Comprise

Comprise brings basic List Comprehension functionality to Ruby.

“A list comprehension is a syntactic construct available in some programming languages for creating a list based on existing lists. It follows the form of the mathematical set-builder notation (set comprehension) as distinct from the use of map and filter functions.” - Wikipedia

Installation

Add this line to your application's Gemfile:

gem 'comprise'

And then execute:

$ bundle

Or install it yourself as:

$ gem install comprise

Usage

Building a comprehension goes something like:

comp = Comprise::ListComprehension.new(y: ->{ (1..5).map { |i| i * 2 } }, x: 1..2)
=> #<Comprise::ListComprehension:70240864145320 generators:[[:y, Proc], [:x, Range]]>

comp.to_a
# => [[2, 1], [2, 2], [4, 1], [4, 2], [6, 1], [6, 2], [8, 1], [8, 2], [10, 1], [10, 2]]

comp.map { x * y }.to_a # to_a to trigger lazy enumeration in Ruby 2.0
# => [2, 4, 4, 8, 6, 12, 8, 16, 10, 20]

Comprise also adds the Kernel method listcomp, so all that could be done more simply with the one-liner:

listcomp(y: ->{ (1..5).map { |i| i * 2 } }, x: 1..2) { x * y }.to_a
=> [2, 4, 4, 8, 6, 12, 8, 16, 10, 20]

Comprehensions can also be self referential; provided that your using a lambda or a Proc & your referencing lists that have already been evaluated.

listcomp(x: 1..3, y: ->{ 1..x }, z: ->{ [x + y] }).to_a
# => [[1, 1, 2], [2, 1, 3], [2, 2, 4], [3, 1, 4], [3, 2, 5], [3, 3, 6]]

Another exmaple; for good measure. Factoring primes:

is_prime     = ->(num) { !listcomp(divisor: 2..(num**0.5).to_i, is_factor: ->{ [num % divisor == 0] }) { is_factor }.any? }
search_space = 2..100
listcomp(x: search_space, test: ->{ is_prime[x] ? [x] : [] }) { x }.take(10).to_a # If this is slow reduce the search space
# => [2, 3, 5, 7, 11, 13, 17, 19, 23, 29]

Important note: In Ruby 2.0.x Comprise makes use of Lazy enumerators.

Contributing

1. Fork it
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Added some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create new Pull Request