add compilable decorators

composer.json

@@ -23,7 +23,8 @@
         },
         "files": [
             "./src/functions.php",
-            "./src/Predicates/index.php"
+            "./src/Predicates/index.php",
+            "./src/Collections/Internal/functions.php"
         ]
     },
     "suggest": {

src/Collections/Internal/functions.php

@@ -0,0 +1,33 @@
+<?php
+/**
+ * Copyright 2026 Kacper Donat
+ *
+ * @author Kacper "Kadet" Donat <kacper@kadet.net>
+ *
+ * Full license available in separate LICENSE file
+ */
+
+namespace Kadet\Functional\Collections\Internal;
+
+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.
+ *
+ * @internal
+ */
+function callback(callable $callable): \Closure
+{
+    $compiled   = compile($callable);
+    $reflection = new \ReflectionFunction($compiled);
+
+    if ($reflection->isInternal() || (!$reflection->isVariadic() && $reflection->getNumberOfParameters() === 1)) {
+        return static fn ($value) => $compiled($value);
+    }
+
+    return $compiled;
+}

src/Compilable.php

@@ -0,0 +1,21 @@
+<?php
+/**
+ * Copyright 2026 Kacper Donat
+ *
+ * @author Kacper "Kadet" Donat <kacper@kadet.net>
+ *
+ * Full license available in separate LICENSE file
+ */
+
+namespace Kadet\Functional;
+
+/**
+ * A callable object that can be flattened into a plain closure.
+ */
+interface Compilable
+{
+    /**
+     * Returns a closure that behaves exactly like invoking this object, but does not go through it on each call.
+     */
+    public function compile(): \Closure;
+}

src/Utils/FixedFunction.php

@@ -11,9 +11,13 @@ namespace Kadet\Functional\Utils;
 
 
 use function Kadet\Functional\apply;
+use Kadet\Functional\Compilable;
+use function Kadet\Functional\compile;
 use Kadet\Functional\Decorator;
+use function Kadet\Functional\partial;
+use function Kadet\Functional\reflect;
 
-class FixedFunction implements Decorator
+class FixedFunction implements Decorator, Compilable
 {
     /**
      * @var callable
@@ -39,4 +43,51 @@ class FixedFunction implements Decorator
     {
         return $this->fixed;
     }
+
+    /**
+     * The compiled closure passes itself as the self-reference, so recursive calls stay compiled too. It applies
+     * arguments like apply(), with the fixed function reflected once here instead of on every call.
+     */
+    public function compile(): \Closure
+    {
+        $fixed = $this->fixed;
+
+        try {
+            $reflection = reflect($fixed);
+        } catch (\ReflectionException $exception) {
+            $self = static function (...$args) use (&$self, $fixed) {
+                return $fixed($self, ...$args);
+            };
+
+            return $self;
+        }
+
+        $variadic = $reflection->isVariadic();
+        $required = $reflection->getNumberOfRequiredParameters();
+        $all      = $reflection->getNumberOfParameters();
+
+        // A curried function decides on its own whether to call or keep currying, so it is kept as is.
+        $call = $fixed instanceof CurriedFunction ? $fixed : compile($fixed);
+
+        $self = static function (...$args) use (&$self, $fixed, $call, $variadic, $required, $all) {
+            array_unshift($args, $self);
+
+            switch (true) {
+                case $variadic:
+                    return partial($fixed, ...$args);
+
+                case count($args) >= $all:
+                    $applied = $call(...array_slice($args, 0, $all));
+                    return is_callable($applied) ? apply($applied, ...array_slice($args, $all)) : $applied;
+
+                case count($args) >= $required:
+                    return $call(...$args);
+
+                default:
+                    return partial($fixed, ...$args);
+            }
+        };
+
+        return $self;
+    }
 }

src/Utils/PartiallyAppliedFunction.php

@@ -10,11 +10,13 @@
 namespace Kadet\Functional\Utils;
 
 use const Kadet\Functional\_ as PLACEHOLDER;
+use Kadet\Functional\Compilable;
 use Kadet\Functional\Decorator;
+use function Kadet\Functional\compile;
 use function Kadet\Functional\not;
 use function Kadet\Functional\Predicates\same;
 
-class PartiallyAppliedFunction implements Decorator
+class PartiallyAppliedFunction implements Decorator, Compilable
 {
     protected $callable;
     protected $arguments = [];
@@ -64,6 +66,28 @@ class PartiallyAppliedFunction implements Decorator
         return $this->callable;
     }
 
+    public function compile(): \Closure
+    {
+        $callable     = compile($this->callable);
+        $arguments    = $this->arguments;
+        $placeholders = array_keys($arguments, PLACEHOLDER, true);
+
+        if (empty($placeholders)) {
+            return static fn (...$args) => $callable(...$arguments, ...array_values($args));
+        }
+
+        return static function (...$args) use ($callable, $arguments, $placeholders) {
+            $args  = array_values($args);
+            $count = min(count($placeholders), count($args));
+
+            for ($i = 0; $i < $count; $i++) {
+                $arguments[$placeholders[$i]] = $args[$i];
+            }
+
+            return $callable(...$arguments, ...array_slice($args, $count));
+        };
+    }
+
     public static function fix(callable $callable, ...$args)
     {
         $applied = new self($callable, ...$args);

src/Utils/index.php

@@ -72,3 +72,12 @@ function memoize(callable $callable)
 {
     return new MemoizedFunction($callable);
 }
+
+/**
+ * Flattens a callable into a plain closure. Compilable decorators are compiled, so calling the result does not go
+ * through them; any other callable is converted with closure().
+ */
+function compile(callable $callable): \Closure
+{
+    return $callable instanceof Compilable ? $callable->compile() : closure($callable);
+}

tests/Collections/CallbackTest.php

@@ -0,0 +1,47 @@
+<?php
+/**
+ * Copyright 2026 Kacper Donat
+ *
+ * @author Kacper "Kadet" Donat <kacper@kadet.net>
+ *
+ * Full license available in separate LICENSE file
+ */
+
+use Kadet\Functional as f;
+use function Kadet\Functional\Collections\Internal\callback;
+use const Kadet\Functional\_;
+
+class CallbackTest extends \PHPUnit\Framework\TestCase
+{
+    public function testInternalFunctionGetsValueOnly()
+    {
+        $this->assertSame('a', callback(trim(...))(' a ', 'xyz'));
+    }
+
+    public function testInternalStringCallableGetsValueOnly()
+    {
+        $this->assertSame('A', callback('strtoupper')('a', 'key'));
+    }
+
+    public function testSingleParamClosureGetsValueOnly()
+    {
+        $this->assertSame([1], callback(function ($v) { return func_get_args(); })(1, 'k'));
+    }
+
+    public function testTwoParamClosureGetsKey()
+    {
+        $this->assertSame('k=v', callback(fn ($v, $k) => "$k=$v")('v', 'k'));
+    }
+
+    public function testVariadicClosureGetsKey()
+    {
+        $this->assertSame(['v', 'k'], callback(fn (...$args) => $args)('v', 'k'));
+    }
+
+    public function testDecoratorIsCompiled()
+    {
+        $callback = callback(f\partial(fn ($prefix, $v, $k) => "$prefix$k=$v", '>'));
+
+        $this->assertSame('>k=v', $callback('v', 'k'));
+    }
+}

tests/CompileTest.php

@@ -0,0 +1,149 @@
+<?php
+/**
+ * Copyright 2026 Kacper Donat
+ *
+ * @author Kacper "Kadet" Donat <kacper@kadet.net>
+ *
+ * Full license available in separate LICENSE file
+ */
+
+use Kadet\Functional as f;
+use Kadet\Functional\Compilable;
+use Kadet\Functional\Decorator;
+use Kadet\Functional\Utils\PartiallyAppliedFunction;
+use const Kadet\Functional\_;
+
+class CompileTest extends \PHPUnit\Framework\TestCase
+{
+    public function testCompileReturnsClosureForPartial()
+    {
+        $compiled = f\partial(str_replace(...), 'a', _, 'banana')->compile();
+
+        $this->assertInstanceOf(\Closure::class, $compiled);
+        $this->assertNotInstanceOf(Decorator::class, $compiled);
+        $this->assertSame('bxnxnx', $compiled('x'));
+    }
+
+    public function testCompiledPartialDoesNotInvokeDecorator()
+    {
+        $spy = new class(fn ($a, $b) => $a . $b, 'a') extends PartiallyAppliedFunction {
+            public int $calls = 0;
+
+            public function __invoke(...$args)
+            {
+                $this->calls++;
+                return parent::__invoke(...$args);
+            }
+
+            protected function prepareArguments($args)
+            {
+                $this->calls++;
+                return parent::prepareArguments($args);
+            }
+        };
+
+        $compiled = $spy->compile();
+
+        $this->assertSame('ab', $compiled('b'));
+        $this->assertSame(0, $spy->calls);
+    }
+
+    public function testCompileMatchesDecoratorForCurried()
+    {
+        $curried  = f\curry(fn ($a, $b, $c) => "$a$b$c");
+        $applied  = $curried('a');
+        $compiled = $applied->compile();
+
+        $this->assertSame('abc', $compiled('b', 'c'));
+        $this->assertSame($applied('b', 'c'), $compiled('b', 'c'));
+    }
+
+    public function testCompiledCurriedDoesNotCurryFurther()
+    {
+        $applied  = f\curry(fn ($a, $b = 'B', $c = 'C') => "$a$b$c", 3, 'a');
+
+        $this->assertSame($applied->call('b'), $applied->compile()('b'));
+    }
+
+    public function testCompileNestedPartial()
+    {
+        $f3 = fn ($a, $b, $c) => [$a, $b, $c];
+
+        $this->assertSame($f3(1, 2, 3), f\compile(f\partial(f\partial($f3, 1), _, 3))(2));
+    }
+
+    public function testCompileAppendsExtraArguments()
+    {
+        $partial = f\partial(fn (...$args) => $args, 1, _, 3);
+
+        $this->assertSame($partial(2, 4, 5), f\compile($partial)(2, 4, 5));
+        $this->assertSame([1, 2, 3, 4, 5], f\compile($partial)(2, 4, 5));
+    }
+
+    public function testCompileKeepsUnfilledPlaceholders()
+    {
+        $partial = f\partial(fn (...$args) => $args, _, 2, _);
+
+        $this->assertSame($partial(1), f\compile($partial)(1));
+        $this->assertSame($partial(), f\compile($partial)());
+    }
+
+    public function testCompileFunctionDelegatesToMethod()
+    {
+        $partial = f\partial(str_replace(...), 'a', _, 'banana');
+
+        $this->assertSame($partial->compile()('o'), f\compile($partial)('o'));
+    }
+
+    public function testCompilePassesThroughClosures()
+    {
+        $closure = fn ($x) => $x;
+
+        $this->assertSame($closure, f\compile($closure));
+    }
+
+    public function testCompileConvertsCallableToClosure()
+    {
+        $compiled = f\compile('strtoupper');
+
+        $this->assertInstanceOf(\Closure::class, $compiled);
+        $this->assertSame('ABC', $compiled('abc'));
+    }
+
+    public function testDecoratorsAreCompilable()
+    {
+        $this->assertInstanceOf(Compilable::class, f\partial('strtoupper'));
+        $this->assertInstanceOf(Compilable::class, f\curry('strtoupper'));
+        $this->assertInstanceOf(Compilable::class, f\fix(fn ($f) => $f));
+    }
+
+    public function testCompiledFixedFunctionReferencesItself()
+    {
+        $compiled = f\fix(function ($f) use (&$compiled) {
+            $this->assertSame($f, $compiled);
+
+            return fn ($x) => $x;
+        })->compile();
+
+        $this->assertSame(1, $compiled(1));
+    }
+
+    public function testCompiledFixedFunctionRecurses()
+    {
+        $compiled = f\compile(f\fix(function ($f) {
+            return function ($x) use ($f) { return $x >= 2 ? $f($x - 1) * $x : 1; };
+        }));
+
+        $this->assertInstanceOf(\Closure::class, $compiled);
+        $this->assertSame(6, $compiled(3));
+        $this->assertSame(24, $compiled(4));
+    }
+
+    public function testCompiledFixedCurriedFunction()
+    {
+        $compiled = f\compile(f\fix(f\curry(function ($f, $x) { return $x >= 2 ? $f($x - 1) * $x : 1; })));
+
+        $this->assertSame(6, $compiled(3));
+        $this->assertSame(24, $compiled(4));
+    }
+}