improvement: add :struct (Ash.Type.struct) type

lib/ash/type/struct.ex

@@ -0,0 +1,48 @@
+defmodule Ash.Type.Struct do
+  @moduledoc """
+  Represents a struct.
+
+  This cannot be loaded from a database, it can only be used to cast input.
+
+  Use the `instance_of` constraint to specify that it must be an instance of a specific struct.
+  """
+  use Ash.Type
+
+  @constraints [
+    instance_of: [
+      type: :atom,
+      doc: "The module the struct should be an instance of",
+      default: false
+    ]
+  ]
+
+  @impl true
+  def constraints, do: @constraints
+
+  @impl true
+  def storage_type, do: :map
+
+  @impl true
+  def cast_input(nil, _), do: {:ok, nil}
+
+  def cast_input(%struct{} = value, constraints) do
+    case constraints[:instance_of] do
+      nil ->
+        {:ok, value}
+
+      ^struct ->
+        {:ok, value}
+
+      _ ->
+        :error
+    end
+  end
+
+  def cast_input(_, _), do: :error
+
+  @impl true
+  def cast_stored(_, _), do: :error
+
+  @impl true
+  def dump_to_native(_, _), do: :error
+end

lib/ash/type/type.ex

@@ -25,6 +25,7 @@ defmodule Ash.Type do
     duration_name: Ash.Type.DurationName,
     function: Ash.Type.Function,
     boolean: Ash.Type.Boolean,
+    struct: Ash.Type.Struct,
     uuid: Ash.Type.UUID,
     binary: Ash.Type.Binary,
     date: Ash.Type.Date,

mix.exs

@@ -220,7 +220,7 @@ defmodule Ash.MixProject do
   # Run "mix help deps" to learn about dependencies.
   defp deps do
     [
-      {:spark, "~> 0.2 and >= 0.2.18"},
+      {:spark, "~> 0.2.18"},
       {:ecto, "~> 3.7"},
       {:ets, "~> 0.8.0"},
       {:decimal, "~> 2.0"},