proc_compose_spec.rb

require 'spec_helper'

describe 'Proc#compose' do
  let(:double) do
    proc {|a| a * 2 }
  end
  let(:triple) do
    proc {|a| a * 3 }
  end
  let(:negate) do
    proc {|a| -a }
  end

  it 'lives on Proc' do
    expect(double.respond_to?(:compose)).to eq(true)
    expect(double.respond_to?(:*)).to eq(true)
  end
  it 'creates new Proc from 2 Procs' do
    expect(double * triple).to be_a Proc
  end
  it 'compose Proc can be evaluated' do
    expect((double * triple).(2)).to eq(triple.(double.(2)))
  end
  it 'compose on & on' do
    expect((double * triple * negate).(2)).to eq(-12)
  end

  context 'lambdas' do
    it 'removes lambda information' do
      a, b = [lambda {|x| x + 2 }, lambda {|y| y + 3 }]
      # works by coercing to Proc
      expect((a * b).(2)).to eq(7)

      # if lambdas were preserved, this would be:
      #expect{ (a * b).() }.to raise_error ArgumentError
      expect{ (a * b).() }.to raise_error NoMethodError
    end
  end
end