NAME

   Lazy::Util - Perl utilities for lazy evaluation

VERSION

   version 0.003

SYNOPSIS

     use Lazy::Util qw/ l_grep l_map /;

     my $lazy = l_map { $_ * 2 } l_grep { /^[0-9]+$/ } 3, 4, 5, sub {
       print "Enter a number: ";
       return scalar readline(STDIN);
     };

     while (defined(my $answer = $lazy->get())) {
       print "Double your number: $answer\n";
     }

DESCRIPTION

   Perl utility functions for lazy evaluation.

FUNCTIONS

   This module has two sets of functions, the l_* functions and the g_*
   functions. The l_* functions are designed to return a Lazy::Iterator
   object which you can get values from, the g_* functions are designed to
   get a value out of a Lazy::Iterator object. Some of the g_* functions
   may never return if the source of values is infinite, but they are for
   the most part designed to not eat up all of your memory at least ;).

   All these functions can be exported, but none are exported by default.
   You can use the :all export tag to export all of them.

l_* functions

   The l_* functions are:

l_concat(@sources)

     my $lazy = l_concat @sources;

   l_concat returns a Lazy::Iterator object which will simply return each
   subsequent value from the list of sources it's given.

l_first($n, @sources)

     my $lazy = l_first $n, @sources;

   l_first will return a Lazy::Iterator object which will only get the
   first $n values from the subsequent arguments. This can be used the
   'break' an otherwise infinite list to only return a certain number of
   results.

l_grep($code, @sources)

     my $lazy = l_grep { ... } @sources;

   l_grep will return a Lazy::Iterator object which will filter out any
   value which doesn't return true from the $code block in the first
   argument.

l_map($code, @sources)

     my $lazy = l_map { ... } @sources;

   l_map will return a Lazy::Iterator object which will transform any
   value using the $code block in the first argument.

   The $code block is evaluated in list context, and each scalar it
   returns will be returned by each subsequent ->get(), not poking the
   @sources again until the list is exhausted. If an empty list is
   returned, the @sources will be poked again until a list of at least one
   element is returned, or the source returns undef.

l_until($code, @sources)

     my $lazy = l_until { ... } @sources;

   l_until will return a Lazy::Iterator object which will return values
   from the @sources until the $code block returns true, after which it
   will be exhausted.

g_* functions

   The g_* functions are:

 g_count(@sources)

     my $count = g_count @sources;

   g_count counts the number of values from the @sources and returns how
   many there were. This has the potential to never return if given a
   source of infinite values.

g_first(@sources)

     my $val = g_first @sources;

   g_first returns the first value from the list of arguments, lazily
   evaluating them. Equivalent to l_concat(...)->get();. If @sources is
   empty, it will return undef.

g_join($sep, @sources)

     my $lines = g_join $str, @sources;

   g_join evaluates all the values it's given and returns them joined into
   a string. This has the potential to never return as well as running out
   of memory if given a source of infinite values. If @sources is empty,
   it will return undef.

g_last(@sources)

     my $val = g_last @sources;

   g_last evaluates all the values it's given and returns the last value.
   This has the potential to never return if given a source of infinite
   values. If @sources is empty, it will return undef.

g_max(@sources)

     my $val = g_max @sources;

   g_max evaluates all the values it's given and returns the highest one.
   This has the potential to never return if given a source of infinite
   values. If @sources is empty, it will return undef.

g_min(@sources)

     my $val = g_min @sources;

   g_min evaluates all the values it's given and returns the lowest one.
   This has the potential to never return if given a source of infinite
   values. If @sources is empty, it will return undef.

g_prod(@sources)

     my $val = g_prod @sources;

   g_prod evaluates all the values it's given and returns the product of
   all of them. This has the potential to never return if given a source
   of infinite values. Unless one of them is 0. If so, it will
   short-circuit and return 0. If @sources is empty, it will return 1.

g_sum(@sources)

     my $val = g_sum @sources;

   g_sum evaluates all the values it's given and returns the sum of all of
   them. This has the potential to never return if given a source of
   infinite values. If @sources is empty, it will return 0.

@sources

   The @sources array that most (all?) of these functions take can be any
   combination of regular scalar values, Lazy::Iterator objects,
   Scalar::Defer variables (see "NOTES"), or subroutine references. Each
   of these will be iterated through from start to finish, and if one of
   them returns undef, the next one will be used instead, until the last
   one returns undef.

   For instance, in the following scenario:

     my @values = qw/ a b c /;
     my $source = sub { shift @values };
     my $lazy = l_concat $source, 1;

     my @results = ($lazy->get(), $lazy->get(), $lazy->get(), $lazy->get());

   What happens when you run $lazy->get() the first time is that the
   subroutine in $source will be executed, and so @values will change to
   only contain qw/ b c /, and a will be returned. The next time @values
   will be changed to only contain qw/ c /, and b will be returned. The
   third $lazy->get() will change @values to qw// (an empty array), and
   return the c.

   So far so good.

   What happens with the next $lazy->get() is that the subroutine in
   $source will be executed one last time, and it will run shift @values,
   but @values is empty, so it will return undef, which will signal that
   $source is exhausted, and so it will be discarded. The next value will
   be taken from the next element in @sources, which is the single scalar
   1.

   This means that at the end, @results will contain qw/ a b c 1 /, and
   any subsequent call to $lazy->get() will return undef.

NOTES

   If Scalar::Defer is installed, it will assume that any variable of type
   0 is a Scalar::Defer variable and will treat it as a source of values.

   Not to be confused with Lazy::Utils.

SEE ALSO

   Lazy::Iterator

   Scalar::Defer

AUTHOR

   Andreas Guldstrand <[email protected]>

COPYRIGHT AND LICENSE

   This software is Copyright (c) 2017 by Andreas Guldstrand.

   This is free software, licensed under:

     The MIT (X11) License

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