gistfile1.rb

require 'rspec'
# TODO: class vars?
# TODO: better just alias_method_chain method_missing for the duration of running the method
module Tdef
  def self.check_args(args, rules)
    rules.each_pair do |name, klass|
      unless args[name.to_s].kind_of?(klass)
        raise "Wrong type: #{name} should be #{klass.to_s}, but is #{args[name].class.to_s}"
      end
    end
  end
  
  def tdef name, args, &block
    self.send(:define_method, name.to_sym) do |*arg_values|
      raise "Wrong number of arguments" if args.count != arg_values.count
      args_map = Hash.new
      (0...arg_values.count).each do |i|
        args_map[args.keys[i].to_s] = arg_values[i]
      end
      Tdef::check_args(args_map, args)
      
      self.class.send(:alias_method, :method_missing_tmp, :method_missing)
      self.class.send(:define_method, :method_missing) do |meth, *args, &block|
        if args_map.keys.include?(meth.to_s)
          args_map[meth.to_s]
        else
          if self.respond_to?(:method_missing_tmp)
            method_missing_tmp meth, *args, &block
          else
            super(meth, *args, &block)
          end
        end
      end
      result = self.instance_eval &block
      self.class.send(:remove_method, :method_missing)
      self.class.send(:alias_method, :method_missing, :method_missing_tmp)
      # No harm in leaving the method_missing_tmp there
      # self.class.send(:remove_method, :method_missing_tmp)
      result
    end
  end
end

# Test class

class TestClass
  extend Tdef

  tdef :hello, :a1 => Integer, :a2 => String do
    a2 = "Goodbye"
    @lala = 999
    [a1, a2, missing_arg]
  end
  
  def method_missing meth, *args, &block
    if meth.to_s == "missing_arg"
      "lolario"
    else
      super
    end
  end
end

# Tests

describe TestClass do
  
  it "works" do
    k = TestClass.new
    expect { k.hello }.should_not raise_error(NoMethodError)
    r1, r2, r3 = k.hello(10, "Hello")
    r1.should eq(10)
    r2.should eq("Goodbye")
    r3.should eq("lolario")
    
    k.instance_variable_get("@lala").should eq(999)
    expect { k.a1 }.should raise_error(NoMethodError)
    expect { k.a2 }.should raise_error(NoMethodError)
    defined?(a1).should be_nil
    defined?(a2).should be_nil
  end
  
  it "detects wrong arguments" do
    k = TestClass.new
    expect { k.hello(10, 20) }.should raise_error
    expect { k.hello("Hello", "World") }.should raise_error
  end
end