I've been playing around some more with Ruby as a result of my work on TweetSum and in an effort to validate the behaviours of some of our code I'm coming to the conclusion that some kind of validation/precondition/postcondition framework might be useful. I really want to be able to do things like the following:

class MyClass
  def my_method()
    pre {
      # Preconditions go here.
    }
    post {
      # Postconditions go here.
    }

    # The method body
  end
end

This is roughly the approach taken by Ruby Design by Contract and I may yet adopt this library. However, along the way I came up with my own solution which I'm going to describe quickly here. I'm sure there are problems with it (apparent from the obvious lack of syntactic sugar) that I haven't anticipated yet, but I feel like posting this anyway.

You just need to require validation.rb and define your functions or methods as follows:

class MyClass
  define_validated_method(:my_method, lambda {|a, b, c, d|
      # Preconditions
      validate_is_string(a, "a must be a string of greater than 5 characters") {a.length > 5}
      validate_is_string(b, "b must be a string")
      validate_is_string(c, "c must be a string")
      validate_is_string(d, "d must be a string")
    }, lambda {|return_value|
      # Postconditions
      validate_is_fixnum(return_value, "return_value must be a Fixnum greater than 0") {return_value > 0}
    }) do |a, b, c, d|

    return a.length + b.length + c.length + d.length
  end
end

Normally you'd use def my_method(a, b, c, d) but this wrapper function takes care of injecting the precondition and postcondition code into the method using the define_method function internally.

I hope this makes some kind of sense to you!