Add parameters option

lib/typed_struct.ex

@@ -35,6 +35,7 @@ defmodule TypedStruct do
       individual fields.
     * `opaque` - if set to true, creates an opaque type for the struct.
     * `module` - if set, creates the struct in a submodule named `module`.
+    * `parameters` - if set, adds these parameters to the generated `t()`.
 
   ## Examples
 
@@ -74,6 +75,42 @@ defmodule TypedStruct do
           field :field_four, atom(), default: :hey
         end
       end
+
+  You can also add type parameters:
+
+      defmodule RepairOrder do
+        use TypedStruct
+
+        @type new :: %{status: :new}
+        @type in_progress :: %{status: :in_progress, progress: integer()}
+        @type completed :: %{status: :completed}
+
+        typedstruct parameters: [state] do
+          field :state, state()
+          customer: String.t()
+        end
+
+        @spec new(String.t()) :: t(new())
+        def new(customer) do
+          %{state: %{status: :new}, customer: customer}
+        end
+
+        @spec start_repair(t(new())) :: t(in_progress())
+        def start_repair(order) do
+          %{order | state: %{status: :in_progress, progress: 0}}
+        end
+
+        @spec advance_repair(t(in_progress()), integer()) :: t(in_progress())
+        def advance_repair(order, progress) do
+          put_in(order.state.progress, progress)
+        end
+
+        @spec finish_repair(t(in_progress())) :: t(completed())
+        def finish_repair(order) do
+          %{order | state: %{status: :completed}}
+        end
+      end
+
   """
   defmacro typedstruct(opts \\ [], do: block) do
     ast = TypedStruct.__typedstruct__(block, opts)
@@ -116,13 +153,19 @@ defmodule TypedStruct do
 
   @doc false
   defmacro __type__(types, opts) do
+    type_parameters = Keyword.get(opts, :parameters, [])
+
     if Keyword.get(opts, :opaque, false) do
-      quote bind_quoted: [types: types] do
-        @opaque t() :: %__MODULE__{unquote_splicing(types)}
+      quote bind_quoted: [types: types, type_parameters: type_parameters] do
+        @opaque t(unquote_splicing(type_parameters)) :: %__MODULE__{
+                  unquote_splicing(types)
+                }
       end
     else
-      quote bind_quoted: [types: types] do
-        @type t() :: %__MODULE__{unquote_splicing(types)}
+      quote bind_quoted: [types: types, type_parameters: type_parameters] do
+        @type t(unquote_splicing(type_parameters)) :: %__MODULE__{
+                unquote_splicing(types)
+              }
       end
     end
   end

test/typed_struct_test.exs

@@ -60,9 +60,21 @@ defmodule TypedStructTest do
       end
     end
 
+  {:module, _name, bytecode_parameterized, _exports} =
+    defmodule ParameterizedTestStruct do
+      use TypedStruct
+
+      typedstruct parameters: [a, b] do
+        field :a, a
+        field :b, b | nil
+        field :c, integer()
+      end
+    end
+
   @bytecode bytecode
   @bytecode_opaque bytecode_opaque
   @bytecode_noalias bytecode_noalias
+  @bytecode_parameterized bytecode_parameterized
 
   # Standard struct name used when comparing generated types.
   @standard_struct_name TypedStructTest.TestStruct
@@ -156,6 +168,31 @@ defmodule TypedStructTest do
     assert type1 == type2
   end
 
+  test "generates parameterized types" do
+    # Define a second struct with the type expected for TestStruct.
+    {:module, _name, bytecode2, _exports} =
+      defmodule TestStruct4 do
+        defstruct [:a, :b, :c]
+
+        @type t(a, b) :: %__MODULE__{
+                a: a,
+                b: b | nil,
+                c: integer()
+              }
+      end
+
+    # Get both types and standardise them (remove line numbers and rename
+    # the second struct with the name of the first one).
+    type1 = @bytecode_parameterized |> extract_first_type() |> standardise()
+
+    type2 =
+      bytecode2
+      |> extract_first_type()
+      |> standardise(TypedStructTest.TestStruct4)
+
+    assert type1 == type2
+  end
+
   test "generates the struct in a submodule if `module: ModuleName` is set" do
     assert TestModule.Struct.__struct__() == %TestModule.Struct{field: nil}
   end