ExUnion is meant to be a lightweight, elixir-y implementation of tagged unions (also called variant, discriminated union, sum type, etc.).

While conventionally Elixir tends to promote using tuples to model tagged unions - the {:ok, ...} | {:error, ...} pattern being a good example of that - this approach arguably lacks expressiveness, especially when modeling non-trivial unions. An alternative is to employ structs to model the individual cases of a tagged union, which works nicely but has the disadvantage of requiring significant boilerplate code.

ExUnion attempts to bridge this gap by generating the necessary boilerplate (and a bit more) through a concise albeit opinionated DSL. Below the following links you can see an example for an idea on how you might use ExUnion.

• GitHub
• Hex.pm
• Docs

defmodule Maybe do
  import ExUnion

  defunion some(value) | none
end

iex> Maybe.none()
%Maybe.None{}

iex> Maybe.some("string!")
%Maybe.Some{value: "string!"}

# Requiring is necessary since `is_maybe` is a guard (defguard)
iex> require Maybe
iex> Maybe.is_maybe("What's the meaning of life, the universe, and everything?")
false
iex> Maybe.is_maybe(42)
false
iex> Maybe.is_maybe(Maybe.none())
true

For more examples take a look at the project’s README.