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Ruby Cookbook by Leonard Richardson, Lucas Carlson

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10.16. Enforcing Software Contracts

Credit: Maurice Codik

Problem

You want your methods to to validate their arguments, using techniques like duck typing and range validation, without filling your code with tons of conditions to test arguments.

Solution

Here's a Contracts module that you can mix in to your classes. Your methods can then define and enforce contracts.

 module Contracts def valid_contract(input) if @user_defined and @user_defined[input] @user_defined[input] else case input when :number lambda { |x| x.is_a? Numeric } when :string lambda { |x| x.respond_to? :to_str } when :anything lambda { |x| true } else lambda { |x| false } end end end class ContractViolation < StandardError end def define_data(inputs={}.freeze) @user_defined ||= {} inputs.each do |name, contract| @user_defined[name] = contract if contract.respond_to? :call end end def contract(method, *inputs) @contracts ||= {} @contracts[method] = inputs method_added(method) end def setup_contract(method, inputs) @contracts[method] = nil method_renamed = "__#{method}".intern conditions = "" inputs.flatten.each_with_index do |input, i| conditions << %{ if not self.class.valid_contract(#{input.inspect}).call(args[#{i}]) raise ContractViolation, "argument #{i+1} of method '#{method}' must" + "satisfy the '#{input}' contract", caller end } end class_eval %{ alias_method #{method_renamed.inspect}, #{method.inspect} def #{method}(*args) #{conditions} return #{method_renamed}(*args) end } end def method_added(method) inputs ...

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