composer.rb

require 'wrong/adapters/rspec'
require 'facets/proc/compose'

# (apply comp fns) in clojure

module Composer
  def compose(lambdas)
    lambdas.reduce(:*) || lambda {|x| x}
  end

  module_function :compose
end

describe Composer do
  describe ".compose" do
    it "returns identity with no fns" do
      empty = Composer.compose([])
      assert { empty.call(3) == 3 }
    end

    it "composes with 1 fns" do
      double = ->(x) { x + x }
      composed_double = Composer.compose([double])
      assert { composed_double.call(3) == 6 }
    end

    it "composes with 2 fns" do
      double = ->(x) { x + x }
      square = ->(x) { x * x }
      square_double = Composer.compose([square, double])
      assert { square_double.call(3) == 36 }
    end

    it "composes with 3 fns" do
      double = ->(x) { x * 2 }
      triple = ->(x) { x * 3 }
      square = ->(x) { x * x }
      square_double_triple = Composer.compose([square, double, triple])
      assert { square_double_triple.call(3) == 18*18 }
    end

    it "composes with no-arg fn at end" do
      double = ->(x) { x * 2 }
      triple = ->(x) { x * 3 }
      square = ->(x) { x * x }
      three = -> (){3}
      square_double_triple = Composer.compose([square, double, triple, three])
      assert { square_double_triple.call == 18*18 }
    end
  end
end

@mattwynne - I think I prefer the original implementation plus @tomstuart's suggestions over reducing with :* since that's not part of Ruby core, and * is not really intention revealing. WDYT?