Fix slice

Closes gleam-lang/gleam#3499

Author: lpil

CHANGELOG.md

@@ -19,6 +19,7 @@
   where the `Intl` API is not supported.
 - Fixed a bug where the behaviour of `uri.percent_decode` would decode `+` as a
   space on JavaScript.
+- Fixed a bug where `string.slice` could return invalid values on Erlang.
 
 ## v0.39.0 - 2024-07-09
 

src/gleam/string.gleam

@@ -230,7 +230,7 @@ pub fn slice(from string: String, at_index idx: Int, length len: Int) -> String
   }
 }
 
-@external(erlang, "string", "slice")
+@external(erlang, "gleam_stdlib", "slice")
 fn do_slice(string: String, idx: Int, len: Int) -> String {
   string
   |> to_graphemes

src/gleam_stdlib.erl

@@ -14,7 +14,7 @@
     decode_tuple5/1, decode_tuple6/1, tuple_get/2, classify_dynamic/1, print/1,
     println/1, print_error/1, println_error/1, inspect/1, float_to_string/1,
     int_from_base_string/2, utf_codepoint_list_to_string/1, contains_string/2,
-    crop_string/2, base16_decode/1, string_replace/3, regex_replace/3
+    crop_string/2, base16_decode/1, string_replace/3, regex_replace/3, slice/3
 ]).
 
 %% Taken from OTP's uri_string module
@@ -547,3 +547,9 @@ base16_decode(String) ->
 
 string_replace(String, Pattern, Replacement) ->
     string:replace(String, Pattern, Replacement, all).
+
+slice(String, Index, Length) ->
+    case string:slice(String, Index, Length) of
+        X when is_binary(X) -> X;
+        X when is_list(X) -> unicode:characters_to_binary(X)
+    end.

test/gleam/string_test.gleam

@@ -465,6 +465,13 @@ pub fn drop_left_test() {
   |> should.equal("gleam")
 }
 
+pub fn drop_left_3499_test() {
+  // https://github.com/gleam-lang/gleam/issues/3499
+  "\r]"
+  |> string.drop_left(1)
+  |> should.equal("]")
+}
+
 pub fn drop_right_test() {
   "gleam"
   |> string.drop_right(up_to: 2)