Improve error messages

3 files changed, 40 insertions, 25 deletions

diff --git a/src/gleam_stdlib.erl b/src/gleam_stdlib.erl
index 98b4a4b..c52e694 100644
--- a/src/gleam_stdlib.erl
+++ b/src/gleam_stdlib.erl
@@ -108,35 +108,39 @@ 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)
+        #{Key := Value} -> {ok, Value};
+        _ -> decode_error_msg(io_lib:format("a map with key `~p`", [Key]), Data)
     end.
 
-decode_element(Data, Index) when is_tuple(Data) ->
-    Error = fun(Size) ->
-        S = case Size of 
-            1 -> "s";
-            _ -> ""
-        end,
-        Msg = list_to_binary(["Tuple of at least ", integer_to_list(Size), S, " elements"]),
-        decode_error_msg(Msg, Data)
+
+decode_tuple_error(Size, Data) ->
+    S = case Size of 
+        1 -> "";
+        _ -> "s"
     end,
+    Msg = list_to_binary([
+        "Tuple of at least ", integer_to_list(Size), " element", S
+    ]),
+    decode_error_msg(Msg, Data).
+
+decode_element(Data, Index) when is_tuple(Data) ->
     Size = tuple_size(Data),
     case Index >= 0 of
         true -> case Index < Size of
             true -> {ok, element(Index + 1, Data)};
-            false -> Error(Index + 1)
+            false -> decode_tuple_error(Index + 1, Data)
         end;
         false -> case abs(Index) < Size of
             true -> {ok, element(Size + Index + 1, Data)};
-            false -> Error(abs(Index))
+            false -> decode_tuple_error(abs(Index), Data)
         end
     end;
-decode_element(Data, _Position) -> 
-    decode_error_msg(<<"Tuple of at least 1 element">>, Data).
+decode_element(Data, Index) -> 
+    Size = case Index < 0 of
+        true -> abs(Index);
+        false -> Index + 1
+    end,
+    decode_tuple_error(Size, Data).
 
 decode_optional(Term, F) ->
     Decode = fun(Inner) ->
diff --git a/src/gleam_stdlib.js b/src/gleam_stdlib.js
index 7ecbe11..b743c03 100644
--- a/src/gleam_stdlib.js
+++ b/src/gleam_stdlib.js
@@ -518,24 +518,27 @@ export function decode_bit_string(data) {
     : decoder_error("BitString", data);
 }
 
+function decode_tuple_error(size, data) {
+  return decoder_error(
+    `Tuple of at least ${size} element${size == 1 ? "" : "s"}`,
+    data
+  );
+}
+
 export function decode_element(data, index) {
-  let error = (size) =>
-    decoder_error(
-      `Tuple of at least ${size} element${size == 1 ? "" : "s"}`,
-      data
-    );
-  if (!Array.isArray(data)) return error(index < 0 ? 1 : index + 1);
+  if (!Array.isArray(data))
+    return decode_tuple_error(index < 0 ? Math.abs(index) : index + 1, data);
   if (index >= 0) {
     if (index < data.length) {
       return new Ok(data[index]);
     } else {
-      return error(index + 1);
+      return decode_tuple_error(index + 1, data);
     }
   } else {
     if (Math.abs(index) <= data.length) {
       return new Ok(data[data.length + index]);
     } else {
-      return error(Math.abs(index));
+      return decode_tuple_error(Math.abs(index), data);
     }
   }
 }
diff --git a/test/gleam/dynamic_test.gleam b/test/gleam/dynamic_test.gleam
index 1bf0eee..21fb98c 100644
--- a/test/gleam/dynamic_test.gleam
+++ b/test/gleam/dynamic_test.gleam
@@ -292,6 +292,14 @@ pub fn element_test() {
 
   1
   |> dynamic.from
+  |> dynamic.element(-3)
+  |> should.equal(Error(DecodeError(
+    expected: "Tuple of at least 3 elements",
+    found: "Int",
+  )))
+
+  1
+  |> dynamic.from
   |> dynamic.element(0)
   |> should.equal(Error(DecodeError(
     expected: "Tuple of at least 1 element",