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HOI::Comprehensions - Higher-Order Imperative "Re"features in Perl: List Comprehensions


  use HOI::Comprehensions;

  my $list = HOI::Comprehensions::comp( sub { $x + $y + $z + $w }, x => [ 1, 2, 3 ], y => [ 4, 5, 6 ], w => HOI::Comprehensions::comp( sub { $u }, u => [ 1, 2, 3 ] )->(), z => sub { (2, 1) } )->( sub { $x > 1 } );

  my ($elt, $done);
  do {
      ($elt, $done) = @{<$list>};
      print "$elt ";
  } while (not $done);
  print "\n";


HOI::Comprehensions offers lazy-evaluated list comprehensions with limited support to generators of an infinite list. It works as if evaluating multi-level loops lazily.

Currently, the generators are handled in sequence of the argument list offered by the user. As a result of such implementation, a list { (x, y) | x belongs to { 0, 1 }, y belongs to natural number set } may be trapped in the inner infinite loop. To avoid such situation, make sure finite generators are subsequent to all the infinite ones.



For creating a list comprehension object. The formula for computing the elements of the list is given as a subroutine, following by the generators, in form of name => arrayref, name => subroutine or name => comprehension. Comp returns a function which takes all guards in form of subroutines. Guard parameters can be left empty if there is no guard.

The variable names for naming the ganerators could be used directly in the computation sub and all guard subs without strict vars enabled. They have local scope as if they were declared with keyword 'local' in Perl.

A hashref which holds generator variables as its keys and value of those variables as its values is passed to the formula subroutine. However, it is recommended to use such variables directly instead of dereference the hashref.

Generators can be arrayrefs, subroutines or list comprehensions. A subroutine generator should return a pair ( elt, done ), where elt is the next element and done is a flag telling whether the iteration is over, or return a single element.

It is possible that some generator A is dependent on another generator B. In that case, B must be subsequent to A. See test cases for details.

The subroutine comp is in EXPORT_OK, but it is not exported by default.



Get the list member of a list comprehension object. It returns a arrayref which holds the actual list evaluated so far.


Returns a boolean which tells if the evaluation of the list is over.


Get a member of a list comprehension object by name. A list comprehension object is actually a blessed hashref.


Eval the comprehension eagerly. Beware - it will be trapped forever in an infinite comprehension. The method returns the evaluated list reference.



List evaluation iterator. Returns the "next" element generated in the sequence in the situation of eager evaluation, and a flag telling whether the evaluation is done, together in a arrayref.


List indexing operator. Takes an integer as the index and a list comprehension object. The operator returns an arrayref [ $elt, $done ], where $elt is the element at the given index and $done is a flag telling whether the evaluation is done.


withering <withering@cpan.org>


Copyright (C) 2014 by withering

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself, either Perl version 5.20.0 or, at your option, any later version of Perl 5 you may have available.