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=head1 TITLE
Builtins: merge() and demerge()
=head1 VERSION
Maintainer: Jeremy Howard <[EMAIL PROTECTED]>
Date: 10 August 2000
Last modified: 29 August 2000
Mailing List: [EMAIL PROTECTED]
Version: 2
Number: 90
Status: Developing
=head1 ABSTRACT
It is proposed that two new functions, C<merge>, and C<demerge>, be added
to Perl. C<merge(@list1, @list2, ...)> would return a list that
interleaved its arguments. C<demerge($num_lists, @list)> would reverse
this operation.
=head1 CHANGES
=head2 Since v1
=over 4
=item *
Moved list to [EMAIL PROTECTED]
=item *
Changed name from zip/unzip
=item *
Pass lists directly in examples, not references
=item *
Change 2nd argument from $list_size to $num_lists
=back
=head1 DESCRIPTION
It is proposed that Perl implement a function called C<merge> that
interleaves the arguments of arrays together, and is evaluated lazily.
For instance:
@a = (1,3,5);
@b = (2,4,6);
@merged_list = merge(@a,@b); # (1,2,3,4,5,6)
This makes it easy to operate on multiple lists using flexible reduction
functions:
$sum_xy = sub {reduce ^last+^x*^y, merge($_[0], $_[1])};
print $sum_xy->(@a, @b); # Prints '44', i.e. 1*2+3*4+5*6
In order to reverse this operation we need an C<demerge> function:
@merged_list = merge(@a,@b); # (1,2,3,4,5,6)
@demerged_list = demerge(2, @merged_list); # ([1,3,5], [2,4,6])
The second argument to C<demerge> is the number of lists that are to be
created (i.e. the number of lists that would have been C<merge>d for this
to reverse the operation).
If the list to be demerged is not an exact multiple of the partition size,
the final list references are not padded--their length is one less than
the list size. For example:
@list = (1..7);
@demerged_list2 = demerge(3, @list); # ([1,4,7], [2,5], [3,6])
=head1 IMPLEMENTATION
The C<merge> and C<demerge> functions should be evaluated lazily.
Effectively, C<merge> creates an iterator over multiple lists. If used as
part of a reduction, the actual interleaved list need never be created.
For instance:
$sum_xy = sub {reduce ^last+^x*^y, merge($_[0], $_[1])};
$answer = $sum_xy->(@a, @b);
should be evaluated as if it read:
$answer = 0;
$answer += $a[$_] * $b[$_] for (0..$#a-1));
which does not need to create an intermediate list.
=head1 REFERENCES
RFC 23: Higher order functions
RFC 76: Builtin: reduce
RFC 91: Builtin: partition
