Optional

3 files changed, 78 insertions, 52 deletions

diff --git a/src/gleam/dynamic.gleam b/src/gleam/dynamic.gleam
index 726fbeb..2175461 100644
--- a/src/gleam/dynamic.gleam
+++ b/src/gleam/dynamic.gleam
@@ -6,10 +6,7 @@ import gleam/option
 import gleam/result
 import gleam/string_builder
 import gleam/map.{Map}
-
-if erlang {
-  import gleam/option.{Option}
-}
+import gleam/option.{Option}
 
 /// `Dynamic` data is data that we don't know the type of yet.
 /// We likely get data like this from interop with Erlang, or from
@@ -339,36 +336,53 @@ pub fn typed_list(
   |> result.then(list.try_map(_, decoder_type))
 }
 
+/// Checks to see if a Dynamic value is a nullable version of a particular
+/// type, and return the Option if it is.
+///
+/// ## Examples
+///
+///    > option(from("Hello"), string)
+///    Ok(Some("Hello"))
+///
+///    > option(from("Hello"), string)
+///    Ok(Some("Hello"))
+///
+///    > option(from(atom.from_string("null")), string)
+///    Ok(None)
+///
+///    > option(from(atom.from_string("nil")), string)
+///    Ok(None)
+///
+///    > option(from(atom.from_string("undefined")), string)
+///    Ok(None)
+///
+///    > option(from(123), string)
+///    Error(DecodeError(expected: "BitString", found: "Int"))
+///
+pub fn optional(
+  from value: Dynamic,
+  of decode: Decoder(inner),
+) -> Result(Option(inner), DecodeError) {
+  decode_optional(value, decode)
+}
+
 if erlang {
-  /// Checks to see if a Dynamic value is a nullable version of a particular
-  /// type, and return the Option if it is.
-  ///
-  /// ## Examples
-  ///
-  ///    > option(from("Hello"), string)
-  ///    Ok(Some("Hello"))
-  ///
-  ///    > option(from("Hello"), string)
-  ///    Ok(Some("Hello"))
-  ///
-  ///    > option(from(atom.from_string("null")), string)
-  ///    Ok(None)
-  ///
-  ///    > option(from(atom.from_string("nil")), string)
-  ///    Ok(None)
-  ///
-  ///    > option(from(atom.from_string("undefined")), string)
-  ///    Ok(None)
-  ///
-  ///    > option(from(123), string)
-  ///    Error(DecodeError(expected: "BitString", found: "Int"))
-  ///
-  pub external fn optional(
-    from: Dynamic,
-    of: Decoder(inner),
-  ) -> Result(Option(inner), DecodeError) =
-    "gleam_stdlib" "decode_optional"
+  external fn decode_optional(
+    Dynamic,
+    Decoder(Option(a)),
+  ) -> Result(a, DecodeError) =
+    "gleam_stdlib" "decode_optionl"
+}
 
+if javascript {
+  external fn decode_optional(
+    Dynamic,
+    Decoder(a),
+  ) -> Result(Option(a), DecodeError) =
+    "../gleam_stdlib.js" "decode_option"
+}
+
+if erlang {
   /// Checks to see if a Dynamic value is a map with a specific field, and return
   /// the value of the field if it is.
   ///
diff --git a/src/gleam_stdlib.js b/src/gleam_stdlib.js
index 4c52daf..37f071a 100644
--- a/src/gleam_stdlib.js
+++ b/src/gleam_stdlib.js
@@ -540,3 +540,15 @@ export function decode_result(data) {
 export function decode_map(data) {
   return data instanceof Map ? new Ok(data) : decoder_error("Map", data);
 }
+
+export function decode_option(data, decoder) {
+  if (data === null || data === undefined || data instanceof None)
+    return new Ok(new None());
+  if (data instanceof Some) data = data[0];
+  let result = decoder(data);
+  if (result.isOk()) {
+    return new Ok(new Some(result[0]));
+  } else {
+    return result;
+  }
+}
diff --git a/test/gleam/dynamic_test.gleam b/test/gleam/dynamic_test.gleam
index 2fb4a2a..c062d91 100644
--- a/test/gleam/dynamic_test.gleam
+++ b/test/gleam/dynamic_test.gleam
@@ -2,11 +2,11 @@ import gleam/should
 import gleam/dynamic.{DecodeError}
 import gleam/bit_string
 import gleam/map
+import gleam/option.{None, Some}
 
 if erlang {
   import gleam/list
   import gleam/result
-  import gleam/option.{None, Some}
 }
 
 pub fn bit_string_test() {
@@ -202,29 +202,29 @@ pub fn typed_list_test() {
   |> should.be_error
 }
 
-if erlang {
-  pub fn optional_test() {
-    1
-    |> dynamic.from
-    |> dynamic.optional(dynamic.int)
-    |> should.equal(Ok(Some(1)))
+pub fn optional_test() {
+  1
+  |> dynamic.from
+  |> dynamic.optional(dynamic.int)
+  |> should.equal(Ok(Some(1)))
 
-    option.None
-    |> dynamic.from
-    |> dynamic.optional(dynamic.int)
-    |> should.equal(Ok(None))
+  option.None
+  |> dynamic.from
+  |> dynamic.optional(dynamic.int)
+  |> should.equal(Ok(None))
 
-    Nil
-    |> dynamic.from
-    |> dynamic.optional(dynamic.int)
-    |> should.equal(Ok(None))
+  Nil
+  |> dynamic.from
+  |> dynamic.optional(dynamic.int)
+  |> should.equal(Ok(None))
 
-    1
-    |> dynamic.from
-    |> dynamic.optional(dynamic.string)
-    |> should.be_error
-  }
+  1
+  |> dynamic.from
+  |> dynamic.optional(dynamic.string)
+  |> should.be_error
+}
 
+if erlang {
   pub fn field_test() {
     map.new()
     |> map.insert("ok", 1)