diff --git a/psalm.xml b/psalm.xml
index 1bcd67b..ae0443a 100644
--- a/psalm.xml
+++ b/psalm.xml
@@ -7,6 +7,7 @@
     xsi:schemaLocation="https://getpsalm.org/schema/config vendor/vimeo/psalm/config.xsd"
 >
     <projectFiles>
+        <directory name="src"/>
         <directory name="tests/static-analysis"/>
         <ignoreFiles>
             <directory name="vendor" />
diff --git a/src/Combinator.php b/src/Combinator.php
index bb1725b..56ccb78 100644
--- a/src/Combinator.php
+++ b/src/Combinator.php
@@ -6,14 +6,16 @@
 
 use Closure;
 use InvalidArgumentException;
+use loophp\combinator\Contract\Combinator as CombinatorInterface;
 
-use function get_called_class;
 use function is_callable;
 
 /**
  * Class Combinator.
+ *
+ * @psalm-immutable
  */
-abstract class Combinator implements \loophp\combinator\Contract\Combinator
+abstract class Combinator implements CombinatorInterface
 {
     /**
      * @return Closure
@@ -26,6 +28,6 @@ public static function with(): Closure
             return Closure::fromCallable($possibleCallable);
         }
 
-        throw new InvalidArgumentException('Implement on() method in ' . get_called_class() . '.');
+        throw new InvalidArgumentException('Implement on() method in ' . static::class . '.');
     }
 }
diff --git a/src/Combinator/A.php b/src/Combinator/A.php
index b4dea88..565fc25 100644
--- a/src/Combinator/A.php
+++ b/src/Combinator/A.php
@@ -12,15 +12,21 @@
  *
  * @psalm-template AType
  * @psalm-template BType
+ *
+ * @psalm-immutable
  */
 final class A extends Combinator
 {
     /**
+     * @psalm-var callable(AType): BType
+     *
      * @var callable
      */
     private $f;
 
     /**
+     * @psalm-var AType
+     *
      * @var mixed
      */
     private $x;
@@ -29,7 +35,7 @@ final class A extends Combinator
      * A constructor.
      *
      * @psalm-param callable(AType): BType $f
-     * @psalm-param BType $x
+     * @psalm-param AType $x
      *
      * @param callable $f
      * @param mixed $x
@@ -49,16 +55,19 @@ public function __invoke()
     }
 
     /**
-     * @psalm-param callable(AType): BType $f
+     * @template NewAType
+     * @template NewBType
      *
-     * @param callable $f
+     * @param callable(NewAType): NewBType $f
      *
-     * @return Closure
+     * @return Closure(NewAType): NewBType
      */
     public static function on(callable $f): Closure
     {
-        return static function ($x) use ($f) {
-            return (new self($f, $x))();
-        };
+        return
+            /** @param NewAType $x */
+            static function ($x) use ($f) {
+                return (new self($f, $x))();
+            };
     }
 }
diff --git a/src/Combinator/B.php b/src/Combinator/B.php
index 6441e45..573d72c 100644
--- a/src/Combinator/B.php
+++ b/src/Combinator/B.php
@@ -13,20 +13,28 @@
  * @psalm-template AType
  * @psalm-template BType
  * @psalm-template CType
+ *
+ * @psalm-immutable
  */
 final class B extends Combinator
 {
     /**
+     * @psalm-var callable(AType): BType
+     *
      * @var callable
      */
     private $f;
 
     /**
+     * @psalm-var callable(CType): AType
+     *
      * @var callable
      */
     private $g;
 
     /**
+     * @psalm-var CType
+     *
      * @var mixed
      */
     private $x;
@@ -58,16 +66,23 @@ public function __invoke()
     }
 
     /**
-     * @param callable $a
+     * @template AParam
+     * @template AReturn
+     *
+     * @param callable(AParam): AReturn $f
      *
-     * @return Closure
+     * @return Closure(callable(mixed): AParam): Closure(mixed): AReturn
      */
-    public static function on(callable $a): Closure
+    public static function on(callable $f): Closure
     {
-        return static function (callable $b) use ($a): Closure {
-            return static function ($c) use ($a, $b) {
-                return (new self($a, $b, $c))();
+        return
+            /** @param callable(mixed): AParam $g */
+            static function (callable $g) use ($f): Closure {
+                return
+                    /** @param mixed $x */
+                    static function ($x) use ($f, $g) {
+                        return (new self($f, $g, $x))();
+                    };
             };
-        };
     }
 }
diff --git a/src/Combinator/C.php b/src/Combinator/C.php
index f2ade47..3235a4d 100644
--- a/src/Combinator/C.php
+++ b/src/Combinator/C.php
@@ -13,20 +13,28 @@
  * @psalm-template AType
  * @psalm-template BType
  * @psalm-template CType
+ *
+ * @psalm-immutable
  */
 final class C extends Combinator
 {
     /**
+     * @psalm-var callable(AType): callable(BType): CType
+     *
      * @var callable
      */
     private $f;
 
     /**
+     * @psalm-var BType
+     *
      * @var mixed
      */
     private $x;
 
     /**
+     * @psalm-var AType
+     *
      * @var mixed
      */
     private $y;
@@ -51,6 +59,8 @@ public function __construct(callable $f, $x, $y)
 
     /**
      * @psalm-return CType
+     *
+     * @return mixed
      */
     public function __invoke()
     {
@@ -58,16 +68,26 @@ public function __invoke()
     }
 
     /**
-     * @param callable $a
+     * @template NewAType
+     * @template NewBType
+     * @template NewCType
+     *
+     * @psalm-param callable(NewAType): callable(NewBType): NewCType $f
+     *
+     * @param callable(NewAType): NewCType $f
      *
-     * @return Closure
+     * @return Closure(NewBType): Closure(NewAType): NewCType
      */
-    public static function on(callable $a): Closure
+    public static function on(callable $f): Closure
     {
-        return static function ($b) use ($a): Closure {
-            return static function ($c) use ($a, $b) {
-                return (new self($a, $b, $c))();
+        return
+            /** @param NewBType $x */
+            static function ($x) use ($f): Closure {
+                return
+                /** @param NewAType $y */
+                static function ($y) use ($f, $x) {
+                    return (new self($f, $x, $y))();
+                };
             };
-        };
     }
 }
diff --git a/src/Combinator/D.php b/src/Combinator/D.php
index 2797e09..46775ac 100644
--- a/src/Combinator/D.php
+++ b/src/Combinator/D.php
@@ -14,48 +14,58 @@
  * @psalm-template BType
  * @psalm-template CType
  * @psalm-template DType
+ *
+ * @psalm-immutable
  */
 final class D extends Combinator
 {
     /**
+     * @psalm-var callable(AType): callable(CType): DType
+     *
      * @var callable
      */
-    private $f;
+    private $a;
 
     /**
-     * @var callable
+     * @psalm-var AType
+     *
+     * @var mixed
      */
-    private $g;
+    private $b;
 
     /**
-     * @var mixed
+     * @psalm-var callable(BType): CType
+     *
+     * @var callable
      */
-    private $x;
+    private $c;
 
     /**
+     * @psalm-var BType
+     *
      * @var mixed
      */
-    private $y;
+    private $d;
 
     /**
      * D constructor.
      *
-     * @psalm-param callable(AType): callable(CType): DType $f
-     * @psalm-param AType $x
-     * @psalm-param callable(BType): CType $g
-     * @psalm-param BType $y
+     * @psalm-param callable(AType): callable(CType): DType $a
+     * @psalm-param AType $b
+     * @psalm-param callable(BType): CType $c
+     * @psalm-param BType $d
      *
-     * @param callable $f
-     * @param callable $g
-     * @param mixed $x
-     * @param mixed $y
+     * @param callable $a
+     * @param mixed $b
+     * @param callable $c
+     * @param mixed $d
      */
-    public function __construct(callable $f, $x, callable $g, $y)
+    public function __construct(callable $a, $b, callable $c, $d)
     {
-        $this->f = $f;
-        $this->x = $x;
-        $this->g = $g;
-        $this->y = $y;
+        $this->a = $a;
+        $this->b = $b;
+        $this->c = $c;
+        $this->d = $d;
     }
 
     /**
@@ -63,22 +73,35 @@ public function __construct(callable $f, $x, callable $g, $y)
      */
     public function __invoke()
     {
-        return (($this->f)($this->x))(($this->g)($this->y));
+        return (($this->a)($this->b))(($this->c)($this->d));
     }
 
     /**
-     * @param callable $a
+     * @template NewAType
+     * @template NewBType
+     * @template NewCType
+     * @template NewDType
+     *
+     * @psalm-param callable(NewAType): callable(NewCType): NewDType $f
+     *
+     * @param callable(NewCType): NewDType $f
      *
-     * @return Closure
+     * @return Closure(NewAType): Closure(Closure(NewBType): NewCType): Closure(NewBType): NewDType
      */
-    public static function on(callable $a): Closure
+    public static function on(callable $f): Closure
     {
-        return static function ($b) use ($a): Closure {
-            return static function (callable $c) use ($a, $b): Closure {
-                return static function ($d) use ($a, $b, $c) {
-                    return (new self($a, $b, $c, $d))();
-                };
+        return
+            /** @param NewAType $x */
+            static function ($x) use ($f): Closure {
+                return
+                    /** @param callable(NewBType): NewCType $g */
+                    static function (callable $g) use ($f, $x): Closure {
+                        return
+                            /** @param NewBType $y */
+                            static function ($y) use ($f, $x, $g) {
+                                return (new self($f, $x, $g, $y))();
+                            };
+                    };
             };
-        };
     }
 }
diff --git a/src/Combinator/E.php b/src/Combinator/E.php
index db8d405..b6647d1 100644
--- a/src/Combinator/E.php
+++ b/src/Combinator/E.php
@@ -15,30 +15,44 @@
  * @psalm-template CType
  * @psalm-template DType
  * @psalm-template EType
+ *
+ * @psalm-immutable
+ *
+ * phpcs:disable Generic.Files.LineLength.TooLong
  */
 final class E extends Combinator
 {
     /**
+     * @psalm-var callable(AType): callable(DType): EType
+     *
      * @var callable
      */
     private $f;
 
     /**
+     * @psalm-var callable(BType): callable(CType): DType
+     *
      * @var callable
      */
     private $g;
 
     /**
+     * @psalm-var AType
+     *
      * @var mixed
      */
     private $x;
 
     /**
+     * @psalm-var BType
+     *
      * @var mixed
      */
     private $y;
 
     /**
+     * @psalm-var CType
+     *
      * @var mixed
      */
     private $z;
@@ -72,26 +86,42 @@ public function __construct(callable $f, $x, callable $g, $y, $z)
      */
     public function __invoke()
     {
-        // Demander à Danny :-)
-        // const E = a => b => c => d => e => a(b)(c(d)(e))
         return (($this->f)($this->x))((($this->g)($this->y))($this->z));
     }
 
     /**
-     * @param callable $a
+     * @template NewAType
+     * @template NewBType
+     * @template NewCType
+     * @template NewDType
+     * @template NewEType
+     *
+     * @psalm-param callable(NewAType): callable(NewDType): NewEType $f
+     *
+     * @param callable $f
+     *
+     * @psalm-return Closure(NewAType): Closure(callable(NewBType): callable(NewCType): NewDType): Closure(NewBType): Closure(NewCType)
      *
      * @return Closure
      */
-    public static function on(callable $a): Closure
+    public static function on(callable $f): Closure
     {
-        return static function ($b) use ($a): Closure {
-            return static function (callable $c) use ($a, $b): Closure {
-                return static function ($d) use ($a, $b, $c): Closure {
-                    return static function ($e) use ($a, $b, $c, $d) {
-                        return (new self($a, $b, $c, $d, $e))();
+        return
+            /** @param NewAType $x */
+            static function ($x) use ($f): Closure {
+                return
+                    /** @psalm-param callable(NewBType): callable(NewCType): NewDType $g */
+                    static function (callable $g) use ($f, $x): Closure {
+                        return
+                            /** @param NewBType $y */
+                            static function ($y) use ($f, $x, $g): Closure {
+                                return
+                                    /** @param NewCType $z */
+                                    static function ($z) use ($f, $x, $g, $y) {
+                                        return (new self($f, $x, $g, $y, $z))();
+                                    };
+                            };
                     };
-                };
             };
-        };
     }
 }
diff --git a/src/Combinator/F.php b/src/Combinator/F.php
index c849bdc..ac24701 100644
--- a/src/Combinator/F.php
+++ b/src/Combinator/F.php
@@ -13,20 +13,28 @@
  * @psalm-template AType
  * @psalm-template BType
  * @psalm-template CType
+ *
+ * @psalm-immutable
  */
 final class F extends Combinator
 {
     /**
+     * @psalm-var callable(BType): callable(AType): CType
+     *
      * @var callable
      */
     private $f;
 
     /**
+     * @psalm-var AType
+     *
      * @var mixed
      */
     private $x;
 
     /**
+     * @psalm-var BType
+     *
      * @var mixed
      */
     private $y;
@@ -58,16 +66,26 @@ public function __invoke()
     }
 
     /**
-     * @param callable $a
+     * @template NewAType
+     * @template NewBType
+     * @template NewCType
+     *
+     * @psalm-param NewAType $a
+     *
+     * @param mixed $a
      *
-     * @return Closure
+     * @return Closure(NewBType): Closure(callable(NewBType): callable(NewAType): NewCType): NewCType
      */
     public static function on($a): Closure
     {
-        return static function ($b) use ($a): Closure {
-            return static function (callable $c) use ($a, $b) {
-                return (new self($a, $b, $c))();
+        return
+            /** @param NewBType $b */
+            static function ($b) use ($a): Closure {
+                return
+                    /** @psalm-param callable(NewBType): callable(NewAType): NewCType $c */
+                    static function (callable $c) use ($a, $b) {
+                        return (new self($a, $b, $c))();
+                    };
             };
-        };
     }
 }
diff --git a/src/Combinator/G.php b/src/Combinator/G.php
index fad7e09..263d468 100644
--- a/src/Combinator/G.php
+++ b/src/Combinator/G.php
@@ -14,25 +14,35 @@
  * @psalm-template BType
  * @psalm-template CType
  * @psalm-template DType
+ *
+ * @psalm-immutable
  */
 final class G extends Combinator
 {
     /**
+     * @psalm-var callable(AType): callable(BType): CType
+     *
      * @var callable
      */
     private $f;
 
     /**
+     * @psalm-var callable(DType): BType
+     *
      * @var callable
      */
     private $g;
 
     /**
+     * @psalm-var DType
+     *
      * @var mixed
      */
     private $x;
 
     /**
+     * @psalm-var AType
+     *
      * @var mixed
      */
     private $y;
@@ -67,18 +77,31 @@ public function __invoke()
     }
 
     /**
-     * @param callable $a
+     * @template NewAType
+     * @template NewBType
+     * @template NewCType
+     * @template NewDType
+     *
+     * @psalm-param callable(NewAType): callable(NewBType): NewCType $f
+     *
+     * @param callable $f
      *
-     * @return Closure
+     * @return Closure(callable(NewDType): NewBType): Closure(NewDType): Closure(NewAType): NewCType
      */
-    public static function on(callable $a): Closure
+    public static function on(callable $f): Closure
     {
-        return static function (callable $b) use ($a): Closure {
-            return static function ($c) use ($a, $b): Closure {
-                return static function ($d) use ($a, $b, $c) {
-                    return (new self($a, $b, $c, $d))();
-                };
+        return
+            /** @param callable(NewDType): NewBType $g */
+            static function (callable $g) use ($f): Closure {
+                return
+                    /** @param NewDType $x */
+                    static function ($x) use ($f, $g): Closure {
+                        return
+                            /** @param NewAType $y */
+                            static function ($y) use ($f, $g, $x) {
+                                return (new self($f, $g, $x, $y))();
+                            };
+                    };
             };
-        };
     }
 }
diff --git a/src/Combinator/H.php b/src/Combinator/H.php
index 3fb80ff..7139967 100644
--- a/src/Combinator/H.php
+++ b/src/Combinator/H.php
@@ -13,20 +13,28 @@
  * @psalm-template AType
  * @psalm-template BType
  * @psalm-template CType
+ *
+ * @psalm-immutable
  */
 final class H extends Combinator
 {
     /**
+     * @psalm-var callable(AType): callable(BType): callable(AType): CType
+     *
      * @var callable
      */
     private $f;
 
     /**
+     * @psalm-var AType
+     *
      * @var mixed
      */
     private $x;
 
     /**
+     * @psalm-var BType
+     *
      * @var mixed
      */
     private $y;
@@ -58,16 +66,26 @@ public function __invoke()
     }
 
     /**
-     * @param callable $a
+     * @template NewAType
+     * @template NewBType
+     * @template NewCType
+     *
+     * @psalm-param callable(NewAType): callable(NewBType): callable(NewAType): NewCType $f
+     *
+     * @param callable $f
      *
-     * @return Closure
+     * @return Closure(NewAType): Closure(NewBType): NewCType
      */
-    public static function on(callable $a): Closure
+    public static function on(callable $f): Closure
     {
-        return static function ($b) use ($a): Closure {
-            return static function ($c) use ($a, $b) {
-                return (new self($a, $b, $c))();
+        return
+            /** @param NewAType $x */
+            static function ($x) use ($f): Closure {
+                return
+                    /** @param NewBType $y */
+                    static function ($y) use ($f, $x) {
+                        return (new self($f, $x, $y))();
+                    };
             };
-        };
     }
 }
diff --git a/src/Combinator/I.php b/src/Combinator/I.php
index dc9ad9b..7bb99ca 100644
--- a/src/Combinator/I.php
+++ b/src/Combinator/I.php
@@ -10,10 +10,14 @@
  * Class I.
  *
  * @psalm-template AType
+ *
+ * @psalm-immutable
  */
 final class I extends Combinator
 {
     /**
+     * @psalm-var AType
+     *
      * @var mixed
      */
     private $x;
@@ -39,12 +43,16 @@ public function __invoke()
     }
 
     /**
-     * @param callable $a
+     * @template NewAType
+     *
+     * @psalm-param NewAType $x
+     *
+     * @param mixed $x
      *
-     * @return mixed
+     * @return NewAType
      */
-    public static function on($a)
+    public static function on($x)
     {
-        return (new self($a))();
+        return (new self($x))();
     }
 }
diff --git a/src/Combinator/J.php b/src/Combinator/J.php
index 3586a4c..66368e3 100644
--- a/src/Combinator/J.php
+++ b/src/Combinator/J.php
@@ -12,25 +12,35 @@
  *
  * @psalm-template AType
  * @psalm-template BType
+ *
+ * @psalm-immutable
  */
 final class J extends Combinator
 {
     /**
+     * @psalm-var callable(AType): callable(BType): BType
+     *
      * @var callable
      */
     private $f;
 
     /**
+     * @psalm-var AType
+     *
      * @var mixed
      */
     private $x;
 
     /**
+     * @psalm-var BType
+     *
      * @var mixed
      */
     private $y;
 
     /**
+     * @psalm-var AType
+     *
      * @var mixed
      */
     private $z;
@@ -65,18 +75,31 @@ public function __invoke()
     }
 
     /**
-     * @param callable $a
+     * @template NewAType
+     * @template NewBType
+     *
+     * @psalm-param callable(NewAType): callable(NewBType): NewBType $f
+     *
+     * @param callable $f
+     *
+     * @psalm-return Closure(NewAType): Closure(NewBType): Closure(NewAType): NewBType
      *
-     * @return Closure
+     * @return NewBType
      */
-    public static function on(callable $a): Closure
+    public static function on(callable $f): Closure
     {
-        return static function ($b) use ($a): Closure {
-            return static function ($c) use ($a, $b): Closure {
-                return static function ($d) use ($a, $b, $c) {
-                    return (new self($a, $b, $c, $d))();
-                };
+        return
+            /** @param NewAType $x */
+            static function ($x) use ($f): Closure {
+                return
+                    /** @param NewBType $y */
+                    static function ($y) use ($f, $x): Closure {
+                        return
+                            /** @param NewAType $z */
+                            static function ($z) use ($f, $x, $y) {
+                                return (new self($f, $x, $y, $z))();
+                            };
+                    };
             };
-        };
     }
 }
diff --git a/src/Combinator/K.php b/src/Combinator/K.php
index d239fbb..cdcbd40 100644
--- a/src/Combinator/K.php
+++ b/src/Combinator/K.php
@@ -12,15 +12,21 @@
  *
  * @psalm-template AType
  * @psalm-template BType
+ *
+ * @psalm-immutable
  */
 final class K extends Combinator
 {
     /**
+     * @psalm-var AType
+     *
      * @var mixed
      */
     private $x;
 
     /**
+     * @psalm-var BType
+     *
      * @var mixed
      */
     private $y;
@@ -41,7 +47,9 @@ public function __construct($x, $y)
     }
 
     /**
-     * @psalm-return BType
+     * @psalm-return AType
+     *
+     * @return mixed
      */
     public function __invoke()
     {
@@ -49,14 +57,21 @@ public function __invoke()
     }
 
     /**
-     * @param callable $a
+     * @template NewAType
+     * @template NewBType
+     *
+     * @psalm-param NewAType $x
+     *
+     * @param mixed $x
      *
-     * @return Closure
+     * @return Closure(NewBType): NewAType
      */
-    public static function on($a): Closure
+    public static function on($x): Closure
     {
-        return static function ($b) use ($a) {
-            return (new self($a, $b))();
-        };
+        return
+            /** @param NewBType $y */
+            static function ($y) use ($x) {
+                return (new self($x, $y))();
+            };
     }
 }
diff --git a/src/Combinator/Ki.php b/src/Combinator/Ki.php
index 649a1d8..d506cf2 100644
--- a/src/Combinator/Ki.php
+++ b/src/Combinator/Ki.php
@@ -10,17 +10,23 @@
 /**
  * Class Ki.
  *
- * @psalm-template XType
- * @psalm-template YType
+ * @psalm-template AType
+ * @psalm-template BType
+ *
+ * @psalm-immutable
  */
 final class Ki extends Combinator
 {
     /**
+     * @psalm-var AType
+     *
      * @var mixed
      */
     private $x;
 
     /**
+     * @psalm-var BType
+     *
      * @var mixed
      */
     private $y;
@@ -28,8 +34,8 @@ final class Ki extends Combinator
     /**
      * Ki constructor.
      *
-     * @psalm-param XType $x
-     * @psalm-param YType $y
+     * @psalm-param AType $x
+     * @psalm-param BType $y
      *
      * @param mixed $x
      * @param mixed $y
@@ -41,7 +47,7 @@ public function __construct($x, $y)
     }
 
     /**
-     * @psalm-return YType
+     * @psalm-return BType
      */
     public function __invoke()
     {
@@ -49,14 +55,21 @@ public function __invoke()
     }
 
     /**
-     * @param callable $a
+     * @template NewAType
+     * @template NewBType
+     *
+     * @psalm-param NewAType $x
+     *
+     * @param mixed $x
      *
-     * @return Closure
+     * @return Closure(NewBType): NewBType
      */
-    public static function on($a): Closure
+    public static function on($x): Closure
     {
-        return static function ($b) use ($a) {
-            return (new self($a, $b))();
-        };
+        return
+            /** @param NewBType $y */
+            static function ($y) use ($x) {
+                return (new self($x, $y))();
+            };
     }
 }
diff --git a/src/Combinator/L.php b/src/Combinator/L.php
index 4ea7d74..853941e 100644
--- a/src/Combinator/L.php
+++ b/src/Combinator/L.php
@@ -10,7 +10,7 @@
 /**
  * Class L.
  *
- * @psalm-template ResultType
+ * @psalm-immutable
  */
 final class L extends Combinator
 {
@@ -37,8 +37,6 @@ public function __construct(callable $f, callable $g)
     }
 
     /**
-     * @psalm-return ResultType
-     *
      * @return mixed
      */
     public function __invoke()
@@ -47,6 +45,8 @@ public function __invoke()
     }
 
     /**
+     * @psalm-suppress MissingClosureReturnType
+     *
      * @param callable $a
      *
      * @return Closure
diff --git a/src/Combinator/M.php b/src/Combinator/M.php
index a596e3f..87b8225 100644
--- a/src/Combinator/M.php
+++ b/src/Combinator/M.php
@@ -4,13 +4,12 @@
 
 namespace loophp\combinator\Combinator;
 
-use Closure;
 use loophp\combinator\Combinator;
 
 /**
  * Class M.
  *
- * @psalm-template ResultType
+ * @psalm-immutable
  */
 final class M extends Combinator
 {
@@ -32,7 +31,7 @@ public function __construct(callable $f)
     }
 
     /**
-     * @psalm-return ResultType
+     * @return mixed
      */
     public function __invoke()
     {
@@ -42,7 +41,7 @@ public function __invoke()
     /**
      * @param callable $a
      *
-     * @return Closure
+     * @return mixed
      */
     public static function on(callable $a)
     {
diff --git a/src/Combinator/O.php b/src/Combinator/O.php
index bdd6690..f84c9fa 100644
--- a/src/Combinator/O.php
+++ b/src/Combinator/O.php
@@ -12,15 +12,21 @@
  *
  * @psalm-template AType
  * @psalm-template BType
+ *
+ * @psalm-immutable
  */
 final class O extends Combinator
 {
     /**
+     * @psalm-var callable(callable(AType): BType): AType
+     *
      * @var callable
      */
     private $f;
 
     /**
+     * @psalm-var callable(AType): BType
+     *
      * @var callable
      */
     private $g;
@@ -28,6 +34,9 @@ final class O extends Combinator
     /**
      * O constructor.
      *
+     * @psalm-param callable(callable(AType): BType): AType $f
+     * @psalm-param callable(AType): BType $g
+     *
      * @param callable(callable(AType): BType): AType $f
      * @param callable(AType): BType $g
      */
@@ -46,14 +55,23 @@ public function __invoke()
     }
 
     /**
-     * @param callable $a
+     * @template NewAType
+     * @template NewBType
+     *
+     * @psalm-param callable(callable(NewAType): NewBType): NewAType $f
+     *
+     * @param callable $f
+     *
+     * @psalm-return Closure(callable(NewAType): NewBType): NewBType
      *
      * @return Closure
      */
-    public static function on(callable $a): Closure
+    public static function on(callable $f): Closure
     {
-        return static function (callable $b) use ($a) {
-            return (new self($a, $b))();
-        };
+        return
+            /** @param callable(NewAType): NewBType $g */
+            static function (callable $g) use ($f) {
+                return (new self($f, $g))();
+            };
     }
 }
diff --git a/src/Combinator/Omega.php b/src/Combinator/Omega.php
index 9d0aaea..ef4e6ce 100644
--- a/src/Combinator/Omega.php
+++ b/src/Combinator/Omega.php
@@ -10,7 +10,7 @@
 /**
  * Class Omega.
  *
- * @psalm-template ResultType
+ * @psalm-immutable
  */
 final class Omega extends Combinator
 {
@@ -22,8 +22,6 @@ final class Omega extends Combinator
     /**
      * Omega constructor.
      *
-     * @psalm-param callable $f
-     *
      * @param callable $f
      */
     public function __construct(callable $f)
@@ -32,7 +30,11 @@ public function __construct(callable $f)
     }
 
     /**
-     * @psalm-return ResultType
+     * @psalm-return mixed
+     *
+     * @return mixed
+     *
+     * @psalm-suppress MixedFunctionCall
      */
     public function __invoke()
     {
@@ -40,12 +42,12 @@ public function __invoke()
     }
 
     /**
-     * @param callable $a
+     * @param callable $f
      *
-     * @return Closure
+     * @return mixed
      */
-    public static function on(callable $a)
+    public static function on(callable $f)
     {
-        return (new self($a))();
+        return (new self($f))();
     }
 }
diff --git a/src/Combinator/Phoenix.php b/src/Combinator/Phoenix.php
index de78b59..defc451 100644
--- a/src/Combinator/Phoenix.php
+++ b/src/Combinator/Phoenix.php
@@ -14,25 +14,37 @@
  * @psalm-template BType
  * @psalm-template CType
  * @psalm-template DType
+ *
+ * @psalm-immutable
+ *
+ * phpcs:disable Generic.Files.LineLength.TooLong
  */
 final class Phoenix extends Combinator
 {
     /**
+     * @psalm-var callable(AType): callable(BType): CType
+     *
      * @var callable
      */
     private $f;
 
     /**
+     * @psalm-var callable(DType): AType
+     *
      * @var callable
      */
     private $g;
 
     /**
+     * @psalm-var callable(DType): BType
+     *
      * @var callable
      */
     private $h;
 
     /**
+     * @psalm-var DType
+     *
      * @var mixed
      */
     private $x;
@@ -67,18 +79,33 @@ public function __invoke()
     }
 
     /**
-     * @param callable $a
+     * @template NewAType
+     * @template NewBType
+     * @template NewCType
+     * @template NewDType
+     *
+     * @psalm-param callable(NewAType): callable(NewBType): NewCType $f
+     *
+     * @param callable $f
+     *
+     * @psalm-return Closure(callable(NewDType): NewAType): Closure(callable(NewDType): NewBType): Closure(NewDType): NewCType
      *
      * @return Closure
      */
-    public static function on(callable $a): Closure
+    public static function on(callable $f): Closure
     {
-        return static function (callable $b) use ($a): Closure {
-            return static function (callable $c) use ($a, $b): Closure {
-                return static function ($d) use ($a, $b, $c) {
-                    return (new self($a, $b, $c, $d))();
-                };
+        return
+            /** @psalm-param callable(NewDType): NewAType $g */
+            static function (callable $g) use ($f): Closure {
+                return
+                    /** @psalm-param callable(NewDType): NewBType $h */
+                    static function (callable $h) use ($f, $g): Closure {
+                        return
+                            /** @psalm-param NewDType $x  */
+                            static function ($x) use ($f, $g, $h) {
+                                return (new self($f, $g, $h, $x))();
+                            };
+                    };
             };
-        };
     }
 }
diff --git a/src/Combinator/Psi.php b/src/Combinator/Psi.php
index 05e0894..82d905f 100644
--- a/src/Combinator/Psi.php
+++ b/src/Combinator/Psi.php
@@ -13,25 +13,35 @@
  * @psalm-template AType
  * @psalm-template BType
  * @psalm-template CType
+ *
+ * @psalm-immutable
  */
 final class Psi extends Combinator
 {
     /**
+     * @psalm-var callable(AType): callable(AType): BType
+     *
      * @var callable
      */
     private $f;
 
     /**
+     * @psalm-var callable(CType): AType
+     *
      * @var callable
      */
     private $g;
 
     /**
+     * @psalm-var CType
+     *
      * @var mixed
      */
     private $x;
 
     /**
+     * @psalm-var CType
+     *
      * @var mixed
      */
     private $y;
@@ -66,18 +76,32 @@ public function __invoke()
     }
 
     /**
-     * @param callable $a
+     * @template NewAType
+     * @template NewBType
+     * @template NewCType
+     *
+     * @psalm-param callable(NewAType): callable(NewAType): NewBType $f
+     *
+     * @param callable $f
+     *
+     * @psalm-return Closure(callable(NewCType): NewAType): Closure(NewCType): Closure(NewCType): NewBType
      *
      * @return Closure
      */
-    public static function on(callable $a): Closure
+    public static function on(callable $f): Closure
     {
-        return static function (callable $b) use ($a): Closure {
-            return static function ($c) use ($a, $b): Closure {
-                return static function ($d) use ($a, $b, $c) {
-                    return (new self($a, $b, $c, $d))();
-                };
+        return
+            /** @psalm-param callable(NewCType): NewAType $g */
+            static function (callable $g) use ($f): Closure {
+                return
+                    /** @psalm-param NewCType $x */
+                    static function ($x) use ($f, $g): Closure {
+                        return
+                            /** @psalm-param NewCType $y */
+                            static function ($y) use ($f, $g, $x) {
+                                return (new self($f, $g, $x, $y))();
+                            };
+                    };
             };
-        };
     }
 }
diff --git a/src/Combinator/Q.php b/src/Combinator/Q.php
index 251b6a2..f0cc467 100644
--- a/src/Combinator/Q.php
+++ b/src/Combinator/Q.php
@@ -13,20 +13,28 @@
  * @psalm-template AType
  * @psalm-template BType
  * @psalm-template CType
+ *
+ * @psalm-immutable
  */
 final class Q extends Combinator
 {
     /**
+     * @psalm-var callable(AType): BType
+     *
      * @var callable
      */
     private $f;
 
     /**
+     * @psalm-var callable(BType): CType
+     *
      * @var callable
      */
     private $g;
 
     /**
+     * @psalm-var AType
+     *
      * @var mixed
      */
     private $x;
@@ -58,16 +66,28 @@ public function __invoke()
     }
 
     /**
-     * @param callable $a
+     * @template NewAType
+     * @template NewBType
+     * @template NewCType
+     *
+     * @psalm-param callable(NewAType): NewBType $f
+     *
+     * @param callable $f
+     *
+     * @psalm-return Closure(callable(NewBType): NewCType): Closure(NewAType): NewCType
      *
      * @return Closure
      */
-    public static function on(callable $a): Closure
+    public static function on(callable $f): Closure
     {
-        return static function (callable $b) use ($a): Closure {
-            return static function ($c) use ($a, $b) {
-                return (new self($a, $b, $c))();
+        return
+            /** @psalm-param callable(NewBType): NewCType $g */
+            static function (callable $g) use ($f): Closure {
+                return
+                    /** @psalm-param NewAType $x */
+                    static function ($x) use ($f, $g) {
+                        return (new self($f, $g, $x))();
+                    };
             };
-        };
     }
 }
diff --git a/src/Combinator/R.php b/src/Combinator/R.php
index 7a0e34f..595cda7 100644
--- a/src/Combinator/R.php
+++ b/src/Combinator/R.php
@@ -13,20 +13,28 @@
  * @psalm-template AType
  * @psalm-template BType
  * @psalm-template CType
+ *
+ * @psalm-immutable
  */
 final class R extends Combinator
 {
     /**
+     * @psalm-var callable(BType): callable(AType): CType
+     *
      * @var callable
      */
     private $f;
 
     /**
+     * @psalm-var AType
+     *
      * @var mixed
      */
     private $x;
 
     /**
+     * @psalm-var BType
+     *
      * @var mixed
      */
     private $y;
@@ -58,16 +66,28 @@ public function __invoke()
     }
 
     /**
-     * @param callable $a
+     * @template NewAType
+     * @template NewBType
+     * @template NewCType
+     *
+     * @psalm-param NewAType $x
+     *
+     * @param NewAType $x
+     *
+     * @psalm-return Closure(callable(NewBType): callable(NewAType): NewCType): Closure(NewBType): NewCType
      *
      * @return Closure
      */
-    public static function on($a): Closure
+    public static function on($x): Closure
     {
-        return static function (callable $b) use ($a): Closure {
-            return static function ($c) use ($a, $b) {
-                return (new self($a, $b, $c))();
+        return
+            /** @psalm-param callable(NewBType): callable(NewAType): NewCType $f */
+            static function (callable $f) use ($x): Closure {
+                return
+                    /** @psalm-param NewBType $y */
+                    static function ($y) use ($x, $f) {
+                        return (new self($x, $f, $y))();
+                    };
             };
-        };
     }
 }
diff --git a/src/Combinator/S.php b/src/Combinator/S.php
index 97d83a3..8daf6c2 100644
--- a/src/Combinator/S.php
+++ b/src/Combinator/S.php
@@ -13,20 +13,28 @@
  * @psalm-template AType
  * @psalm-template BType
  * @psalm-template CType
+ *
+ * @psalm-immutable
  */
 final class S extends Combinator
 {
     /**
+     * @psalm-var callable(AType): callable(BType): CType
+     *
      * @var callable
      */
     private $f;
 
     /**
+     * @psalm-var callable(AType): BType
+     *
      * @var callable
      */
     private $g;
 
     /**
+     * @psalm-var AType
+     *
      * @var mixed
      */
     private $x;
@@ -58,16 +66,28 @@ public function __invoke()
     }
 
     /**
-     * @param callable $a
+     * @template NewAType
+     * @template NewBType
+     * @template NewCType
+     *
+     * @psalm-param callable(NewAType): callable(NewBType): NewCType $f
+     *
+     * @param callable $f
+     *
+     * @psalm-return Closure(callable(NewAType): NewBType): Closure(NewAType): NewCType
      *
      * @return Closure
      */
-    public static function on(callable $a): Closure
+    public static function on(callable $f): Closure
     {
-        return static function (callable $b) use ($a): Closure {
-            return static function ($c) use ($a, $b) {
-                return (new self($a, $b, $c))();
+        return
+            /** @psalm-param callable(NewAType): NewBType $g */
+            static function (callable $g) use ($f): Closure {
+                return
+                    /** @psalm-param NewAType $x */
+                    static function ($x) use ($f, $g) {
+                        return (new self($f, $g, $x))();
+                    };
             };
-        };
     }
 }
diff --git a/src/Combinator/T.php b/src/Combinator/T.php
index f726151..672dc13 100644
--- a/src/Combinator/T.php
+++ b/src/Combinator/T.php
@@ -12,15 +12,21 @@
  *
  * @psalm-template AType
  * @psalm-template BType
+ *
+ * @psalm-immutable
  */
 final class T extends Combinator
 {
     /**
+     * @psalm-var callable(AType): BType
+     *
      * @var callable
      */
     private $f;
 
     /**
+     * @psalm-var AType
+     *
      * @var mixed
      */
     private $x;
@@ -49,14 +55,23 @@ public function __invoke()
     }
 
     /**
-     * @param callable $a
+     * @template NewAType
+     * @template NewBType
+     *
+     * @psalm-param NewAType $x
+     *
+     * @param mixed $x
+     *
+     * @psalm-return Closure(callable(NewAType): NewBType): NewBType
      *
      * @return Closure
      */
-    public static function on($a): Closure
+    public static function on($x): Closure
     {
-        return static function (callable $b) use ($a) {
-            return (new self($a, $b))();
+        return
+            /** @psalm-param callable(NewAType): NewBType $f */
+            static function (callable $f) use ($x) {
+            return (new self($x, $f))();
         };
     }
 }
diff --git a/src/Combinator/U.php b/src/Combinator/U.php
index 91d1029..1a33516 100644
--- a/src/Combinator/U.php
+++ b/src/Combinator/U.php
@@ -10,11 +10,13 @@
 /**
  * Class U.
  *
- * @psalm-template ResultType
+ * @psalm-immutable
  */
 final class U extends Combinator
 {
     /**
+     * @psalm-var callable(callable): callable
+     *
      * @var callable
      */
     private $f;
@@ -27,8 +29,7 @@ final class U extends Combinator
     /**
      * U constructor.
      *
-     * @psalm-param callable $f
-     * @psalm-param callable $g
+     * @psalm-param callable(callable): callable $f
      *
      * @param callable $f
      * @param callable $g
@@ -40,7 +41,7 @@ public function __construct(callable $f, callable $g)
     }
 
     /**
-     * @psalm-return ResultType
+     * @return mixed
      */
     public function __invoke()
     {
@@ -51,6 +52,10 @@ public function __invoke()
     }
 
     /**
+     * @psalm-suppress MissingClosureReturnType
+     * @psalm-suppress MissingClosureParamType
+     * @psalm-suppress MixedArgumentTypeCoercion
+     *
      * @param callable $a
      *
      * @return Closure
diff --git a/src/Combinator/V.php b/src/Combinator/V.php
index 7a4594f..c562f2c 100644
--- a/src/Combinator/V.php
+++ b/src/Combinator/V.php
@@ -13,20 +13,28 @@
  * @psalm-template AType
  * @psalm-template BType
  * @psalm-template CType
+ *
+ * @psalm-immutable
  */
 final class V extends Combinator
 {
     /**
+     * @psalm-var callable(AType): callable(BType): CType
+     *
      * @var callable
      */
     private $f;
 
     /**
+     * @psalm-var AType
+     *
      * @var mixed
      */
     private $x;
 
     /**
+     * @psalm-var BType
+     *
      * @var mixed
      */
     private $y;
@@ -58,16 +66,28 @@ public function __invoke()
     }
 
     /**
-     * @param callable $a
+     * @template NewAType
+     * @template NewBType
+     * @template NewCType
+     *
+     * @psalm-param NewAType $x
+     *
+     * @param mixed $x
+     *
+     * @psalm-return Closure(NewBType): Closure(callable(NewAType): callable(NewBType): NewCType): NewCType
      *
      * @return Closure
      */
-    public static function on($a): Closure
+    public static function on($x): Closure
     {
-        return static function ($b) use ($a): Closure {
-            return static function (callable $c) use ($a, $b) {
-                return (new self($a, $b, $c))();
+        return
+            /** @psalm-param NewBType $y */
+            static function ($y) use ($x): Closure {
+                return
+                    /** @psalm-param callable(NewAType): callable(NewBType): NewCType $f */
+                    static function (callable $f) use ($x, $y) {
+                        return (new self($x, $y, $f))();
+                    };
             };
-        };
     }
 }
diff --git a/src/Combinator/W.php b/src/Combinator/W.php
index 64397b8..62af4d5 100644
--- a/src/Combinator/W.php
+++ b/src/Combinator/W.php
@@ -12,15 +12,21 @@
  *
  * @psalm-template AType
  * @psalm-template BType
+ *
+ * @psalm-immutable
  */
 final class W extends Combinator
 {
     /**
+     * @psalm-var callable(AType): callable(AType): BType
+     *
      * @var callable
      */
     private $f;
 
     /**
+     * @psalm-var AType
+     *
      * @var mixed
      */
     private $x;
@@ -49,14 +55,23 @@ public function __invoke()
     }
 
     /**
-     * @param callable $a
+     * @template NewAType
+     * @template NewBType
+     *
+     * @psalm-param callable(NewAType): callable(NewAType): NewBType $f
+     *
+     * @param callable $f
+     *
+     * @psalm-return Closure(NewAType): NewBType
      *
      * @return Closure
      */
-    public static function on(callable $a): Closure
+    public static function on(callable $f): Closure
     {
-        return static function ($b) use ($a) {
-            return (new self($a, $b))();
-        };
+        return
+            /** @psalm-param NewAType $x */
+            static function ($x) use ($f) {
+                return (new self($f, $x))();
+            };
     }
 }
diff --git a/src/Combinator/Y.php b/src/Combinator/Y.php
index 076c3cc..f988dd8 100644
--- a/src/Combinator/Y.php
+++ b/src/Combinator/Y.php
@@ -10,11 +10,13 @@
 /**
  * Class Y.
  *
- * @psalm-template ResultType
+ * @psalm-immutable
  */
 final class Y extends Combinator
 {
     /**
+     * @psalm-var callable(callable): Closure
+     *
      * @var callable
      */
     private $f;
@@ -22,7 +24,7 @@ final class Y extends Combinator
     /**
      * Y constructor.
      *
-     * @psalm-param callable $f
+     * @psalm-param callable(callable): Closure $f
      *
      * @param callable $f
      */
@@ -32,24 +34,38 @@ public function __construct(callable $f)
     }
 
     /**
-     * @psalm-return Closure(Closure(callable): mixed): mixed
+     * @psalm-suppress MixedInferredReturnType
+     *
+     * @return Closure
      */
     public function __invoke()
     {
         $callable = $this->f;
 
-        $f = static function (callable $f) use ($callable): Closure {
-            return $callable(
-                static function (...$arguments) use ($f) {
-                    return M::with()($f)(...$arguments);
-                }
-            );
-        };
-
-        return M::with()($f);
+        /**
+         * @psalm-suppress MissingClosureReturnType
+         * @psalm-suppress MissingClosureParamType
+         * @psalm-suppress MixedArgumentTypeCoercion
+         * @psalm-suppress ImpureMethodCall
+         * @psalm-suppress MixedFunctionCall
+         * @psalm-suppress MixedReturnStatement
+         */
+        return M::with()(
+            static function (callable $f) use ($callable): Closure {
+                return $callable(
+                    static function (...$arguments) use ($f) {
+                        return M::with()($f)(...$arguments);
+                    }
+                );
+            }
+        );
     }
 
     /**
+     * @psalm-suppress MissingClosureReturnType
+     * @psalm-suppress MissingClosureParamType
+     * @psalm-suppress MixedArgumentTypeCoercion
+     *
      * @param callable $a
      *
      * @return Closure
diff --git a/src/Combinator/Z.php b/src/Combinator/Z.php
index a4b1eb4..af6676a 100644
--- a/src/Combinator/Z.php
+++ b/src/Combinator/Z.php
@@ -10,7 +10,7 @@
 /**
  * Class Z.
  *
- * @psalm-template ResultType
+ * @psalm-immutable
  */
 final class Z extends Combinator
 {
@@ -32,26 +32,40 @@ public function __construct(callable $f)
     }
 
     /**
-     * @psalm-return Closure(Closure(callable): mixed): mixed
+     * @psalm-suppress MixedInferredReturnType
+     *
+     * @return Closure
      */
     public function __invoke()
     {
         $callable = $this->f;
 
-        $f = static function (callable $f) use ($callable): Closure {
-            return $callable(
-                static function () use ($f) {
-                    return static function (...$arguments) use ($f) {
-                        return M::with()($f)(...$arguments);
-                    };
-                }
-            );
-        };
-
-        return M::with()($f);
+        /**
+         * @psalm-suppress MissingClosureReturnType
+         * @psalm-suppress MissingClosureParamType
+         * @psalm-suppress MixedArgumentTypeCoercion
+         * @psalm-suppress ImpureMethodCall
+         * @psalm-suppress MixedFunctionCall
+         * @psalm-suppress MixedReturnStatement
+         */
+        return M::with()(
+            static function (callable $f) use ($callable): Closure {
+                return $callable(
+                    static function () use ($f) {
+                        return static function (...$arguments) use ($f) {
+                            return M::with()($f)(...$arguments);
+                        };
+                    }
+                );
+            }
+        );
     }
 
     /**
+     * @psalm-suppress MissingClosureReturnType
+     * @psalm-suppress MissingClosureParamType
+     * @psalm-suppress MixedArgumentTypeCoercion
+     *
      * @param callable $a
      *
      * @return Closure
diff --git a/tests/src/BadCombinator.php b/tests/src/BadCombinator.php
index 0f28a27..0b694ba 100644
--- a/tests/src/BadCombinator.php
+++ b/tests/src/BadCombinator.php
@@ -6,6 +6,11 @@
 
 use loophp\combinator\Combinator;
 
+/**
+ * Class BadCombinator.
+ *
+ * @psalm-immutable
+ */
 class BadCombinator extends Combinator
 {
 }
diff --git a/tests/static-analysis/A.php b/tests/static-analysis/A.php
index 0e3c578..abed6dc 100644
--- a/tests/static-analysis/A.php
+++ b/tests/static-analysis/A.php
@@ -8,7 +8,7 @@
 
 /**
  * @param callable(int): string $f
- * @param string $x
+ * @param int $x
  *
  * @return string
  */
diff --git a/tests/static-analysis/K.php b/tests/static-analysis/K.php
index faf13c4..13aaf6b 100644
--- a/tests/static-analysis/K.php
+++ b/tests/static-analysis/K.php
@@ -10,7 +10,7 @@
  * @param string $x
  * @param int $y
  *
- * @return int
+ * @return string
  */
 function test($x, $y)
 {
diff --git a/tests/static-analysis/U.php b/tests/static-analysis/U.php
index 9a45a12..eeeda5f 100644
--- a/tests/static-analysis/U.php
+++ b/tests/static-analysis/U.php
@@ -7,7 +7,7 @@
 use loophp\combinator\Combinator\U as Combinator;
 
 /**
- * @param callable(int): mixed $f
+ * @param callable(callable):callable $f
  * @param callable(int): mixed $g
  *
  * @return mixed
diff --git a/tests/static-analysis/Y.php b/tests/static-analysis/Y.php
index 06b11d1..042ce34 100644
--- a/tests/static-analysis/Y.php
+++ b/tests/static-analysis/Y.php
@@ -4,14 +4,15 @@
 
 namespace loophp\combinator\Tests\StaticAnalysis;
 
+use Closure;
 use loophp\combinator\Combinator\Y as Combinator;
 
 /**
- * @param callable(int): callable $f
+ * @param callable(callable): Closure $f
  *
- * @return callable
+ * @return Closure
  */
-function test(callable $f)
+function test(callable $f): Closure
 {
     return (new Combinator($f))();
 }
diff --git a/tests/static-analysis/Z.php b/tests/static-analysis/Z.php
index 9237705..d538b82 100644
--- a/tests/static-analysis/Z.php
+++ b/tests/static-analysis/Z.php
@@ -4,14 +4,15 @@
 
 namespace loophp\combinator\Tests\StaticAnalysis;
 
+use Closure;
 use loophp\combinator\Combinator\Z as Combinator;
 
 /**
- * @param callable(int): callable $f
+ * @param callable(callable): Closure $f
  *
- * @return callable
+ * @return Closure
  */
-function test(callable $f)
+function test(callable $f): Closure
 {
     return (new Combinator($f))();
 }