A quick and crazy go at implementing contracts using Ruby 2.1 and naive evals.

trifle_contracts_spec.rb

require 'rspec'
require 'pry'

module Trifle
  class BrokenContract < Exception ; end

  def contract(*contracts, method_name)
    old_method_name = :"__contractually_obligated_#{method_name}"
    alias_method old_method_name, method_name

    named_parameters = instance_method(method_name).parameters.collect { |dep, name| name }
    parameter_call = named_parameters.join(', ')

    guards = Hash[named_parameters.zip contracts].collect { |argument_name, klass|
      "raise BrokenContract unless #{argument_name}.is_a?(#{klass})"
    }
    class_eval <<-RUBBY, __FILE__, __LINE__ + 1
      def #{method_name}(#{parameter_call})
        #{guards.join("\n")}
    #{old_method_name}(#{parameter_call})
      end
    RUBBY
  end
end


class Tower
  extend Trifle

  contract Integer, Integer,
  def add(a, b)
    a + b
  end

  contract String, String,
  def concat(two, things)
    "#{two} #{things}"
  end
end


describe 'simple class checking' do
  subject { Tower.new }

  context 'Integers' do
    specify  do
      expect{ subject.add(3,'f') }.to raise_exception Trifle::BrokenContract
    end

    specify do
      expect{ subject.add('a', 2) }.to raise_exception Trifle::BrokenContract
    end

    specify do
      expect(subject.add(3,3)).to eq 6
    end
  end

  context 'Strings' do
    specify do
      expect{ subject.concat('CON', nil) }.to raise_exception Trifle::BrokenContract
    end

    specify do
      expect{ subject.concat(Object.new, 'CAT') }.to raise_exception Trifle::BrokenContract
    end

    specify do
      expect(subject.concat('CON', 'CAT')).to eq 'CON CAT'
    end
  end


end