I came across Limbic~Region's article on iterators and thought I write up some quick perl examples using the more generic unfold (code reuse in action). Fold is the list deconstructor, while unfold is its dual, the list generator.
#!/usr/bin/perl -w
use strict;
#Demonstrations of iterators using 'unfold'
#the natural numbers 1,2,3... my $nats = unfold(\&false, #loop forever \&id, #identity function sub{$_[0]+1}, #increment value 1 ); #initial seed
# 0,2,4,6,8 my $evens_5 = unfold( sub{$_[0]>8}, \&id, sub{$_[0]+2}, 0);
#Factorials my $facs = unfold(\&false, \&fst, sub{[fst($_[0])*snd($_[0]),snd($_[0])+1]}, [1,1]);
#Fibonacci's famous sequence my $fibs = unfold(\&false, \&fst, sub{[snd($_[0]),fst($_[0])+snd($_[0])]},[0,1]);
#Enumeration of DNA sequences from... # http://www.perl.com/pub/a/2005/06/16/iterators.html # my $dna = unfold(sub{toDNA($_[0]) eq "CTTTT"}, sub{my $x = toDNA($_[0]); $x=~s/^.(.*)$/$1/; $x}, sub{$_[0]+1}, fromDNA("CAAAA"));
#print out some samples... my (@n, @e, @a, @f); push @n, $nats->() for (1..10); push @e, $evens_5->() for (1..5); push @a, $facs->() for (1..10); push @f, $fibs->() for (1..10); print "@n\n@e\n@a\n@f\n";
print $dna->() ."\n" for (1..10);
# --Haskell version of unfold... # # unfold p f g x = # if p x # then [] # else f x : unfold p f g (g x)
sub unfold { # function $p is a predicate to indicate when to stop iterator # function $f takes the seed ($x) and formats it before returning it # function $g massages $x for the next iteration # $x is the initial value
my ($p, $f, $g, $x) = @_; sub{ ($p->($x)) ? undef : do{my $val = $f->($x); $x=$g->($x); $val}} }
#Helper functions sub fst { $_[0]->[0] } sub snd { $_[0]->[1] } sub false { 0 } sub id { $_[0] }
# These subs convert back and forth between strings of "ACGT" and # integers, using base-4 arithmetic. Something with pack/unpack # might be cleaner.
sub fromDNA { my $s = shift; $s =~ tr/ACGT/0-3/;
my $n=0; $n = $n*4+$_ for (split(//, $s));
return $n; }
sub toDNA { my $n = shift; my $acc = ""; while($n) { my $d = $n % 4; $n = int($n/4); $acc = "$d" . $acc; } ($acc="$acc") =~ tr/0-3/ACGT/; return $acc; }
