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Re: Multiple values passed as single argument to procedure
From: |
Chris Marusich |
Subject: |
Re: Multiple values passed as single argument to procedure |
Date: |
Sun, 11 Jun 2017 17:19:47 -0700 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/25.2 (gnu/linux) |
Hi Mark,
Thank you for the detailed response! I learn something new every day.
Mark H Weaver <address@hidden> writes:
> Use 'call-with-values', 'let-values', or 'receive' to call a procedure
> that returns multiple values (or no values).
>
> If you do not use one of the above forms (or a macro that expands to one
> of them) to call a procedure that returns multiple values, then Guile
> will discard all but the first result. Note that this is a
> Guile-specific extension. Other Scheme implementations may behave
> differently (e.g. report an error) if multiple values (or no values) are
> returned to a procedure call that was not done using one of the forms
> listed above.
I see. So, this behavior is implementation-specific for Guile scheme.
Is this behavior documented in the Guile reference manual? I looked,
but I couldn't find information about it. So, it is not clear to me if
one should rely on this behavior, or if it is likely to change in the
future. I was hoping to find this behavior documented in either
"(guile) Multiple Values" or somewhere in "(guile) About Procedures".
Perhaps there's a better location. In any case, I think it would be
helpful if this were documented in the manual.
Here's another question. I've also noticed that when the 'list'
procedure is composed with a procedure f that returns multiple values,
the list that gets returned when calling the composition differs from
the list that results when "manually" invoking the same composition. An
example will clarify what I mean:
--8<---------------cut here---------------start------------->8---
$ guile
GNU Guile 2.2.2
Copyright (C) 1995-2017 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)> (define (f . _) (values 1 2))
scheme@(guile-user)> (f)
$1 = 1
$2 = 2
scheme@(guile-user)> (define g (compose list f))
scheme@(guile-user)> (g)
$3 = (1 2)
scheme@(guile-user)> (list (f))
$4 = (1)
scheme@(guile-user)>
--8<---------------cut here---------------end--------------->8---
In the above, I was surprised to find that $3 was not the same as $4.
To put this another way, I was surprised to find that the composition
via 'compose' (which returned $3) did not behave the same as the
'manual' composition (which returned $4). What's going on here? I
couldn't find the answer by looking at the documentation for the compose
((guile) Higher-Order Functions) or list ((guile) List Constructors)
procedures in the Guile manual.
Thank you for taking the time to help me understand this better.
--
Chris
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- Multiple values passed as single argument to procedure, Chris Marusich, 2017/06/11
- Re: Multiple values passed as single argument to procedure, David Kastrup, 2017/06/11
- Re: Multiple values passed as single argument to procedure, Mark H Weaver, 2017/06/11
- Re: Multiple values passed as single argument to procedure, Mark H Weaver, 2017/06/11
- Re: Multiple values passed as single argument to procedure,
Chris Marusich <=
- Re: Multiple values passed as single argument to procedure, David Kastrup, 2017/06/12
- Re: Multiple values passed as single argument to procedure, Mark H Weaver, 2017/06/12
- Re: Multiple values passed as single argument to procedure, David Kastrup, 2017/06/12
- Re: Multiple values passed as single argument to procedure, Mark H Weaver, 2017/06/12
- Re: Multiple values passed as single argument to procedure, Mark H Weaver, 2017/06/12
- Re: Multiple values passed as single argument to procedure, Mark H Weaver, 2017/06/12