design_by_contract.rb

$LOAD_PATH.unshift(File.join(File.dirname(__FILE__),'..','lib'))
require'aspector'

# Design by contract example class
classExampleClass
definitialize
@transactions=[]
@total=0
end

defbuy(price)
@transactions << price
@total += price
end

defsell(price)
@transactions << price# Wrong
@total -= price
end
end

# Object extensions
classObject
defassert(bool,message='Assertion failure')
returnifbool

 $stderr.putsmessage
 $stderr.putscaller
end
end

# Aspect that we will apply
classContractExample < Aspector::Base
beforedo |price, &_block|
assertprice > 0,"Price is #{price}, should be greater than 0"
end

afterresult_arg: falsedo |*_, &_block|
sum=@transactions.reduce(&:+)
assert@total == sum,"Total(#{@total}) and sum of transactions(#{sum}) do not match"
end
end

ContractExample.applyExampleClass,methods: %w(buysell)

instance=ExampleClass.new
instance.buy(10)
instance.sell(10)
instance.sell(-10)