Daniel Daniel - 2 months ago 6
Ruby Question

Ruby metaprogramming, how does RSpec's 'should' work?

I was reading up on RSpec and I was trying to figure out how RSpec's "should" was implemented.

Could someone give a hand on how the meta nature of this function works?

The code is located here:

http://github.com/dchelimsky/rspec/blob/master/lib/spec/expectations/extensions/kernel.rb

TIA,

-daniel

Clarification:

target.should == 5


How did target's value get passed along to "should", which in turn was "=="'d against 5?

Answer

Take a look at class OperatorMatcher.

It all boils down to Ruby allowing you to leave out periods and parenthesis. What you are really writing is:

target.should.send(:==, 5)

That is, send the message should to the object target, then send the message == to whatever should returns.

The method should is monkey patched into Kernel, so it can be received by any object. The Matcher returned by should holds the actual which in this case is target.

The Matcher implements the method == which does the comparison with the expected which, in this case, is the number 5. A cut down example that you can try yourself:

module Kernel
  def should
    Matcher.new(self)
  end
end

class Matcher
  def initialize(actual)
    @actual = actual
  end

  def == expected
    if @actual == expected
      puts "Hurrah!"
    else
      puts "Booo!"
    end
  end
end

target = 4
target.should == 5
=> Booo!

target = 5
target.should == 5
=> Hurrah!