From: David Kastrup <dak@gnu.org>
To: Mark H Weaver <mhw@netris.org>
Cc: guile-user@gnu.org
Subject: Re: Multiple values passed as single argument to procedure
Date: Mon, 12 Jun 2017 16:24:25 +0200 [thread overview]
Message-ID: <87h8zl7086.fsf@fencepost.gnu.org> (raw)
In-Reply-To: <87wp8h1lyh.fsf@netris.org> (Mark H. Weaver's message of "Mon, 12 Jun 2017 07:31:34 -0400")
Mark H Weaver <mhw@netris.org> writes:
> I'm sorry David, but _everything_ that you wrote below is incorrect.
Well, let me try again. It's not all that easy to understand.
> David Kastrup <dak@gnu.org> writes:
>
>> Chris Marusich <cmmarusich@gmail.com> writes:
>>
>>> I think I'm missing something here. In (list (f)), the call to f
>>> certainly looks like it's happening at a position that one might
>>> intuitively call a "tail" position.
>>
>> It is,
>
> No it isn't.
>
>> but list does not take multiple values
>
> Yes it does. 'list' accepts an arbitrary number of values
> (arguments).
In my book, multiple values/arguments are different things but I admit
that this is a quagmire to me and strictly speaking what I call
"multiple values" (namely the unmitigated return value from calling
values) never occurs in the position of an argument (at least when not
looking at the C API and/or Guile 1.x).
dak@lola:/usr/local/tmp/lilypond$ guile-2.0
GNU Guile 2.0.13
Copyright (C) 1995-2016 Free Software Foundation, Inc.
Guile comes with ABSOLUTELY NO WARRANTY; for details type `,show w'.
This program is free software, and you are welcome to redistribute it
under certain conditions; type `,show c' for details.
Enter `,help' for help.
scheme@(guile-user)> (list (values 1 2 3))
$1 = (1)
scheme@(guile-user)>
>> and thus discards additional values returned by f.
>
> 'list' does not discard anything. The additional values are discarded
> before 'list' is called.
Ok. Because a function call cannot take multiple values. It does take
multiple _arguments_.
A function call can appear to return multiple values by using call/cc
(which is essentially what (values ...) does) and the construct
call-with-values can salvage them as long as they are only passed
through tail calls (or rather tail returns):
(call-with-values
(lambda () (call/cc (lambda (c) (c 1 2 3))))
list) => (1 2 3)
Basically call-with-values is a mechanism that lets the continuation
delivered by call/cc be a function accepting multiple arguments. An
ordinary function call doesn't.
> I will try to clarify all of this in another message.
Well, I probably messed up things a whole lot more now. But at least it
should be an excellent opportunity for correcting the half-truths and
misconceptions that Scheme's procedure/continuation model might inspire
in people (like me) without a firm grasp of its concepts.
--
David Kastrup
next prev parent reply other threads:[~2017-06-12 14:24 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-06-11 7:56 Multiple values passed as single argument to procedure Chris Marusich
2017-06-11 8:28 ` David Kastrup
2017-06-11 20:36 ` Mark H Weaver
2017-06-11 21:31 ` Mark H Weaver
2017-06-12 0:19 ` Chris Marusich
2017-06-12 4:25 ` Mark H Weaver
2017-06-12 8:19 ` Chris Marusich
2017-06-12 8:55 ` Neil Jerram
2017-06-12 9:48 ` Neil Jerram
2017-06-12 9:39 ` David Kastrup
2017-06-12 11:31 ` Mark H Weaver
2017-06-12 14:24 ` David Kastrup [this message]
2017-06-13 2:26 ` Mark H Weaver
2017-06-13 3:09 ` Mark H Weaver
2017-06-13 3:45 ` Mark H Weaver
2017-06-13 11:17 ` dsmich
2017-06-26 11:25 ` Alex Vong
-- strict thread matches above, loose matches on Subject: below --
2017-09-01 19:39 Chris Marusich
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