add warning comment to foldl2.

Author: KtorZ

CHANGELOG.md

@@ -1,5 +1,12 @@
 # Changelog
 
+## v2.2.1 - 2025-12-16
+
+### Fixed
+
+- Add warning comment to `foldl2`.
+- Expose `Foldl2` as a public type.
+
 ## v2.2.0 - 2024-12-13
 
 ### Added

aiken.lock

@@ -13,4 +13,4 @@ requirements = []
 source = "github"
 
 [etags]
-"aiken-lang/fuzz@v2" = [{ secs_since_epoch = 1734106349, nanos_since_epoch = 450591000 }, "64a32283418d58cade34059d3855b857e84505541158c541c460cafa0d355475"]
+"aiken-lang/fuzz@v2" = [{ secs_since_epoch = 1765902744, nanos_since_epoch = 267815000 }, "9843473958e51725a9274b487d2d4aac0395ec1a2e30f090724fa737226bc127"]

lib/aiken/collection/list.ak

@@ -1203,9 +1203,15 @@ pub fn foldl(self: List<a>, zero: b, with: fn(a, b) -> b) -> b {
   }
 }
 
-type Fold2<a, b, result> =
+pub type Fold2<a, b, result> =
   fn(a, b) -> result
 
+/// > [!CAUTION]
+/// > This function was released by mistake as part of the v2 series of updates. However, unlike the name suggests, it isn't a left-fold but a right-fold.
+/// >
+/// > It cannot be removed without changing its semantic and possibly breaking existing code that relies on it. Sorry for the inconvenience.
+/// >
+/// > Be also aware that starting from `v3.0.0`, this has been fixed. So be careful if you've been using `foldl2` when upgrading to `v3.0.0`; you very likely want to use v3's `foldr2` instead.
 pub fn foldl2(
   self: List<elem>,
   zero_a: a,