scheme. [...]
IMO implementations that do some half-baked reification of multiple
values as first-class values are broken. Things like
(list (values 1 2) 3) or (define x (values 1 2)) should throw an
error. (Note to potential flamers: this has nothing to do with the
question of having MV in the language.)
One relevant by-product of this is that debating whether `set!' can
return 0 values looks very different in such an implementation.
> Chibi actually implements multiple purely in Scheme by returning a
> list with a unique CAR, which prints as "(value)".
Ugh. If the above is broken, then this is a train wreck.
> (cadr (values 1 2))
> (length (values '(1) 2))
> (define (foo x)
;; this function always returns 3...?
(length (call-with-values (lambda () (list x 1 2)) list)))
> (foo (car (values)))
I'm sure that there are reasons for implementing things this way, but
making an argument out of it sounds insane. (Or a camouflaged attemt
to make multiple values so useless that it's as if they're not in...)
((lambda (x) (x x)) (lambda (x) (x x))) Eli Barzilay:
http://barzilay.org/ Maze is Life!
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