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Here's one example. The Hamming sequence is the sequence of numbers whose prime factors are only 2, 3 and 5, in ascending order. Writing a program to print out the sequence is non-trivial (we exclude "solutions" which must store the entire sequence in a large array and the like). It used to be the poster example of co-routines, but current style more likely favours a functional program using streams and (somewhat) lazy evaluation.

Read about streams in this chapter of Structure and Interpretation of Computer Programs. The program below is a translation to Perl of a streams-based program to print out the Hamming sequence.

#!/usr/local/bin/perl -w # Output the Hamming sequence use strict; package Stream; sub new { my ($class, $hd, $tl) = @_; $class = ref $class || $class; bless { _hd => $hd, _tl => $tl }, $class } sub head { my $self=shift; $self->{_hd} } sub tail { my $self = shift; $self->{_tl} = $self->{_tl}->() if ref $self->{_tl} eq 'CODE'; $self->{_tl} } sub exec { my ($self, $code, $n) = @_; for my $i (1..$n) { $code->($self->head); $self = $self->tail } } package main; sub merge { my ($a, $b) = @_; return ($a->head == $b->head) ? new Stream($a->head, sub { merge($a->tail, $b->tail) }) : ($a->head < $b->head) ? new Stream($a->head, sub { merge($a->tail, $b) }) : new Stream($b->head, sub { merge($a, $b->tail) }); } sub mult { my ($a, $s) = @_; return new Stream($a*$s->head, sub { mult($a,$s->tail) }); } my $hamming; $hamming = new Stream(1, sub { merge( mult(2, $hamming), merge(mult(3, $hamming), mult(5, $hamming))) } ); my @x; $hamming->exec(sub { push @x, shift }, 1000); print "@x\n";

EDIT (20020506): fix indentation (Perlmonks does horrible things to tabs; glad it's not Pythonmonks).

In reply to Re: Functional perl please by ariels
in thread Functional perl please by rje

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