is_valid

documentation/how_to/use_diffo_type.livemd

@@ -9,7 +9,7 @@ SPDX-License-Identifier: MIT
 ```elixir
 Mix.install(
   [
-    {:diffo, "~> 0.2.0"},
+    {:diffo, "~> 0.2.0"}
   ],
   consolidate_protocols: false
 )
@@ -47,7 +47,7 @@ Built-in implementations:
 | `Diffo.Type.Primitive` | returns the primitive value     |
 | `Diffo.Type.Dynamic`   | delegates to inner `:value`     |
 | `Ash.CiString`         | returns the comparable string   |
-| `Ash.NotLoaded`        | raises — field was not loaded   |
+| `Ash.NotLoaded`        | raises — field was not loaded |
 | `List`                 | unwraps each element            |
 | `Any`                  | returns the value unchanged     |
 
@@ -254,6 +254,22 @@ Diffo.Unwrap.unwrap(result)
 :ok
 ```
 
+### Checking type compatibility
+
+`Dynamic.is_valid?/1` lets you check whether a module is usable as a Dynamic type before constructing a value. It returns `true` only for `Ash.Type.NewType` modules with `storage_type: :map`:
+
+```elixir
+Dynamic.is_valid?(MyApp.Patch)
+```
+
+```elixir
+Dynamic.is_valid?(Ash.Type.Date)
+```
+
+```elixir
+Dynamic.is_valid?(NonExistent.Module)
+```
+
 ### Constraint validation
 
 Dynamic enforces the constraints defined on the inner type during casting. Here `MyApp.Patch` requires both fields to be `>= 0`, so passing a negative value returns an error:

lib/diffo/type/dynamic.ex

@@ -26,6 +26,28 @@ defmodule Diffo.Type.Dynamic do
       iex> Ash.Type.cast_stored(Diffo.Type.Dynamic, nil, [])
       {:ok, nil}
 
+  ## Invalid types
+
+  Scalar Ash types and modules that are not `Ash.Type.NewType` with `storage_type: :map` are
+  rejected at cast time:
+
+      iex> Ash.Type.cast_input(Diffo.Type.Dynamic, %Diffo.Type.Dynamic{type: Ash.Type.Date, value: ~D[2026-01-01]}, [])
+      {:error, "Dynamic type Ash.Type.Date must be an Ash.Type.NewType with storage_type :map"}
+
+      iex> Ash.Type.cast_input(Diffo.Type.Dynamic, %Diffo.Type.Dynamic{type: Diffo.Type.NonExistent, value: nil}, [])
+      {:error, "Dynamic type Diffo.Type.NonExistent must be an Ash.Type.NewType with storage_type :map"}
+
+  ## Checking type compatibility
+
+  Use `is_valid?/1` to check whether a module is usable as a Dynamic type before
+  constructing a `%Diffo.Type.Dynamic{}`:
+
+      iex> Diffo.Type.Dynamic.is_valid?(Ash.Type.Date)
+      false
+
+      iex> Diffo.Type.Dynamic.is_valid?(Diffo.Type.NonExistent)
+      false
+
   ## Constraints
 
       iex> Diffo.Type.Dynamic.dynamic_constraints(nil)
@@ -68,7 +90,7 @@ defmodule Diffo.Type.Dynamic do
 
   def dynamic_constraints(type) when is_atom(type) do
     cond do
-      Ash.Type.NewType.new_type?(type) ->
+      is_valid?(type) ->
         [
           fields: [
             type: @type_field_constraints,
@@ -85,23 +107,31 @@ defmodule Diffo.Type.Dynamic do
   def dynamic_constraints(_), do: []
 
   @impl true
-  def apply_constraints(%__MODULE__{} = value, _constraints), do: {:ok, value}
+  def apply_constraints(%__MODULE__{type: type} = value, _constraints) do
+    if is_valid?(type) do
+      {:ok, value}
+    else
+      {:error, "Dynamic type #{inspect(type)} must be an Ash.Type.NewType with storage_type :map"}
+    end
+  end
+
   def apply_constraints(nil, _constraints), do: {:ok, nil}
   def apply_constraints(value, _constraints), do: {:error, "is invalid: #{inspect(value)}"}
 
   @impl true
   def cast_input(nil, _constraints), do: {:ok, nil}
 
   def cast_input(%__MODULE__{type: type, value: value}, _constraints) do
-    constraints = dynamic_constraints(type)
-    result = Ash.Type.cast_input(type, value, constraints[:fields][:value][:constraints] || [])
-
-    case result do
-      {:ok, cast_value} ->
-        {:ok, %__MODULE__{type: type, value: cast_value}}
-
-      error ->
-        error
+    if is_valid?(type) do
+      constraints = dynamic_constraints(type)
+      result = Ash.Type.cast_input(type, value, constraints[:fields][:value][:constraints] || [])
+
+      case result do
+        {:ok, cast_value} -> {:ok, %__MODULE__{type: type, value: cast_value}}
+        error -> error
+      end
+    else
+      {:error, "Dynamic type #{inspect(type)} must be an Ash.Type.NewType with storage_type :map"}
     end
   end
 
@@ -142,6 +172,21 @@ defmodule Diffo.Type.Dynamic do
     end
   end
 
+  @doc """
+  Returns true if the module is a valid Dynamic type — an `Ash.Type.NewType` with
+  `storage_type: :map`. Returns false for unloaded modules, non-NewTypes, and scalar
+  Ash types such as `Ash.Type.Date`.
+  """
+  def is_valid?(type) when is_atom(type) do
+    Code.ensure_loaded?(type) and
+      Ash.Type.NewType.new_type?(type) and
+      Ash.Type.storage_type(type, []) == :map
+  rescue
+    _ -> false
+  end
+
+  def is_valid?(_), do: false
+
   defimpl Diffo.Unwrap do
     def unwrap(%{value: value}), do: Diffo.Unwrap.unwrap(value)
   end

test/type/dynamic_test.exs

@@ -10,6 +10,51 @@ defmodule Diffo.Type.DynamicTest do
   alias Diffo.Test.Patch
   alias Diffo.Test.CardValue
 
+  describe "dynamic type validation" do
+    test "cast_input rejects non-NewType scalar Ash type" do
+      value = %Dynamic{type: Ash.Type.Date, value: ~D[2026-01-01]}
+      assert {:error, msg} = Ash.Type.cast_input(Dynamic, value, [])
+      assert msg =~ "storage_type :map"
+    end
+
+    test "cast_input rejects unloaded module" do
+      value = %Dynamic{type: Diffo.Type.NonExistent, value: nil}
+      assert {:error, msg} = Ash.Type.cast_input(Dynamic, value, [])
+      assert msg =~ "storage_type :map"
+    end
+
+    test "apply_constraints rejects invalid type" do
+      value = %Dynamic{type: Ash.Type.Date, value: ~D[2026-01-01]}
+      assert {:error, msg} = Ash.Type.apply_constraints(Dynamic, value, [])
+      assert msg =~ "storage_type :map"
+    end
+
+    test "is_valid? returns false for non-NewType" do
+      assert Dynamic.is_valid?(Ash.Type.Date) == false
+    end
+
+    test "is_valid? returns false for unloaded module" do
+      assert Dynamic.is_valid?(Diffo.Type.NonExistent) == false
+    end
+
+    test "is_valid? returns true for valid map-storage NewType" do
+      assert Dynamic.is_valid?(Patch) == true
+    end
+
+    test "dynamic_constraints returns [] for non-NewType" do
+      assert Dynamic.dynamic_constraints(Ash.Type.Date) == []
+    end
+
+    test "dynamic_constraints returns [] for unloaded module" do
+      assert Dynamic.dynamic_constraints(Diffo.Type.NonExistent) == []
+    end
+
+    test "valid map-storage NewType still works" do
+      value = %Dynamic{type: Patch, value: %Patch{aEnd: 1, zEnd: 42}}
+      assert {:ok, %Dynamic{type: Patch}} = Ash.Type.cast_input(Dynamic, value, [])
+    end
+  end
+
   describe "dynamic cast and dump" do
     test "cast_input from struct" do
       value = %Dynamic{type: Patch, value: %Patch{aEnd: 1, zEnd: 42}}