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Functional perl please

by rje (Deacon)
on Jan 03, 2002 at 21:54 UTC ( #136017=perlquestion: print w/ replies, xml ) Need Help??
rje has asked for the wisdom of the Perl Monks concerning the following question:

So, one of the first things I learned about when I first
came to perlmonks is map. It immediately struck me as
being yet another example of LISP bleeding over to perl.

So I was wondering if you can code in purely 'functional' perl?
Has anyone tried? And when I say functional programming, I
mean non-one-liners. map makes me think this might be
possible. Has anyone out there done it? Are there examples
posted out there?

rje

Comment on Functional perl please
Re: Functional perl please
by giulienk (Curate) on Jan 03, 2002 at 22:02 UTC
Re: Functional perl please
by danger (Priest) on Jan 04, 2002 at 01:11 UTC

    There is the Language::Functional module which provides a reasonable number Haskell(ish) functions.

Re: Functional perl please
by ariels (Curate) on Jan 06, 2002 at 12:56 UTC

    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).
Re (tilly) 1: Functional perl please
by tilly (Archbishop) on Jan 07, 2002 at 17:19 UTC
    A good resource on functional techniques in Perl is Dominus. He has several past articles on it, and is currently writing a book on the topic.

    Of course, after obvious substitutions, if you know how to do functional programming in any language, you can in any other which supports the necessary notions. Which most scripting languages these days (Perl, Python, Ruby, JavaScript, etc) seem to in addition to all of the usual suspects.

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