Eric Strom > List-Gen > List::Gen::Cookbook

Download:
List-Gen-0.974.tar.gz

Dependencies

Annotate this POD

CPAN RT

New  2
Open  1
View/Report Bugs
Source  

NAME ^

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

COOKBOOK ^

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

iteration

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 (<$gen>) {...}   # iterated generators must be reset with `$gen->reset`
    ... while <$gen>       # before each loop, 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 also use the do method:

    $gen->do(sub {...}); # which calls sub on every element of $gen

list creation

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).

inline generators

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

normal generators

the range and makegen functions are the most primitive generators, range producing a lazy list, and makegen wrapping a perl 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.

iterative generators

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 = ...;
        }
    }

you can also use the ->from method to write an iterator that builds from an initial value:

    say iterate{$_*2}->from(1)->str(10); # 1 2 4 8 16 32 64 128 256 512

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;

composite generators

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};

variable length generators

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;
    }

recursive generators

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.

more ways to write the fibonacci sequence

    my $fib = <0, 1, *+*...>; >>

    my $fib = <0, 1, {$^a + $^b}...>; >>

    my $fib = ([0, 1] + iterate {sum self($_, $_ + 1)})->rec; >>

    my $fib = ([0, 1] + iterate {sum fib($_, $_ + 1)})->rec('fib'); >>

    my $fib = (iterate {$_ < 2 ? $_ : sum self($_ - 1, $_ - 2)})->rec; >>

    my $fib; $fib = cache gen {$_ < 2 ? $_ : sum $fib->($_ - 1, $_ - 2)}; >>

a few ways to write the factorial sequence

    my $fac = <[*..] 1, 1..>;

    my $fac = <1, 1..>->scan('*');

    my $fac = 1 + repeat(1)->scan('+')->scan('*');

    my $fac = <1, **_...>;

stream generators

here is an example of a sieve of eratosthenes implemented with generators:

    my $primes = do {
        my $src = <2..>;
        iterate {
            my ($x, $xs) = $src->x_xs;
            $src = $xs->grep_stream(sub {$_ % $x});
            $x
        }
    };

in this example, the list is filtered with grep_stream/filter_stream since the algorithm only reads the source once, and reads it in order. a regular filter/grep call could be used, but it would unnecessarily use up a lot of memory since each call would have to build up a new random-access cache.

the inefficiency addressed above could also be fixed by modifying the filtering function itself:

    my $primes = do {
        my @p;
        <2..>->grep(sub {
            my $i = $_;
            $i % $_ or return for @p;
            push @p, $i;
        })
    };

of course if you want prime numbers, just use the primes function:

    my $primes = List::Gen::primes;

which is implemented as a precomputed sieve of eratosthenes in a string buffer. initially it is ready to test the primality of numbers below 1000. if a higher number is checked, the sieve will grow to 10 times larger than that value. beyond 1e7 primes are checked with simple trial division.

printing generators

there are a variety of methods available for printing out the contents of a generator:

    my $gen = <1..5>;

    say $gen->str;  # 1 2 3 4 5
    $gen->say;      # 1 2 3 4 5
    $gen->print;    # same as: print $gen;

    say $gen->perl; # [1, 2, 3, 4, 5]
    $gen->dump;     # [1, 2, 3, 4, 5]

if your generator is longer than you would like to print, such as an infinite generator, passing a number to any of the methods above will limit the number of elements printed.

    <1..>->say(5); # 1 2 3 4 5

which is the same as

    <1..>->take(5)->say;  # 1 2 3 4 5

if passed an additional argument, that string will be included in the output whenever the printing method needs to truncate a generator.

    say <1..>->perl(5, '...');  # [1, 2, 3, 4, 5, ...]

these methods are recursive and will expand elements that are generators or array references.

    list(<1..>, <a..>, <A..>)->dump(3, '...');

    # [[1, 2, 3, ...], ['a', 'b', 'c', ...], ['A', 'B', 'C', ...]]

    <1..>->tuples(<a..>)->dump(3); # [[1, 'a'], [2, 'b'], [3, 'c']]

a target file handle can be passed as the first argument:

    <1..>->dump(*STDERR, 5);

the say, print, and dump methods all return the generator they were called on for easy chaining.

    <1..>->say(5)->map('**2')->say(5);
    # 1 2 3 4 5
    # 1 4 9 16 25

debugging generators

in addition to the methods to print generators, there are several methods dedicated to debugging:

    <0..>->debug;
    # debug:   List::Gen::erator::_20=ARRAY(0x2d07bfc)
    # type:    List::Gen::Range
    # source:  none
    # mutable: no
    # stream:  no
    # range:   [0 .. inf]
    # index:   0
    # perl:    [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, ...]
    #  at file.pl line 12

pass debug a number to control how many elements are printed.

    my $gen = <0..>->watch('range')
                   ->grep('even')->watch('grep')
                   ->map('**2')->watch('map')
                   ->map('"[$_]"');

    local $\ = ', '; # watch ends lines with $\ if defined or with $/

    say $gen->(0); # range: 0, range: 1, range: 2, grep: 0, map: 0, [0]
    say $gen->(1); # range: 3, range: 4, grep: 2, map: 4, [4]
    say $gen->(2); # range: 5, range: 6, grep: 4, map: 16, [16]

watch can also be passed a file handle to print to.

AUTHOR ^

Eric Strom, <asg at cpan.org>

COPYRIGHT & LICENSE ^

copyright 2009-2011 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.

syntax highlighting: