Proof of concept for Pre/Post condition assertions based on RSpec/Given

assertions.rb

require 'given/natural_assertion'
require 'given/line_extractor'

class Object
  def self._Gvn_location_of(block)
    eval "[__FILE__, __LINE__]", block.binding
  end
end

module Given
  AssertionError = Class.new(StandardError)
  PreconditionError = Class.new(AssertionError)
  PostconditionError = Class.new(AssertionError)

  module Assertions
    def Assert(&block)
      unless instance_eval(&block)
        na = Given::NaturalAssertion.new("Assert", block, self, Given::LineExtractor.new)
        fail AssertionError, na.message
      end
    end

    def Precondition(&block)
      unless instance_eval(&block)
        na = Given::NaturalAssertion.new("Precondition", block, self, Given::LineExtractor.new)
        fail PreconditionError, na.message
      end
    end

    def Postcondition(&block)
      unless instance_eval(&block)
        na = Given::NaturalAssertion.new("Postcondition", block, self, Given::LineExtractor.new)
        fail PostconditionError, na.message
      end
    end
  end
end

output

$ ruby -Ilib try_precondition.rb 
/Users/jim/pgm/ruby/given_assertions/lib/given/assertions.rb:26:in `Precondition': Precondition expression failed at try_precondition.rb:10 (Given::PreconditionError)
Failing expression: Precondition { n >= 0 }
                 expected: -1
to be greater or equal to: 0
  false   <- n >= 0
  -1      <- n

  from try_precondition.rb:10:in `sqrt'
	from try_precondition.rb:19:in `<main>'

try_precondition.rb

require 'given/assertions'
require 'given/fuzzy_number'

include Given::Assertions
include Given::Fuzzy

def sqrt(n)
  Precondition { n >= 0 }
  result = Math.sqrt(n)
  Postcondition { result ** 2 == about(n) }
  result
end

sqrt(1)
sqrt(2)
sqrt(0)
sqrt(-1)