JS decode field for objects

Author: lpil

src/gleam/dynamic.gleam

@@ -382,25 +382,34 @@ if javascript {
     "../gleam_stdlib.js" "decode_option"
 }
 
+/// Checks to see if a Dynamic value is a map with a specific field, and return
+/// the value of the field if it is.
+///
+/// This will not succeed on a record.
+///
+/// ## Examples
+///
+///    > import gleam/map
+///    > field(from(map.new("Hello", "World")), "Hello")
+///    Ok(Dynamic)
+///
+///    > field(from(123), "Hello")
+///    Error(DecodeError(expected: "Map", found: "Int"))
+///
+pub fn field(from value: Dynamic, named name: a) -> Result(Dynamic, DecodeError) {
+  decode_field(value, name)
+}
+
 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.
-  ///
-  /// This will not succeed on a record.
-  ///
-  /// ## Examples
-  ///
-  ///    > import gleam/map
-  ///    > field(from(map.new("Hello", "World")), "Hello")
-  ///    Ok(Dynamic)
-  ///
-  ///    > field(from(123), "Hello")
-  ///    Error(DecodeError(expected: "Map", found: "Int"))
-  ///
-  pub external fn field(from: Dynamic, named: a) -> Result(Dynamic, DecodeError) =
+  external fn decode_field(Dynamic, name) -> Result(Dynamic, DecodeError) =
     "gleam_stdlib" "decode_field"
 }
 
+if javascript {
+  external fn decode_field(Dynamic, name) -> Result(Dynamic, DecodeError) =
+    "../gleam_stdlib.js" "decode_field"
+}
+
 /// Checks to see if the Dynamic value is a tuple large enough to have a certain
 /// index, and return the value of that index if it is.
 ///

src/gleam_stdlib.erl

@@ -94,7 +94,7 @@ decode_list(Data) -> decode_error_msg(<<"List">>, Data).
 decode_field(Data, Key) ->
     case Data of
         #{Key := Value} -> {ok, Value};
-        _ -> decode_error_msg(io_lib:format("a map with key `~p`", [Key]), Data)
+        _ -> decode_error_msg(io_lib:format("Value with field `~p`", [Key]), Data)
     end.
 
 size_of_tuple(Data) -> tuple_size(Data).

src/gleam_stdlib.js

@@ -473,6 +473,8 @@ export function decode64(sBase64) {
 export function classify_dynamic(data) {
   if (typeof data === "string") {
     return "String";
+  } else if (Result.isResult(data)) {
+    return "Result";
   } else if (List.isList(data)) {
     return "List";
   } else if (Number.isInteger(data)) {
@@ -552,3 +554,9 @@ export function decode_option(data, decoder) {
     return result;
   }
 }
+
+export function decode_field(value, name) {
+  return name in value
+    ? new Ok(value[name])
+    : decoder_error(`Value with field ${inspect(name)}`, value);
+}

test/gleam/dynamic_test.gleam

@@ -224,6 +224,28 @@ pub fn optional_test() {
   |> should.be_error
 }
 
+if javascript {
+  pub fn javascript_object_field_test() {
+    Ok(123)
+    |> dynamic.from
+    |> dynamic.field("0")
+    |> should.equal(Ok(dynamic.from(123)))
+
+    Ok(123)
+    |> dynamic.from
+    |> dynamic.field(0)
+    |> should.equal(Ok(dynamic.from(123)))
+
+    Ok(123)
+    |> dynamic.from
+    |> dynamic.field("Nope")
+    |> should.equal(Error(DecodeError(
+      expected: "Value with field \"Nope\"",
+      found: "Result",
+    )))
+  }
+}
+
 if erlang {
   pub fn field_test() {
     map.new()