Oath provides a system for utilizing design by contract in elixir.

Pre and Post conditions

You can decorate any of your functions with preconditions and postconditions.

@decorator pre("inputs are ints", & is_integer(&1) && is_integer(&2))
@decorate post("the result must be greater than a or b", fn a, b, result ->
  result >= a && result >= b
end)
def add(a, b) do
  a + b
end

If your callers provide incorrect data or your function returns incorrect repsonses, an exception will be thrown.