//erl[1,2|3] output for inspecting improper Erlang lists

3 files changed, 38 insertions, 18 deletions

diff --git a/src/gleam_stdlib.erl b/src/gleam_stdlib.erl
index abb3b45..0d817fe 100644
--- a/src/gleam_stdlib.erl
+++ b/src/gleam_stdlib.erl
@@ -349,7 +349,10 @@ inspect(Binary) when is_binary(Binary) ->
             ["<<", lists:join(", ", Segments), ">>"]
     end;
 inspect(Elements) when is_list(Elements) ->
-    ["[", inspect_list(Elements), "]"];
+    case is_proper_list(Elements) of
+        true -> ["[", inspect_proper_list(Elements), "]"];
+        false -> ["//erl[", inspect_improper_list(Elements), "] %% improper list"]
+    end;
 inspect(Any) when is_tuple(Any) % Record constructors
   andalso is_atom(element(1, Any))
   andalso element(1, Any) =/= false
@@ -374,14 +377,27 @@ inspect(Any) when is_function(Any) ->
 inspect(Any) ->
     ["//erl(", io_lib:format("~p", [Any]), ")"].
 
-inspect_list([])  ->
+is_proper_list([])  ->
+    true;
+is_proper_list([_Head | Tail]) ->
+  is_proper_list(Tail);
+is_proper_list(ImproperTail) when not is_list(ImproperTail) ->
+    false.
+
+inspect_proper_list(List) ->
+    lists:join(<<", ">>, lists:map(fun inspect/1, List)).
+
+inspect_improper_list(Elements) ->
+	do_inspect_improper_list(firstCall, Elements).
+
+do_inspect_improper_list(_NthCall, []) ->
     [];
-inspect_list([Head]) ->
-    [inspect(Head)];
-inspect_list([First | Rest]) ->
-    [inspect(First), <<", ">> | inspect_list(Rest)];
-inspect_list(ImproperTail) ->
-    [<<"..">>, inspect(ImproperTail)].
+do_inspect_improper_list(firstCall, [First | Rest]) ->
+    [inspect(First) | do_inspect_improper_list(nthCall, Rest)];
+do_inspect_improper_list(nthCall, [First | Rest]) ->
+    [ <<",">>, inspect(First) | do_inspect_improper_list(nthCall, Rest)];
+do_inspect_improper_list(nthCall, 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 0637bdd..7c83c84 100644
--- a/test/gleam/string_test.gleam
+++ b/test/gleam/string_test.gleam
@@ -795,11 +795,15 @@ if erlang {
 
 if erlang {
   pub fn improper_list_inspect_test() {
-    let list = improper_list_append(1, 2)
-    assert "[1, ..2]" = string.inspect(list)
+    let list = improper_list_append(1, 2, 3)
+    assert "//erl[1,2|3] %% improper list" = string.inspect(list)
   }
 
   // Warning: The type of this function is incorrect
-  external fn improper_list_append(anything1, anything2) -> List(anything) =
+  external fn improper_list_append(
+    anything1,
+    anything2,
+    anything3,
+  ) -> 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 c26a879..c34cb5e 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, improper_list_append/2
+    should_be_error/1, improper_list_append/3
 ]).
 
 -include_lib("eunit/include/eunit.hrl").
@@ -26,18 +26,18 @@ filepath_to_module(Path0) ->
     Path5 = list_to_binary(Path4),
     binary_to_atom(Path5).
 
-should_equal(Actual, Expected) -> 
+should_equal(Actual, Expected) ->
     ?assertEqual(Expected, Actual),
     nil.
-should_not_equal(Actual, Expected) -> 
+should_not_equal(Actual, Expected) ->
     ?assertNotEqual(Expected, Actual),
     nil.
-should_be_ok(A) -> 
+should_be_ok(A) ->
     ?assertMatch({ok, _}, A),
     nil.
-should_be_error(A) -> 
+should_be_error(A) ->
     ?assertMatch({error, _}, A),
     nil.
 
-improper_list_append(X, Y) ->
-    [X | Y].
+improper_list_append(X, Y, Z) ->
+    [X, Y | Z].