improve performance of string.join

Author: yoshi-monster

CHANGELOG.md

@@ -4,7 +4,8 @@
 
 - The `debug` function in the `io` module has been deprecated in favour of
   the `echo` keyword.
-- The performance of `string.append` and `string.concat` have been improved.
+- The performance of `string.append`, `string.join`, and `string.concat` have
+  been improved.
 
 ## v0.58.0 - 2025-03-23
 

src/gleam/string.gleam

@@ -432,11 +432,23 @@ fn repeat_loop(string: String, times: Int, acc: String) -> String {
 /// // -> "home/evan/Desktop"
 /// ```
 ///
-@external(javascript, "../gleam_stdlib.mjs", "join")
 pub fn join(strings: List(String), with separator: String) -> String {
-  strings
-  |> list.intersperse(with: separator)
-  |> concat
+  case strings {
+    [] -> ""
+    [first, ..rest] -> do_join(rest, separator, first)
+  }
+}
+
+fn do_join(
+  strings: List(String),
+  separator: String,
+  accumulator: String,
+) -> String {
+  case strings {
+    [] -> accumulator
+    [string, ..strings] ->
+      do_join(strings, separator, accumulator <> separator <> string)
+  }
 }
 
 /// Pads the start of a `String` until it has a given length.

src/gleam_stdlib.mjs

@@ -205,17 +205,6 @@ export function split(xs, pattern) {
   return List.fromArray(xs.split(pattern));
 }
 
-export function join(xs, separator) {
-  const iterator = xs[Symbol.iterator]();
-  let result = iterator.next().value || "";
-  let current = iterator.next();
-  while (!current.done) {
-    result = result + separator + current.value;
-    current = iterator.next();
-  }
-  return result;
-}
-
 export function concat(xs) {
   let result = "";
   for (const x of xs) {