Return index of undecodable element in dynamic.list DecodeError path

src/gleam/dynamic.gleam

@@ -290,6 +290,36 @@ pub fn result(
   }
 }
 
+fn do_try_map_with_index(
+  list: List(a),
+  fun: fn(a) -> Result(b, e),
+  acc: List(b),
+  index: Int,
+) -> Result(List(b), #(Int, e)) {
+  case list {
+    [] -> Ok(list.reverse(acc))
+    [x, ..xs] ->
+      case fun(x) {
+        Ok(y) -> do_try_map_with_index(xs, fun, [y, ..acc], index + 1)
+        Error(error) -> Error(#(index, error))
+      }
+  }
+}
+
+/// This is the same as list.try_map but rather than just returning an error it
+/// returns a tuple of the index of the element that failed and the error.
+///
+/// ```gleam
+/// try_map_with_index([[1], [], [2]], first)
+/// // -> Error(#(1, Nil))
+/// ```
+fn try_map_with_index(
+  over list: List(a),
+  with fun: fn(a) -> Result(b, e),
+) -> Result(List(b), #(Int, e)) {
+  do_try_map_with_index(list, fun, [], 0)
+}
+
 /// Checks to see whether a `Dynamic` value is a list of a particular type, and
 /// returns that list if it is.
 ///
@@ -309,7 +339,7 @@ pub fn result(
 ///
 /// ```gleam
 /// from([1, 2, 3]) |> list(of: string)
-/// // -> Error([DecodeError(expected: "String", found: "Int", path: ["*"])])
+/// // -> Error([DecodeError(expected: "String", found: "Int", path: ["0"])])
 /// ```
 ///
 /// ```gleam
@@ -322,9 +352,11 @@ pub fn list(
 ) -> Decoder(List(inner)) {
   fn(dynamic) {
     use list <- result.try(shallow_list(dynamic))
-    list
-    |> list.try_map(decoder_type)
-    |> map_errors(push_path(_, "*"))
+    let result = try_map_with_index(list, decoder_type)
+    case result {
+      Ok(values) -> Ok(values)
+      Error(#(index, errors)) -> Error(list.map(errors, push_path(_, index)))
+    }
   }
 }
 

test/gleam/dynamic_test.gleam

@@ -265,14 +265,14 @@ pub fn list_test() {
   |> dynamic.from
   |> dynamic.list(dynamic.int)
   |> should.equal(
-    Error([DecodeError(expected: "Int", found: "String", path: ["*"])]),
+    Error([DecodeError(expected: "Int", found: "String", path: ["0"])]),
   )
 
   [dynamic.from(1), dynamic.from("not an int")]
   |> dynamic.from
   |> dynamic.list(dynamic.int)
   |> should.equal(
-    Error([DecodeError(expected: "Int", found: "String", path: ["*"])]),
+    Error([DecodeError(expected: "Int", found: "String", path: ["1"])]),
   )
 }