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Re: 'or' and multiple values

From: Mark H Weaver
Subject: Re: 'or' and multiple values
Date: Tue, 25 Mar 2014 22:20:05 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux)

Hi Neil,

Neil Jerram <address@hidden> writes:

> In the following, is the last result a bug?
>   GNU Guile 2.0.9-deb+1-1
>   Copyright (C) 1995-2013 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)> (values 'a 'b)
>   $1 = a
>   $2 = b
>   scheme@(guile-user)> (or (values 'a 'b))
>   $3 = a
>   $4 = b
>   scheme@(guile-user)> (or #f (values 'a 'b))
>   $5 = a
>   $6 = b
>   scheme@(guile-user)> (or (values 'a 'b) (values 'c 'd))
>   $7 = a
> In other words it seems 'or' doesn't propagate multiple values in a
> non-tail position.  Is that expected?

I think so, yes.

> The manual section 'Returning and Accepting Multiple Values' does
> mention tail position, but only in passing, and it isn't obvious to me
> why that restriction should exist.

'or' needs to test whether the results of the non-final expressions are
true or false.  It's not clear how this should be generalized to
multiple values.


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