NAME
Lazy::Util - Perl utilities for lazy evaluation
VERSION
version 0.001
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 evalutation.
FUNCTIONS
This module has two sets of functions, the l_* functions and the g_* functions. The l_* functions are designed to return a Lazy::Util object which you can get values from, the g_* functions are designed to get a value out of a Lazy::Util 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 - l_concat(@sources)
my $lazy = l_concat @sources;l_concat returns a Lazy::Util object which will simply return each subsequent value from the list of sources it's given.
l_first - l_first($n, @sources)
my $lazy = l_first $n, @sources;l_first will return a Lazy::Util object which will only get the first $n values from the subsequent arguments. This can be used the 'break' an otherwise infinite list to only return a certain number of results.
l_grep - l_grep($code, @sources)
my $lazy = l_grep { ... } @sources;l_grep will return a Lazy::Util object which will filter out any value which doesn't return true from the $code block in the first argument.
l_map - l_map($code, @sources)
my $lazy = l_map { ... } @sources;l_map will return a Lazy::Util object which will transform any value using the $code block in the first argument.
l_nuniq - l_nuniq(@sources)
my $lazy = l_nuniq @sources;l_nuniq will return a Lazy::Util object which will only return numerically unique values from the sources. This has the potential to consume all of your memory if the @sources are infinite.
l_uniq - l_uniq(@sources)
my $lazy = l_uniq @sources;l_uniq will return a Lazy::Util object which will only return unique values from the sources. This has the potential to consume all of your memory if the @sources are infinite.
l_until l_until($code, @sources)
my $lazy = l_until { ... } @sources;l_until will return a Lazy::Util 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 - 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 - g_first(@sources)
my $val = g_first @sources;g_first returns the first value from the list of arguments, lazily evaluating them. Equivalent to l_concat(...)->get();. If @sources is empty, it will return undef.
g_join - g_join($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 - 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 - 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 - 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 - 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 - 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::Util 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 $lazy->get() will return undef.
Lazy::Util objects
Lazy::Util objects encapsulate a set of lazy evaluation functions, meaning you can combine them using the l* functions listed above.
new - Lazy::Util->new($source)
my $lazy = Lazy::Util->new(sub { $i++ });Lazy::Util->new takes a code reference which will be used as the source for all the values and returns a Lazy::Util object encapsulating that source.
exhausted - $lazy->exhausted()
my $exhausted = $lazy->exhausted();$lazy->exhausted() checks if there's any more values left in the source, and caches any such value for the next $lazy->get() call. It returns 0 if there are values left, and 1 if the source is exhausted.
An exhausted Lazy::Util object will always return undef from a $lazy->get() call.
get - $lazy->get()
my $next = $lazy->get();$lazy->get returns the next value from the source it encapsulates. When there are no more values it returns undef.
get_all - $lazy->get_all()
my @crazy = $lazy->get_all();$lazy->get_all returns all the values from the source, if it can. This has the potential to never return as well as running out of memory if given a source of infinite values.
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
AUTHOR
Andreas Guldstrand <andreas.guldstrand@gmail.com>
COPYRIGHT AND LICENSE
This software is Copyright (c) 2017 by Andreas Guldstrand.
This is free software, licensed under:
The MIT (X11) License