The topic of lazy code came up recently in regards to Perl6. But with a little extra effort, we can do lazy lists in Perl5. So I thought I present a little taste of my favorite use for laziness: infinite lists...

`#!/usr/bin/perl
#make a finite lazy list (1,2,12,3.14)
$g=cons(1,cons(2,cons(12,cons(3.14,undef))));
#double the values
$twice = lazyMap(sub{2*$_[0]}, $g);
#print values to check correctness...
#should be 2,4,24,6.28
#don't try unlazyMap on infinite lists...
print "twice: ",join(",", unlazyMap(sub{$_[0]}, $twice))."\n";
$z = zeros(); #create infinite list of zeros (a la /dev/zero)
$inf=cons(3,$z); #a infinite list of zeros with a 3 on the front
$double_plus1 = lazyMap(sub{2*$_[0]+1}, $inf);
$take8 = take(8, $double_plus1); #grab first 8 values of infinite list
print "double_plus1: ",join(",", unlazyMap(sub{$_[0]}, $take8))."\n";
$nums = take(10, nums());
print "integers: ",join(",", unlazyMap(sub{$_[0]}, $nums))."\n";
$fibs = take(10, fibs(1,0));
print "fibs: ",join(",", unlazyMap(sub{$_[0]}, $fibs))."\n";
print "1000th Fibonacci: ",Nth(1000,fibs(1,0)),"\n";
$primes = take(10, primes(3,2));
print "primes: ",join(",", unlazyMap(sub{$_[0]}, $primes))."\n";
sub zeros { sub{ (0,zeros()) } }
sub nums { sub{ (0, sub{ (1, lazyMap(sub{$_[0]+2},nums())) })}}
sub fibs { my ($x, $y)=@_; sub{ ($x+$y, fibs($y,$x+$y)) } }
sub primes{ my ($n, @ps) = @_;
(scalar grep !($n % $_), @ps)
? primes($n+2,@ps)
: sub{ ($n, primes($n+2,$n,@ps)) } }
###### Helper subroutines ###############
sub cons { my ($val, $list) = @_;
sub { return ($val, $list) } }
sub head { my ($h, $t)=$_[0]->() if defined($_[0]);
return $h; }
sub tail { my ($h, $t)=$_[0]->() if defined($_[0]);
return $t; }
sub unlazyMap{ my ($f, $list) = @_;
my ($h, $t) = $list->() if defined($list);
defined($t) ? ($f->($h),unlazyMap($f,$t))
: $f->($h) }
sub lazyMap { my ($f, $list) = @_;
my ($h, $t) = $list->() if defined($list);
defined($t) ? sub{ ($f->($h),lazyMap($f,$t)) }
: sub{ ($f->($h),undef ) } }
sub take { my ($n, $list) = @_;
return undef if ($n <= 0 or !defined($list));
(my $h, my $t) = $list->();
sub{ ($h, take($n-1, $t)) } }
sub Nth { my ($n, $list) = @_;
return undef if (!defined($list));
(my $h, my $t) = $list->();
$n<=1 ? $h : Nth($n-1, $t); }
`

-- All code is 100% tested and functional unless otherwise noted.

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