Beefy Boxes and Bandwidth Generously Provided by pair Networks
Welcome to the Monastery

Comment on

( #3333=superdoc: print w/replies, xml ) Need Help??

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

Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post; it's "PerlMonks-approved HTML":

  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.
  • Log In?

    What's my password?
    Create A New User
    and the web crawler heard nothing...

    How do I use this? | Other CB clients
    Other Users?
    Others avoiding work at the Monastery: (8)
    As of 2018-08-20 16:12 GMT
    Find Nodes?
      Voting Booth?
      Asked to put a square peg in a round hole, I would:

      Results (192 votes). Check out past polls.