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);        }

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