correct foldr for Traversable

marcosh · Sep 27, 2023 · 8fc58a4 · 8fc58a4
1 parent 3c4d2a5
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diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -12,7 +12,7 @@ and this project adheres to [Semantic versioning](http://semver.org/).
 - do not flip arguments on `ListL/ConcatenationMonoid`
 - introduce `Alt` and `Plus` typeclasses
 - define `ConstantSemigroup`
-- fix `foldr` behavior for lists
+- correct `foldr` implementation for `ListL` and `Traversable`
 
 ## [2.0.0] - 2023-03-21
 
@@ -22,6 +22,7 @@ and this project adheres to [Semantic versioning](http://semver.org/).
 
 - introduce `OppositeSemigroup` and `OppositMonoid`
 - do not flip arguments on `ListL/ConcatenationMonoid`
+- correct `foldr` implementation for `ListL` and `Traversable`
 
 ## [1.3.0] - 2023-08-23
 

diff --git a/src/Instances/Traversable/TraversableFoldable.php b/src/Instances/Traversable/TraversableFoldable.php
@@ -32,8 +32,15 @@ public function foldr(callable $f, mixed $b, HK1 $a): mixed
 
         $ret = $b;
 
+        $reverseArrayA = [];
+
         /** @psalm-suppress ImpureMethodCall */
         foreach ($arrayA as $element) {
+            $reverseArrayA = array_merge([$element], $reverseArrayA);
+        }
+
+        /** @psalm-suppress ImpureMethodCall */
+        foreach ($reverseArrayA as $element) {
             /** @psalm-suppress ImpureFunctionCall */
             $ret = $f($element, $ret);
         }