List::Gen::Cookbook - how to get the most out of List::Gen

this document contains tips and tricks for working with and combining generators

given the generator my $gen = gen {2**$_} 100; which computes the first hundred powers of two, here are a few was to iterate over it (that all maintain lazy evaluation):

    for (@$gen) {...}       # no need to reset generator between calls
    for my $p (@$gen) {...}
    ... for @$gen;

    while (<$gen>) {...}    # generator must be reset with `$gen->reset`
    ... while <$gen>        # between calls, also be sure to `local $_` before
                            # while loops that modify `$_`
    while (my $p = <$gen>) {...}
    while (defined(my $p = $gen->())) {...}
    while ($gen->more) {do something with $gen->next}

since all of these iteration examples remain lazy (only generating values on demand), you can last at any time to break out of the loop.

you can dereference finite length generators to pass all of their elements to a function:

    say sum @$gen;

but it is usually faster to write it this way:

    say sum $gen->all;

generators interpolate in strings like normal arrays:

    say "@$gen[0 .. 10]";

do not call ->all or use array dereferencing on infinite generators. in some places you may get an error, others, it will loop forever (and probably run out of memory at some point).

the generators without code blocks, range and glob , can be directly dereferenced with @{...}

    for (@{range 0.345, -21.5, -0.5}) {...}

    for (@{< 1 .. 10 by 2 >}) {...}

for those with code blocks, perl needs a little help to figure out whats going on:

    for (@{ +gen {$_**2} 1, 10 }) {...}  # a '+' or ';' before it does the trick

if you export the glob function from List::Gen, that function and the <*.glob> operator will have one special case overridden. if given an argument that matches the following pattern:

   /^ ( .+ : )? number .. number ( (by | += | -= | [-+,]) number )?
                                 ( (if | when) .+ )? $/

then the arguments will be passed to range , gen , and filter as appropriate. any argument that doesn't match that pattern will be passed to perl's builtin glob function. here are a few examples:

    <1 .. 10>                    ~~  range 1, 10
    <1 .. 10 by 2>               ~~  range 1, 10, 2
    <10 .. 1 -= 2>               ~~  range 10, 1, -2
    <x * x: 1 .. 10>             ~~  gen {$_ * $_} 1, 10
    <sin: 0 .. 3.14 += 0.01>     ~~  gen {sin} 0, 3.14, 0.01
    <1 .. 10 if x % 2>           ~~  filter {$_ % 2} 1, 10
    <sin: 0 .. 100 by 3 if /5/>  ~~  filter {/5/} gen {sin} 0, 100, 3

    for (@{< 0 .. 1_000_000_000 by 2 >}) { # starts instantly
        print "$_\n";
        last if $_ >= 100;        # exits the loop with only 51 values generated
    }

    my @files = <*.txt>;  # works as normal

the range and makegen functions are the most primitive generators, range producing a lazy list, and makegen storing an array.

you build upon these with the other generator functions/methods. many generator functions will pass their arguments along to range or makegen as needed, so you rarely need to use them directly.

    gen {$_**2} 100             ~~  gen {$_**2} range 0, 100

    my @names = qw/bob alice eve/;
    gen {"hello $_!"} \@names   ~~  gen {"hello $_!"} makegen @names

those were two examples of gen the generator equivalent of map that attaches a code block to a generator.

there is one other primitive generator type, the iterate generator, which is used when your algorithm is iterative in nature. iterative generators come in two flavors, single element per iteration, and multi element per iteration.

    my $fib = do {
        my ($an, $bn) = (0, 1);
        iterate {
            my $return = $an;
            ($an, $bn) = ($bn, $an + $bn);
            $return
        }
    };

    my $multi = do {
        my $var;
        iterate_multi {
            my @return = ...;
        }
    }

the iterative generators have some syntactic sugar you can use, in the form of gather {...} and take(...) :

    my $fib = do {
        my ($x, $y) = (0, 1);
        gather {
            ($x, $y) = ($y, take($x) + $y)
        }
    };

don't confuse this implementation of gather/take with the perl6 implementation, or the implementation of yield in python. since perl5 does not have continuations, take can't pause the execution of the gather block. instead, it saves the value passed to it, and the gather block returns it when the block ends. you can use gather_multi to take multiple times.

all iterative generators implicitly cache their generated elements in an internal array. this allows random access within the generator. unlike other caching generators, you can not purge the iterator's cache (except by letting all references to the generator fall out of scope, like a normal variable). if you want an iterator that throws its values away, just write a subroutine:

    my $fib = do {
        my ($an, $bn) = (0, 1);
        sub {
            my $return = $an;
            ($an, $bn) = ($bn, $an + $bn);
            $return
        }
    };

    say $fib->() for 1 .. 10;

there are many ways to modify generators.

    my $odd = filter {$_ % 2};
    my $squares_of_odd = gen {$_**2} $odd;

    my $less_than_1000 = While {$_ < 1000} $squares_of_odd;

    say for @$less_than_1000;

    my $this_is_same = While {$_ < 1000} gen {$_**2} filter {$_ % 2};

    say for @$this_is_same;

here is a sub that returns a generator producing the fibonacci sequence to a given magnitude:

    sub fibonacci {
        my $limit   = 10**shift;
        my ($x, $y) = (0, 1);

        While {$_ < $limit} gather {
            ($x, $y) = ($y, take($x) + $y)
        }
    }

    say for @{fibonacci 15};

to implement grep (as filter ) or while (as While ) on a generator means that the generator no longer knows its exact size at all times. care has been taken to make sure that this doesn't bite you too much.

    my $pow = While {$_ < 20} gen {$_**2};

    say for @$pow;     # checks size on every iteration, works fine
    say while <$pow>;  # also works
    say $pow->all;     # ok too

each prints:

    0
    1
    4
    9
    16

but, if instead of say for @$pow you had written map {say} @$pow , perl will try to expand @$pow in list context, and it will not know when to stop, since it only checks at the beginning. the solution, in short, is to only dereference variable length generators in slice @$gen[0 .. 10] or iterator ... for @$gen; context, and never in list context.

in general, it makes more sense (and is faster) to build your constraint into the calling code:

    my $pow = gen {$_**2};
    for (@$gen) {
        last if $_ > 20;
        say;
    }

the fibonacci sequence can be generated from the following definition:

    f[0] = 0;
    f[1] = 1;
    f[n] = f[n-1] + f[n-2];

here are a few ways to write that definition as a generator:

    my $fib; $fib = cache gen {$_ < 2  ? $_ : $$fib[$_ - 1] + $$fib[$_ - 2]};

    my $fib = gen {$_ < 2 ? $_ : self($_ - 1) + self($_ - 2)}
              ->cache
              ->recursive;

    my $fib; $fib = gen {$fib->($_ - 1) + $fib->($_ - 2)}
                  ->overlay( 0 => 0, 1 => 1 )
                  ->cache;

    my $fib; $fib = gen {$$fib[$_ - 1] + $$fib[$_ - 2]}->cache->overlay;
    @$fib[0, 1] = (0, 1);

bringing all those techniques together:

    my $fib = gen {self($_ - 1) + self($_ - 2)}
            ->overlay( 0 => 0, 1 => 1 )
            ->cache
            ->recursive;

the cache function is used in each example because the recursive definition of the fibonacci sequence would generate an exponentially increasing number of calls to itself as the list grows longer. cache prevents any index from being calculated more than once.

Eric Strom, <ejstrom at gmail.com>

copyright 2009-2010 Eric Strom.

this program is free software; you can redistribute it and/or modify it under the terms of either: the GNU General Public License as published by the Free Software Foundation; or the Artistic License.

see http://dev.perl.org/licenses/ for more information.