RFC 82 (v1) Make operators behave consistently in a

This and other RFCs are available on the web at
  http://dev.perl.org/rfc/

=head1 TITLE

Make operators behave consistently in a list context

=head1 VERSION

  Maintainer: Jeremy Howard <[email protected]>
  Date: 10 August 2000
  Version: 1.00
  Mailing List: [email protected]
  Number: 82

=head1 ABSTRACT

It is proposed that in a list context, operators are applied
component-wise to their arguments. Furthermore, it is proposed that
this behaviour be extended to functions that do not provide a specific
list context.

=head1 DESCRIPTION

Currently, operators applied to lists in a list context behave
counter-intuitively:

  @b = (1,2,3);
  @c = (2,4,6);
  @d = @b * @c;   # Returns (9) == scalar @b * scalar @c

This RFC proposes that operators in a list context should be applied
component-wise to the elements of their arguments:

  @d = @b * @c;   # Returns (2,8,18)

If the lists are not of equal length, the shorter lists should be treated
as if they were padded with undefs at the right-hand end. However, this
implicit padding should not cause these elements in the shorter lists to
auto-vivify.

Functions that do not return a list should be treated in the same way:

  @e = (-1,1,-3);
  @f = abs(@e);   # Returns (1,1,3)

=head1 IMPLEMENTATION

These operators and functions should be evaluated lazily. For instance:

  @b = (1,2,3);
  @c = (2,4,6);
  @d = (-2,-4,-6);
  $sum = reduce ^_+^_, @b * @c + @d;

should be evaluated as if it read:

  $sum = 0;
  $sum += $b[$_] * $c[$_] + $d[_] for (0..$#b-1));

That is, no temporary list is created, and only one loop is required.

The proposal to handle functions is tricky, since there is currently no
obvious way to see whether a function is going to return a list. Either we
need some kind of more advanced prototyping (or other way of creating a
signature), or we need to manually change functions provided with Perl to
check for list context and Do The Right Thing.

=head1 REFERENCES

  The Mathematica Navigator, Heikki Ruskeepää,
    Academic Press, ISBN 0-12-603640-3, p383.

  Expression Templates (C++ Implementation):

http://extreme.indiana.edu/~tveldhui/papers/techniques/techniques01.html#l32

  Implementation in Perl Data Language:
    http://pdl.perl.org