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Re: Functional perl please

by ariels (Curate)
on Jan 06, 2002 at 12:56 UTC ( #136663=note: print w/replies, xml ) Need Help??


in reply to Functional perl please

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).

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[Corion]: The fun show at $work continues, as The Big Project is now in its second week of frantic live-bugfixing and weekend releases where nobody knows what went live. Nothing has been tested anyway.
erix mutters cantankerously under his breath
Corion watches from the sidelines. Or rather, from behind, as my system only gets output from that process and my programs adhere strictly to the GIGO design principle.
[erix]: ah, that's nice to hear Corion :)
[Corion]: erix: Yeah, the sad thing is that all I can do is document things, so I can point fingers when the auditors come :-/
[Corion]: "I'm here to open tickets and point fingers. And I'm all out of tickets."
[erix]: didn't Sybase have pretty good auditing? :) (this is a vague memory)
[erix]: (culprits often are upstream of db of course)
[Corion]: Ah, how I missed it. After some years, I revisit slashdot on a click-bait link, and it provides the usual humor instantly: "I didn't know Drupal had rules for sex. It must be a plug-in"
[Corion]: erix: This is not for sybase, but for the input data files, resp. their contents.

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