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From: pavel@parc.xerox.COM (Pavel Curtis)
Newsgroups: comp.lang.scheme
Subject: Re: Multiple return values
Message-ID: <91Apr4.094933pst.40706@lambda.parc.xerox.com>
Date: 4 Apr 91 17:49:31 GMT
References: <JINX.91Apr3160152@chamarti.ai.mit.edu>
Sender: daemon@athena.mit.edu (Mr Background)
Organization: The Internet
Lines: 95

"Guillermo J. Rozas" writes:
> There are a lot of issues in multiple values that haven't been
> mentioned.  I like to classify them by code.  Here are some of the
> more controversial points.

I give below the answers for SchemeXerox.

> A: Are the following two expressions equivalent?
> (values 1)
> 1

Yes.

> B: Is the following expression legal?
> (begin
>   (values 1 2 3)
>   4)

Yes.

> C: Is the following expression legal?
> (begin
>   (values)
>   4)

Yes.

> D: Is the following expression legal, and, if so, what does it return?
> (let ((x (values 1 2)))
>   x)

No; it signals an error about a wrong number of arguments (to the implicit
continuation).

> E: Is the following expression legal, and, if so, what does it return?
> (let ((x (values)))
>   x)

No; same error as above.

> F: Is the following expression legal?
> (call-with-values
>   (lambda ()
>     (values 1 2 3))
>   (lambda (x y)
>     (list x y)))

Ditto.

> G: Is the following expression legal, and, if so, what does it return?
> (call-with-values
>   (lambda ()
>     (values 1))
>   (lambda (x y)
>     (list x y)))

Ditto.

> H: Does the following work?
> (call-with-values
>   (lambda ()
>     (call-with-current-continuation
>       (lambda (k)
>         (k 1 2))))
>   (lambda (x y)
>     (list x y)))

Yes.

> My preferences are:
> A: Yes.
> B: Yes.
> C: Yes.
> D: Yes, 1.
> E: No.
> F: No.
> G: No.
> H: Yes.

Well, Jinx, what a wonder.  We appear to disagree only on D, where you want to
silently ignore extra returned values and SchemeXerox signals an error.  Were
we to agree to leave D unspecified, though, I'd want to do the same with E, F,
and G: passing the wrong number of arguments to a continuation is passing the
wrong number of arguments to a continuation.

On the other hand, we could also say that the implicit single-argument
continuations can have either of the following (implicit) argument list
structures:

	(x)			; The SchemeXerox approach
	(x . ignored)		; The Jinx approach

I could probably live with this compromise as well.

		Pavel Curtis