RFC 76 (v2) Builtin: reduce

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=head1 TITLE

Builtin: reduce

=head1 VERSION

  Maintainer: Damian Conway <[email protected]>
  Date: 10 Aug 2000
  Last Modified: 19 Sep 2000
  Mailing List: [email protected]
  Number: 76
  Version: 2
  Status: Frozen

=head1 ABSTRACT

This RFC proposes a builtin C<reduce> function, modelled after Graham Barr's
C<reduce> subroutine from builtin.pm

=head1 DESCRIPTION

A new builtin -- C<reduce> -- is proposed. 

This function would take an block, subroutine reference, or curried function 
(hereafter referred to as I<the reduction subroutine>),
and call it repeatedly to reduce the remaining arguments
(hereafter, referred to as C<the list>).

If the reduction subroutine has a prototype, that prototype
determines how many items are reduced at a time. If the reduction subroutine
is a block or has no prototype, two items are reduced each time.

The first call to the reduction subroutine will be passed the first N
elements of the list, and subsequent calls will be passed the result of
the previous call and the next N-1 elements in the list, until no more
elements remain in the list. If fewer than N-1 elements remain on the
final call, all the remaining elements are passed. All calls to the
reduction subroutine are made in a scalar context.

In all cases, the elements of the list are lazily evaluated. That is,
any element of the list that is not a literal value is only evaluated
when that element is passed to (and actually used by) the reduction
subroutine.

If the original list has no elements, C<reduce> immediately throws an
exception. If the original list has a single element, that element is
immediately returned (without ever calling the reduction subroutine).
Otherwise, in a scalar context, the result of the final reduction call
is the result returned by C<reduce>. In a list context, a list of all
the interim values, plus the final value, would be returned.

If the reduction subroutine is ever terminated by a call to C<last>, the
enclosing C<reduce> immediately returns the last reduction value -- or
list of partial values -- to that point. If the C<last> occurs during
the first reduction, an exception is thrown.


=head1 EXAMPLES

Summation:

        $sum = reduce {$_[0]+$_[1]}     0, @numbers;
        $sum = reduce sub{$_[0]+$_[1]}, 0, @numbers;
        $sum = reduce ^_+^_,            0, @numbers;

Note that the first element of the list -- zero in this case, 1 in the next
example -- represents the default value if the list is empty.


Production:

        $prod = reduce {$_[0]*$_[1]}     1, @numbers;
        $prod = reduce sub{$_[0]*$_[1]}, 1, @numbers;
        $prod = reduce ^_*^_,            1, @numbers;


Minimization:

        $min = reduce ^x <= ^y ? ^x : ^y,  @numbers
        $min = reduce ^x le ^y ? ^x : ^y,  @strings


Minimization to zero:

        $min = reduce any(^x,^y)<0 && last || ^x<^y && ^x || ^y,  @numbers


Collection:

        @triples = @{ reduce sub($;$$$){ [@{shift},[@_] }, [], @singles };


Separation:

        $sorted = reduce { push @{$_[0][$_[1]%2]}, $_[1]; $_[0] }
                         [[],[]],
                         @numbers;

Accumulative sequence generation:

        @increase = reduce ^value + ^delta, $original, @bonuses;

        @growth = reduce ^value * ^rate, $principal, @annual_interest_rates;


=head1 IMPLEMENTATION

Extend Graham's builtin, I'd imagine.


=head1 REFERENCES

builtin.pm

RFC: Higher Order Functions