feat: decode option

src/toy.gleam

@@ -232,7 +232,7 @@ pub fn list(item: Decoder(a)) -> Decoder(List(a)) {
 @external(javascript, "./toy_ffi.mjs", "is_nullish")
 fn is_nullish(data: a) -> Bool
 
-pub fn nullable(dec: Decoder(a)) -> Decoder(Option(a)) {
+pub fn nullable(of dec: Decoder(a)) -> Decoder(Option(a)) {
   fn(data) {
     case is_nullish(data) {
       True -> #(None, Ok(None))
@@ -246,6 +246,29 @@ pub fn nullable(dec: Decoder(a)) -> Decoder(Option(a)) {
   }
 }
 
+@external(erlang, "toy_ffi", "decode_option")
+@external(javascript, "./toy_ffi.mjs", "decode_option")
+fn decode_option(value: dynamic.Dynamic) -> Result(Option(dynamic.Dynamic), Nil)
+
+pub fn option(of dec: Decoder(a)) -> Decoder(Option(a)) {
+  fn(data) {
+    case decode_option(data) {
+      Ok(Some(value)) ->
+        case dec(value) {
+          #(_default, Ok(value)) -> #(None, Ok(Some(value)))
+          #(_default, Error(errors)) -> #(None, Error(errors))
+        }
+      Ok(None) -> #(None, Ok(None))
+      Error(_) -> #(
+        None,
+        Error([
+          ToyError(error: InvalidType("Option", string.inspect(data)), path: []),
+        ]),
+      )
+    }
+  }
+}
+
 // String validation
 
 pub fn string_email(dec: Decoder(String)) -> Decoder(String) {

src/toy_ffi.erl

@@ -1,5 +1,5 @@
 -module(toy_ffi).
--export([index/2, is_nullish/1]).
+-export([index/2, is_nullish/1, decode_option/1]).
 
 index(Tuple, Index) when is_tuple(Tuple) andalso is_integer(Index) ->
     try
@@ -23,6 +23,13 @@ is_nullish(Value) ->
     undefined -> true;
     null -> true;
     nil -> true;
-    none -> true;
     _ -> false
   end.
+
+decode_option(Value) ->
+  case Value of
+    {some, Inner} -> {ok, {some, Inner}};
+    none -> {ok, none};
+    _ -> {error, nil}
+  end.
+

src/toy_ffi.mjs

@@ -34,3 +34,15 @@ export function index(data, key) {
 export function is_nullish(value) {
   return value === null || value === undefined;
 }
+
+export function decode_option(value) {
+  if (value instanceof None) {
+    return new Ok(new None());
+  }
+
+  if (value instanceof Some) {
+    return new Ok(value);
+  }
+
+  return new Error(undefined);
+}

test/toy_test.gleam

@@ -92,6 +92,37 @@ pub fn int_map_test() {
   |> should.equal(Ok(43))
 }
 
+pub fn nullable_nil_test() {
+  let decoder = toy.int |> toy.nullable
+  let data = dynamic.from(Nil)
+
+  toy.decode(data, decoder) |> should.equal(Ok(None))
+}
+
+pub fn nullable_validated_test() {
+  let decoder = toy.int |> toy.int_min(10) |> toy.nullable
+
+  dynamic.from(11)
+  |> toy.decode(decoder)
+  |> should.equal(Ok(Some(11)))
+}
+
+pub fn option_none_test() {
+  let decoder = toy.int |> toy.option
+
+  dynamic.from(None)
+  |> toy.decode(decoder)
+  |> should.equal(Ok(None))
+}
+
+pub fn option_validated_test() {
+  let decoder = toy.int |> toy.int_min(10) |> toy.option
+
+  dynamic.from(Some(11))
+  |> toy.decode(decoder)
+  |> should.equal(Ok(Some(11)))
+}
+
 pub type Address {
   Address(street: String, city: String, zip: Int)
 }