P O S T M O D E R N

List Comprehensions in Ruby

— comprehension, list, ruby

Recently I have been interested in Haskell and been skimming the whimsical (yet informative) Learn a Haskell, for Great Good!. One feature I really like from Haskell is their implementation of list comprehensions:

Prelude> [(x,y) | x <- [1..10], y <- [2..8]]
[(1,2),(1,3),(1,4),(1,5),(1,6),(1,7),(1,8),
(2,2),(2,3),(2,4),(2,5),(2,6),(2,7),(2,8),
(3,2),(3,3),(3,4),(3,5),(3,6),(3,7),(3,8),
(4,2),(4,3),(4,4),(4,5),(4,6),(4,7),(4,8),
(5,2),(5,3),(5,4),(5,5),(5,6),(5,7),(5,8),
(6,2),(6,3),(6,4),(6,5),(6,6),(6,7),(6,8),
(7,2),(7,3),(7,4),(7,5),(7,6),(7,7),(7,8),
(8,2),(8,3),(8,4),(8,5),(8,6),(8,7),(8,8),
(9,2),(9,3),(9,4),(9,5),(9,6),(9,7),(9,8),
(10,2),(10,3),(10,4),(10,5),(10,6),(10,7),(10,8)]

This list comprehension is essentially a Set definition using first-order predicate logic, with universal quantifiers on x and y (where the Set contains the tuples composed of every x and every y from the ranges 1..10 and 2..8). Amazingly, the Haskell version looks almost exactly like the Set definitions from my Discrete Structures, Logic, and Computability (2nd edition) book, except without the curly-braces or universal quantifiers (universal quantifier).

One does not have to learn functional programming and Haskell to use list comprehensions, Python has them as well:

>>> [(x,y) for x in range(0,11) for y in range(2,9)]
[(0, 2), (0, 3), (0, 4), (0, 5), (0, 6), (0, 7), (0, 8),
(1, 2), (1, 3), (1, 4), (1, 5), (1, 6), (1, 7), (1, 8),
(2, 2), (2, 3), (2, 4), (2, 5), (2, 6), (2, 7), (2, 8),
(3, 2), (3, 3), (3, 4), (3, 5), (3, 6), (3, 7), (3, 8),
(4, 2), (4, 3), (4, 4), (4, 5), (4, 6), (4, 7), (4, 8),
(5, 2), (5, 3), (5, 4), (5, 5), (5, 6), (5, 7), (5, 8),
(6, 2), (6, 3), (6, 4), (6, 5), (6, 6), (6, 7), (6, 8),
(7, 2), (7, 3), (7, 4), (7, 5), (7, 6), (7, 7), (7, 8),
(8, 2), (8, 3), (8, 4), (8, 5), (8, 6), (8, 7), (8, 8),
(9, 2), (9, 3), (9, 4), (9, 5), (9, 6), (9, 7), (9, 8),
(10, 2), (10, 3), (10, 4), (10, 5), (10, 6), (10, 7), (10, 8)]

Granted, using the for loop syntax as constraints for the list comprehension is not as sexy as the Haskell version, but it gets the job done.

Unfortunately, Ruby does not support list comprehensions, and only has a couple methods for doing Combinatorics (Array#combination and Array#permutation). So, I implemented list comprehensions for the Ruby Array class:

>> require './array_comprehension'
=> true
>> [(1..10), (2..8)].comprehension.to_a
=> [[1, 2], [1, 3], [1, 4], [1, 5], [1, 6], [1, 7], [1, 8],
    [2, 2], [2, 3], [2, 4], [2, 5], [2, 6], [2, 7], [2, 8],
    [3, 2], [3, 3], [3, 4], [3, 5], [3, 6], [3, 7], [3, 8],
    [4, 2], [4, 3], [4, 4], [4, 5], [4, 6], [4, 7], [4, 8],
    [5, 2], [5, 3], [5, 4], [5, 5], [5, 6], [5, 7], [5, 8],
    [6, 2], [6, 3], [6, 4], [6, 5], [6, 6], [6, 7], [6, 8],
    [7, 2], [7, 3], [7, 4], [7, 5], [7, 6], [7, 7], [7, 8],
    [8, 2], [8, 3], [8, 4], [8, 5], [8, 6], [8, 7], [8, 8],
    [9, 2], [9, 3], [9, 4], [9, 5], [9, 6], [9, 7], [9, 8],
    [10, 2], [10, 3], [10, 4], [10, 5], [10, 6], [10, 7], [10, 8]]

I used yield to quickly pass results to the given block, to not stress the Garbage Collector with building a huge Arrays. The method also returns an Enumerator object if no block is given, to fake Haskells lazy evaluation.

Note, that the Array may contain:

  • Enumerable objects:

      >> [('a'..'f'),(0..10).step(2)].comprehension.to_a
  => [["A", 0], ["A", 2], ["A", 4], ["A", 6], ["A", 8], ["A", 10],
      ["B", 0], ["B", 2], ["B", 4], ["B", 6], ["B", 8], ["B", 10],
      ["C", 0], ["C", 2], ["C", 4], ["C", 6], ["C", 8], ["C", 10],
      ["D", 0], ["D", 2], ["D", 4], ["D", 6], ["D", 8], ["D", 10],
      ["E", 0], ["E", 2], ["E", 4], ["E", 6], ["E", 8], ["E", 10],
      ["F", 0], ["F", 2], ["F", 4], ["F", 6], ["F", 8], ["F", 10]]

  • Non-enumerable objects:

      >> [5,(6..9),10].comprehension.to_a
  => [[5, 6, 10], [5, 7, 10], [5, 8, 10], [5, 9, 10]]

  • Even other list comprehensions:

      >> syllable = (['a'..'z'] * 2).comprehension
  >> word = [syllable] * 4
  >> number = (1..100)
  >> [*word, number].comprehension { |*s| puts s.join }

This code does require Ruby 1.9 since Enumerator#peek was not back-ported to Ruby 1.8.7.

$ rvm install 1.9.2