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From: jinx@zurich.ai.mit.edu (Guillermo J. Rozas)
Newsgroups: comp.lang.scheme
Subject: Re: Multiple return values
Message-ID: <JINX.91Apr3160152@chamarti.ai.mit.edu>
Date: 3 Apr 91 21:01:50 GMT
References: <1991Mar26.155905.12906@daffy.cs.wisc.edu> <kiran.670017354@copper>
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In-reply-to: carlton@husc10.harvard.edu's message of 3 Apr 91 03:25:54 GMT

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.


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


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


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


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


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


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


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)))


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


PS: 
 - By equivalent I mean that either can be substituted for the other
   without changing the meaning of the program (ignoring capture of
   free variables).
 - By legal I mean it is not in error.
 - By work I mean it is legal and the "obvious" value(s) is(are)
   returned.

My preferences are:
A: Yes.
B: Yes.
C: Yes.
D: Yes, 1.
E: No.
F: No.
G: No.
H: Yes.
But would accept a compromise where any of the above are left unspecified.
My arm would need a lot of twisting to accept a proposal that requires
any of those decisions to be otherwise.

Although this set of choices may appear inconsistent, the crucial
observation, that points out the asymmetry between APPLY and VALUES is
that there are no implicit procedures, but there are implicit
continuations.  Furthermore, the continuation that VALUES is invoking
is not apparent in the VALUES expression, but the (expression
evaluating to) the procedure in APPLY expressions is.
Given these asymmetries, the decisions need not be the same.