Use different syntax for improper lists

3 files changed, 23 insertions, 10 deletions

diff --git a/src/gleam_stdlib.erl b/src/gleam_stdlib.erl
index 9fc1c4a..b2aa135 100644
--- a/src/gleam_stdlib.erl
+++ b/src/gleam_stdlib.erl
@@ -348,9 +348,8 @@ inspect(Binary) when is_binary(Binary) ->
             Segments = [erlang:integer_to_list(X) || <<X>> <= Binary],
             ["<<", lists:join(", ", Segments), ">>"]
     end;
-inspect(List) when is_list(List) ->
-    Elements = lists:join(<<", ">>, do_map_listish(fun inspect/1, List)),
-    ["[", Elements, "]"];
+inspect(Elements) when is_list(Elements) ->
+    ["[", inspect_list(Elements), "]"];
 inspect(Any) when is_tuple(Any) % Record constructors
   andalso is_atom(element(1, Any))
   andalso element(1, Any) =/= false
@@ -375,13 +374,15 @@ inspect(Any) when is_function(Any) ->
 inspect(Any) ->
     ["//erl(", io_lib:format("~p", [Any]), ")"].
 
-do_map_listish(_Fn, [])  ->
+
+inspect_list([])  ->
     [];
-do_map_listish(Fn, List) when is_list(List) ->
-    [Head | Tail] = List,
-    [Fn(Head) | do_map_listish(Fn, Tail)];
-do_map_listish(Fn, Tail) when not is_list(Tail) ->
-    [Fn(Tail)].
+inspect_list([Head]) ->
+    [inspect(Head)];
+inspect_list([First | Rest]) ->
+    [inspect(First), <<", ">> | inspect_list(Rest)];
+inspect_list(ImproperTail) ->
+    [<<"...">>, inspect(ImproperTail)].
 
 float_to_string(Float) when is_float(Float) ->
     erlang:iolist_to_binary(io_lib_format:fwrite_g(Float)).
diff --git a/test/gleam/string_test.gleam b/test/gleam/string_test.gleam
index 46b5e91..acc00d7 100644
--- a/test/gleam/string_test.gleam
+++ b/test/gleam/string_test.gleam
@@ -792,3 +792,12 @@ if erlang {
     |> should.equal("\"abc\"")
   }
 }
+
+pub fn improper_list_inspect_test() {
+  let list = improper_list_append(1, 2)
+  assert "[1, ...2]" = string.inspect(list)
+}
+
+// Warning: The type of this function is incorrect
+external fn improper_list_append(anything1, anything2) -> List(anything) =
+  "gleam_stdlib_test_ffi" "improper_list_append"
diff --git a/test/gleam_stdlib_test_ffi.erl b/test/gleam_stdlib_test_ffi.erl
index 6be4cb6..c26a879 100644
--- a/test/gleam_stdlib_test_ffi.erl
+++ b/test/gleam_stdlib_test_ffi.erl
@@ -2,7 +2,7 @@
 
 -export([
     main/0, should_equal/2, should_not_equal/2, should_be_ok/1,
-    should_be_error/1
+    should_be_error/1, improper_list_append/2
 ]).
 
 -include_lib("eunit/include/eunit.hrl").
@@ -38,3 +38,6 @@ should_be_ok(A) ->
 should_be_error(A) -> 
     ?assertMatch({error, _}, A),
     nil.
+
+improper_list_append(X, Y) ->
+    [X | Y].