pass value only to decorated internal callbacks

src/Collections/Internal/functions.php

@@ -9,14 +9,16 @@
 
 namespace Kadet\Functional\Collections\Internal;
 
+use Kadet\Functional\Decorator;
+use function Kadet\Functional\closure;
 use function Kadet\Functional\compile;
 
 /**
  * Compiles a collection callback, adapted to its arity.
  *
  * The returned closure is called with ($value, $key). Internal functions (including first-class callables like
- * `trim(...)`) and non-variadic closures declaring exactly one parameter receive only the value, so the key is
- * never mistaken for an optional argument.
+ * `trim(...)`, also when wrapped in decorators) and non-variadic closures declaring exactly one parameter receive
+ * only the value, so the key is never mistaken for an optional argument.
  *
  * @internal
  */
@@ -24,10 +26,38 @@ function callback(callable $callable): \Closure
 {
     $compiled   = compile($callable);
     $reflection = new \ReflectionFunction($compiled);
+    $target     = new \ReflectionFunction(closure(target($callable)));
 
-    if ($reflection->isInternal() || (!$reflection->isVariadic() && $reflection->getNumberOfParameters() === 1)) {
+    if ($target->isInternal() || (!$reflection->isVariadic() && $reflection->getNumberOfParameters() === 1)) {
         return static fn ($value) => $compiled($value);
     }
 
     return $compiled;
 }
+
+/**
+ * Finds the function a callable ultimately calls, by unwrapping decorators, also when they were turned into closures.
+ *
+ * @internal
+ */
+function target(callable $callable): callable
+{
+    while (true) {
+        if ($callable instanceof Decorator) {
+            $callable = $callable->getDecorated();
+            continue;
+        }
+
+        if ($callable instanceof \Closure) {
+            $reflection = new \ReflectionFunction($callable);
+            $bound      = $reflection->getClosureThis();
+
+            if ($bound instanceof Decorator && $reflection->getName() === '__invoke') {
+                $callable = $bound;
+                continue;
+            }
+        }
+
+        return $callable;
+    }
+}

src/Compilable.php

@@ -15,7 +15,10 @@ namespace Kadet\Functional;
 interface Compilable
 {
     /**
-     * Returns a closure that behaves exactly like invoking this object, but does not go through it on each call.
+     * Returns a closure that behaves like invoking this object, but does not go through it on each call.
+     *
+     * The one difference: when the call ends in a non-variadic internal function, arguments past its parameter count
+     * are dropped, where invoking this object would throw an ArgumentCountError.
      */
     public function compile(): \Closure;
 }

tests/Collections/CallbackTest.php

@@ -43,6 +43,21 @@ class CallbackTest extends \PHPUnit\Framework\TestCase
         $this->assertSame('a', callback(f\partial(trim(...), _, 'x'))('xax', 'key'));
     }
 
+    public function testPartialOfInternalFunctionDoesNotFillOptionalParameterWithKey()
+    {
+        $this->assertSame(['a', 'b'], callback(f\partial(explode(...), ','))('a,b', 0));
+    }
+
+    public function testPartialOfInternalFunctionWithoutArgumentsGetsValueOnly()
+    {
+        $this->assertSame('a', callback(f\partial(trim(...)))(' a ', 'k'));
+    }
+
+    public function testCompiledMemoizedInternalFunctionGetsValueOnly()
+    {
+        $this->assertSame('a', callback(f\compile(f\memoize(trim(...))))(' a ', 'k'));
+    }
+
     public function testDecoratorIsCompiled()
     {
         $callback = callback(f\partial(fn ($prefix, $v, $k) => "$prefix$k=$v", '>'));

tests/CompileTest.php

@@ -103,6 +103,22 @@ class CompileTest extends \PHPUnit\Framework\TestCase
         $this->assertSame('aa', f\compile(f\curry(str_repeat(...), null, 'a'))(2, 'extra'));
     }
 
+    public function testCompileWithFewerArgumentsThanPlaceholdersMatchesDecorator()
+    {
+        $partial = f\partial(str_repeat(...), _, _);
+
+        $this->assertSame($this->outcome(fn () => $partial('a')), $this->outcome(fn () => f\compile($partial)('a')));
+    }
+
+    private function outcome(callable $call)
+    {
+        try {
+            return ['result', $call()];
+        } catch (\Throwable $error) {
+            return ['error', get_class($error), $error->getMessage()];
+        }
+    }
+
     public function testCompileFunctionDelegatesToMethod()
     {
         $partial = f\partial(str_replace(...), 'a', _, 'banana');