test.rb

require "bundler/inline"

gemfile do
  source "https://rubygems.org"
  gem "contracts"
end

include Contracts

Contract 0 => 0
def fib(_)
  0
end

Contract 1 => 1
def fib(_)
  1
end

Contract Num => Num
def fib(n)
  fib(n-1) + fib(n-2)
end

puts fib(0)
puts fib(1)
puts fib(15)