pass only carry and value to two-parameter reducers

src/Collections/queries.php

@@ -25,7 +25,6 @@ use function Kadet\Functional\Internal\callback;
 use function Kadet\Functional\Collections\Internal\extremum;
 use function Kadet\Functional\Collections\Internal\firstOf;
 use function Kadet\Functional\Collections\Internal\reversedEntries;
-use function Kadet\Functional\compile;
 
 /**
  * Returns the first value for which `$predicate($value, $key)` is truthy, or the first value without a predicate.
@@ -288,6 +287,8 @@ function max(?callable $selector = null): \Closure
 /**
  * Folds the collection into a single value with `$fn($carry, $value, $key)`, starting from `$initial`.
  *
+ * Internal functions (e.g. `max(...)`) and functions declaring two parameters get only `($carry, $value)`.
+ *
  * @template TKey = mixed
  * @template TValue = mixed
  * @template TCarry = mixed
@@ -299,8 +300,8 @@ function max(?callable $selector = null): \Closure
  */
 function reduce(callable $fn, mixed $initial = null): \Closure
 {
-    // The reducer takes the carry first, so it is compiled without the ($value, $key) adaptation.
-    $fn = compile($fn);
+    // The reducer takes the carry first: an internal one, or one declaring two parameters, gets ($carry, $value).
+    $fn = callback($fn, 2);
 
     return static function (iterable $data) use ($fn, $initial) {
         $carry = $initial;

tests/Collections/QueriesTest.php

@@ -9,6 +9,7 @@
 
 use Ds\Map;
 use Ds\Seq;
+use Kadet\Functional as f;
 use Kadet\Functional\Collections as c;
 use Kadet\Functional\Exception\EmptyCollectionException;
 
@@ -204,6 +205,24 @@ class QueriesTest extends \PHPUnit\Framework\TestCase
         $this->assertNull(c\reduce(fn ($carry, $v) => $v)([]));
     }
 
+    public function testReduceWithInternalFunctionGetsCarryAndValueOnly()
+    {
+        $this->assertSame(2, c\reduce(max(...), 0)([5 => 1, 9 => 2]));
+    }
+
+    public function testReduceWithDecoratedTwoParameterClosureGetsCarryAndValueOnly()
+    {
+        $calls   = [];
+        $reducer = f\memoize(function ($carry, $v) use (&$calls) {
+            $calls[] = func_get_args();
+
+            return $carry + $v;
+        });
+
+        $this->assertSame(3, c\reduce($reducer, 0)(['a' => 1, 'b' => 2]));
+        $this->assertSame([[0, 1], [1, 2]], $calls);
+    }
+
     public function testSum()
     {
         $this->assertSame(6, c\sum()([1, 2, 3]));