For is a Ruby library providing list comprehensions similar to those found in Clojure and many other languages. It is my celebration of the the fact that the August 2011 LinkedIn Hackday coincided with _whyday 2011.

For takes one or more sequence specifications and a block and returns an enumerable comprehension object. When the comprehension is enumerated, the block is executed once for each combination that can be formed by taking an element from each sequence:

For({ :x => (1..3) }, { :y => [:a, :b] }) do
  [x, y]
end.to_a
# => [[1, :a], [1, :b], [2, :a], [2, :b], [3, :a], [3, :b]]

Filter conditions may optionally be specified for the sequences:

For(
  { :x => (1..3), :where => Proc.new { x.odd? } },
  { :y => [:a, :b] }
) do
  [x, y]
end.to_a
# => [[1, :a], [1, :b], [3, :a], [3, :b]]

Conditions may also optionally be specified to the comprehension as a whole:

For(
  { :x => (1..4) },
  { :y => (1..4) },
  :where => Proc.new { x < y }
) do
  [x, y]
end.to_a
# => [[1, 2], [1, 3], [1, 4], [2, 3], [2, 4], [3, 4]]

For has been tested with

• Ruby 1.8.7
• Ruby 1.9.2
• JRuby 1.6.3

For is released under the MIT license. See License.txt for the details.