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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.

In reply to Infinitely Lazy by sleepingsquirrel

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