'or' and multiple values

'or' and multiple values

[Neil Jerram]

Hi there!

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?

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.

Thanks,
        Neil

Re: 'or' and multiple values

[Mark H Weaver]

Hi Neil,

Neil Jerram <neil@ossau.homelinux.net> 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.

     Mark