Convert list of values to type specification in Elixir

list_to_typespec.ex

@doc """
Converts a list of values into an AST for defining a type.

Source: https://elixirforum.com/t/dynamically-generate-typespecs-from-module-attribute-list/7078/16

## Usage

    @categories [:one, :two, :three]

    @type category :: unquote(list_to_typespec(@categories))
"""
def list_to_typespec(list) when is_list(list) do
  Enum.reduce(list, &{:|, [], [&1, &2]})
end