# `AshSumType`

A Spark DSL for defining Ash types that behave like algebraic sum types.

Each module that `use`s `AshSumType` becomes an Ash type. Values are
represented in memory as tuples whose first element is the variant tag (must be an atom):

    defmodule DrinkSize do
      use AshSumType, variants: [:small, :large]
    end

    defmodule Result do
      use AshSumType

      variant :ok do
        field :value, :integer
      end

      variant :error do
        field :message, :string, allow_nil?: true
      end
    end

    Result.new(:ok, value: 1)
    # {:ok, {:ok, 1}}
    
    Result.new({:ok, 1})
    # {:ok, {:ok, 1}}

This library stores sum-type values in the database as maps using the reserved
`__variant__` key, while exposing atoms/tuples in memory.

`fields` values are not nullable by default.

### Options

* `:variants` (list of `t:atom/0`) - Shorthand nullary variants to define when using `AshSumType`. The default value is `[]`.

* `:extensions` (list of module that adopts `Spark.Dsl.Extension`) - A list of DSL extensions to add to the `Spark.Dsl`

* `:otp_app` (`t:atom/0`) - The otp_app to use for any application configurable options

* `:fragments` (list of `t:module/0`) - Fragments to include in the `Spark.Dsl`. See the fragments guide for more.

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*Consult [api-reference.md](api-reference.md) for complete listing*