This project was originally called "Vow", but it looks like someone snagged that on hex. So, I had to change it to Oath.
Oath provides a system for design by contract in elixir. It uses the excellent,
:decorator
library by @arjan.
You will first need to enable contracts in whichever envs you want to use them. Typically this will be development and test only.
# config/dev.exs
config :oath,
enable_contracts: true
After enabling contracts, you can add pre
and post
conditions to your functions.
Preconditions are always run before your function body. Postconditions are run after
your function body executes and will be provided the result from your function
call.
We'll use an example function with a broken implementation as a demonstration:
defmodule Mod do
use Oath
@decorate 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
if a == 7 do
(a * -1) + b
else
a + b
end
end
end
Calling this function with contracts enabled will cause an exception to be raised.
Mod.add(7, 2)
** (Oath.ContractError) Mod.add/2 precondition: 'the result must be greater then a or b' failed with input:
Arguments:
a
=> 7
b
=> 2
result
=> -5
Contract predicates don't have to be pure. Its often useful to use contracts as a way of validating the functions environment and any of the functions side-effects.
@doc """
Stores a name in the database.
"""
@decorate pre("name must not be in db", fn name -> !in_database?(name) end)
@decorate post("Name must be normalized in the db", fn name, _result ->
fetch_from_db(name) == String.capitalize(name)
end)
def store_in_db(name) do
#...
end
Classically, you should only enable contracts in dev and test modes. You want to avoid enabling contracts in production as this will lower your production performance due to additional checking. It also means that you won't be able to write side-effecting contracts, which you'll want to be able to do in testing modes.
By default, contracts are disabled and will be compiled away so you won't need to do anything for production builds.
def deps do
[
{:oath, "~> 0.1.0"},
]
end