Simplest Contract System in Ruby

contract.rb

# A Contract is one of:
# - Proc (Any -> Boolean)
# - Hash { Contract => Contract }

def assert_predicate(predicate, value)
  if predicate.(value)
    value
  else
    fail "contract error"
  end
end

def assert_function(signature, function)
  -> (x) do
    assert(signature.values[0], function.(assert(signature.keys[0], x)))
  end
end

def assert(contract, value)
  case contract
  when Hash
    assert_function(contract, assert(-> (v) { value.is_a?(Proc) }, value))
  when Proc
    assert_predicate(contract, value)
  end
end

is_even = -> (n) do
  n % 2 == 0
end

p assert(is_even, 2)
# p assert(is_even, 3)  # Contract error.

func = assert({is_even => is_even}, -> (x) { x + 2 })
p func.(2)
# p func.(3)  # Contract error.

high_func = assert({{is_even => is_even} => is_even}, -> (x) { x.(4)})
p high_func.(-> (x) { x + 6 })
# p high_func.(-> (x) { x + 7 })  # Contract error.