Fix `dynamic.field` behaviour

3 files changed, 35 insertions, 15 deletions

diff --git a/src/gleam/dynamic.gleam b/src/gleam/dynamic.gleam
index a845b1c..4f0708f 100644
--- a/src/gleam/dynamic.gleam
+++ b/src/gleam/dynamic.gleam
@@ -474,20 +474,29 @@ if javascript {
 ///
 pub fn field(named name: a, of inner_type: Decoder(t)) -> Decoder(t) {
   fn(value) {
-    value
-    |> decode_field(name)
-    |> result.try(inner_type)
-    |> map_errors(push_path(_, name))
+    case decode_field(value, name) {
+      Error(not_a_map_errors) -> Error(not_a_map_errors)
+      Ok(dynamic_field_result) ->
+        dynamic_field_result
+        |> result.try(inner_type)
+        |> map_errors(push_path(_, name))
+    }
   }
 }
 
 if erlang {
-  external fn decode_field(Dynamic, name) -> Result(Dynamic, DecodeErrors) =
+  external fn decode_field(
+    Dynamic,
+    name,
+  ) -> Result(Result(Dynamic, DecodeErrors), DecodeErrors) =
     "gleam_stdlib" "decode_field"
 }
 
 if javascript {
-  external fn decode_field(Dynamic, name) -> Result(Dynamic, DecodeErrors) =
+  external fn decode_field(
+    Dynamic,
+    name,
+  ) -> Result(Result(Dynamic, DecodeErrors), DecodeErrors) =
     "../gleam_stdlib.mjs" "decode_field"
 }
 
diff --git a/src/gleam_stdlib.erl b/src/gleam_stdlib.erl
index 33c9609..4c59893 100644
--- a/src/gleam_stdlib.erl
+++ b/src/gleam_stdlib.erl
@@ -78,12 +78,14 @@ decode_bool(Data) -> decode_error_msg(<<"Bool">>, Data).
 decode_list(Data) when is_list(Data) -> {ok, Data};
 decode_list(Data) -> decode_error_msg(<<"List">>, Data).
 
-decode_field(Data, Key) ->
+decode_field(Data, Key) when is_map(Data) ->
     case Data of
-        #{Key := Value} -> {ok, Value};
+        #{Key := Value} -> {ok, {ok, Value}};
         _ ->
-            decode_error(<<"field"/utf8>>, <<"nothing"/utf8>>)
-    end.
+            {ok, decode_error(<<"field"/utf8>>, <<"nothing"/utf8>>)}
+    end;
+decode_field(Data, _) ->
+    decode_error_msg(<<"Map">>, Data).
 
 size_of_tuple(Data) -> tuple_size(Data).
 
diff --git a/src/gleam_stdlib.mjs b/src/gleam_stdlib.mjs
index 2b0a6e1..52fbc89 100644
--- a/src/gleam_stdlib.mjs
+++ b/src/gleam_stdlib.mjs
@@ -663,14 +663,23 @@ export function decode_option(data, decoder) {
 }
 
 export function decode_field(value, name) {
-  let error = () => decoder_error_no_classify("field", "nothing");
-  if (value instanceof PMap) {
+  let missing_field_error = () => decoder_error_no_classify("field", "nothing");
+  let not_a_map_error = () => decoder_error("Map", value);
+
+  if (value instanceof PMap || value instanceof WeakMap || value instanceof Map) {
     let entry = map_get(value, name);
-    return entry.isOk() ? entry : error();
+    return new Ok(entry.isOk() ? entry : missing_field_error());
+  } else if (Object.getPrototypeOf(value) == Object.prototype) {
+    return try_get_field(value, name, () => new Ok(missing_field_error()));
+  } else {
+    return try_get_field(value, name, not_a_map_error);
   }
+}
+
+function try_get_field(value, field, or_else) {
   try {
-    return name in value ? new Ok(value[name]) : error();
+    return field in value ? new Ok(new Ok(value[field])) : or_else();
   } catch {
-    return error();
+    return or_else();
   }
 }