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Only 70 lines. Add a optional type system to ruby using runtime checks and contracts . make it crash early, know exactly what a function (or method) returns and specify the 'type' of the parameters using good old ducktyping
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 Dependencies
 Project Readme

a monkey typing

Gem Version

Monkey Type!

a microgem to add a micro type system to ruby

Why?

Adding just a bit of typing to ruby is just great. It makes you, as a programmer have more control of what gets passed to your classes, methods or functions and you know what your functions (and methods) returns.
And crash earlier. ( thx "the pragmatic programmer" )
Check out what Matz says

Why name it MonkeyType?

Well. The gem is just a elegant monkeyPatch of Object and Class classes. And it does types.
So... MonkeyTypes!

Installation

gem install monkey_type

Usage

well. I tried to simplify the usage as hard as i can.
There is just two new things to learn. the method #is and the class macro contract.
the method is used like this:
(remember to you need both 'require' and 'using' lines!)

require 'monkey_type'
using MonkeyType

def accepts_only_nums(num)
  num.is Numeric
  #things here
end

the method #is will raise an exception in the form of:

error: String is not an Numeric (TypeError)

if accepts_only_nums is called and num isn't numeric or something like numeric. Remember, we are duck typing here. Simple isn't it?

require 'monkey_type'
using MonkeyType

class Returner
  def return1
    1
  end

  def return2
    []
  end

  contract :return1, Numeric
  contract :return2, Array
end  

Remember to call contract after defining the methods.
calling this contract will invoke metaprogramming deities and will ensure that the method will return something like a Numeric in case of return1 and a Array in case of return2. Also, you can call the functions on a typeless manner, just add "_typeless" to the end of the name of the method (like return1_typeless).

I have also included some types for your commodity. if you want to check if something is a Numeric or nil use option(Numeric) (you can change Numeric for Hash or even for you own class names) if you want to check if something is true or false use boolean

def i_accept_nil_or_array(t)
  t.is option(Array)
end

class Booled
  def i_return_true_or_false
    #boolean logic
  end

  contract :i_return_true_or_false, boolean
end

that is all.

Running in irb

The interactive ruby interpreter irb is unable to handle using modules in the method required by monkey_type. This is a known limitation. See Stack Overflow for more details.

Contributing

If you want to help focus on the method is_duck? and how to make easier to add new "types" as Boolean or Option. Or, make better error messages.
Just send Pull Requests.