diff --git a/composer.json b/composer.json
index 4558879..4109d26 100644
--- a/composer.json
+++ b/composer.json
@@ -24,7 +24,8 @@
         "phpspec/phpspec": "^5.1.2 || ^6.1.1",
         "phpstan/phpstan": "^0.12.5",
         "phpstan/phpstan-strict-rules": "^0.12",
-        "scrutinizer/ocular": "^1.6"
+        "scrutinizer/ocular": "^1.6",
+        "vimeo/psalm": "^3.8"
     },
     "config": {
         "sort-packages": true
diff --git a/grumphp.yml.dist b/grumphp.yml.dist
index 2159c87..5651950 100644
--- a/grumphp.yml.dist
+++ b/grumphp.yml.dist
@@ -4,6 +4,7 @@ imports:
 parameters:
     process_timeout: 600
     extra_tasks:
+        psalm: ~
         phpspec:
             verbose: true
             metadata:
diff --git a/psalm.xml b/psalm.xml
new file mode 100644
index 0000000..9d2a538
--- /dev/null
+++ b/psalm.xml
@@ -0,0 +1,15 @@
+<?xml version="1.0"?>
+<psalm
+    totallyTyped="false"
+    resolveFromConfigFile="true"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xmlns="https://getpsalm.org/schema/config"
+    xsi:schemaLocation="https://getpsalm.org/schema/config vendor/vimeo/psalm/config.xsd"
+>
+    <projectFiles>
+        <directory name="src" />
+        <ignoreFiles>
+            <directory name="vendor" />
+        </ignoreFiles>
+    </projectFiles>
+</psalm>
diff --git a/src/Combinator/A.php b/src/Combinator/A.php
index 31aed1a..b47120a 100644
--- a/src/Combinator/A.php
+++ b/src/Combinator/A.php
@@ -8,6 +8,9 @@
 
 /**
  * Class A.
+ *
+ * @psalm-template XType
+ * @psalm-template ResultType
  */
 final class A extends Combinator
 {
@@ -24,6 +27,9 @@ final class A extends Combinator
     /**
      * A constructor.
      *
+     * @psalm-param callable(XType) : ResultType $f
+     * @psalm-param XType $x
+     *
      * @param callable $f
      * @param mixed $x
      */
@@ -34,6 +40,8 @@ public function __construct(callable $f, $x)
     }
 
     /**
+     * @psalm-return ResultType
+     *
      * @return mixed
      */
     public function __invoke()
diff --git a/src/Combinator/B.php b/src/Combinator/B.php
index 22cf87e..1545a3e 100644
--- a/src/Combinator/B.php
+++ b/src/Combinator/B.php
@@ -8,6 +8,10 @@
 
 /**
  * Class B.
+ *
+ * @psalm-template GOutput
+ * @psalm-template XType
+ * @psalm-template ResultType
  */
 final class B extends Combinator
 {
@@ -29,6 +33,10 @@ final class B extends Combinator
     /**
      * B constructor.
      *
+     * @psalm-param callable(GOutput) : ResultType $f
+     * @psalm-param callable(XType) : GOutput $g
+     * @psalm-param XType $x
+     *
      * @param callable $f
      * @param callable $g
      * @param mixed $x
@@ -41,7 +49,7 @@ public function __construct(callable $f, callable $g, $x)
     }
 
     /**
-     * @return mixed
+     * @psalm-return ResultType
      */
     public function __invoke()
     {
diff --git a/src/Combinator/C.php b/src/Combinator/C.php
index 2a2aa92..b6f1c94 100644
--- a/src/Combinator/C.php
+++ b/src/Combinator/C.php
@@ -8,6 +8,9 @@
 
 /**
  * Class C.
+ *
+ * @psalm-template XType
+ * @psalm-template ResultType
  */
 final class C extends Combinator
 {
@@ -29,6 +32,10 @@ final class C extends Combinator
     /**
      * C constructor.
      *
+     * @psalm-param callable(XType) : ResultType $f
+     * @psalm-param XType $x
+     * @psalm-param XType $y
+     *
      * @param callable $f
      * @param mixed $x
      * @param mixed $y
@@ -41,7 +48,7 @@ public function __construct(callable $f, $x, $y)
     }
 
     /**
-     * @return mixed
+     * @psalm-return ResultType
      */
     public function __invoke()
     {
diff --git a/src/Combinator/D.php b/src/Combinator/D.php
index 7deaf7f..2b029cb 100644
--- a/src/Combinator/D.php
+++ b/src/Combinator/D.php
@@ -8,6 +8,9 @@
 
 /**
  * Class D.
+ *
+ * @psalm-template XType
+ * @psalm-template ResultType
  */
 final class D extends Combinator
 {
@@ -34,6 +37,11 @@ final class D extends Combinator
     /**
      * D constructor.
      *
+     * @psalm-param callable(XType) : ResultType $f
+     * @psalm-param callable(XType) : ResultType $g
+     * @psalm-param XType $x
+     * @psalm-param XType $y
+     *
      * @param callable $f
      * @param callable $g
      * @param mixed $x
@@ -48,7 +56,7 @@ public function __construct(callable $f, $x, callable $g, $y)
     }
 
     /**
-     * @return mixed
+     * @psalm-return ResultType
      */
     public function __invoke()
     {
diff --git a/src/Combinator/E.php b/src/Combinator/E.php
index fa5ec11..a9ea4f7 100644
--- a/src/Combinator/E.php
+++ b/src/Combinator/E.php
@@ -8,6 +8,9 @@
 
 /**
  * Class E.
+ *
+ * @psalm-template XType
+ * @psalm-template ResultType
  */
 final class E extends Combinator
 {
@@ -39,6 +42,12 @@ final class E extends Combinator
     /**
      * E constructor.
      *
+     * @psalm-param callable(XType) : ResultType $f
+     * @psalm-param XType $x
+     * @psalm-param callable(XType) : ResultType $g
+     * @psalm-param XType $y
+     * @psalm-param XType $z
+     *
      * @param callable $f
      * @param mixed $x
      * @param callable $g
@@ -55,7 +64,7 @@ public function __construct(callable $f, $x, callable $g, $y, $z)
     }
 
     /**
-     * @return mixed
+     * @psalm-return ResultType
      */
     public function __invoke()
     {
diff --git a/src/Combinator/F.php b/src/Combinator/F.php
index 912ae31..a41fbd1 100644
--- a/src/Combinator/F.php
+++ b/src/Combinator/F.php
@@ -8,6 +8,9 @@
 
 /**
  * Class F.
+ *
+ * @psalm-template XType
+ * @psalm-template ResultType
  */
 final class F extends Combinator
 {
@@ -22,13 +25,17 @@ final class F extends Combinator
     private $x;
 
     /**
-     * @var callable
+     * @var mixed
      */
     private $y;
 
     /**
      * F constructor.
      *
+     * @psalm-param XType $x
+     * @psalm-param XType $y
+     * @psalm-param callable(XType) : ResultType $f
+     *
      * @param mixed $x
      * @param mixed $y
      * @param callable $f
@@ -41,7 +48,7 @@ public function __construct($x, $y, callable $f)
     }
 
     /**
-     * @return mixed
+     * @psalm-return ResultType
      */
     public function __invoke()
     {
diff --git a/src/Combinator/G.php b/src/Combinator/G.php
index 1b2a0cf..06b8ab3 100644
--- a/src/Combinator/G.php
+++ b/src/Combinator/G.php
@@ -8,6 +8,9 @@
 
 /**
  * Class G.
+ *
+ * @psalm-template XType
+ * @psalm-template ResultType
  */
 final class G extends Combinator
 {
@@ -27,13 +30,18 @@ final class G extends Combinator
     private $x;
 
     /**
-     * @var callable
+     * @var mixed
      */
     private $y;
 
     /**
      * G constructor.
      *
+     * @psalm-param callable(XType) : callable $f
+     * @psalm-param callable(XType) : ResultType $g
+     * @psalm-param XType $x
+     * @psalm-param XType $y
+     *
      * @param callable $f
      * @param callable $g
      * @param mixed $x
@@ -48,7 +56,7 @@ public function __construct(callable $f, callable $g, $x, $y)
     }
 
     /**
-     * @return mixed
+     * @psalm-return ResultType
      */
     public function __invoke()
     {
diff --git a/src/Combinator/H.php b/src/Combinator/H.php
index 04370e8..2063d7b 100644
--- a/src/Combinator/H.php
+++ b/src/Combinator/H.php
@@ -8,6 +8,9 @@
 
 /**
  * Class H.
+ *
+ * @psalm-template XType
+ * @psalm-template ResultType
  */
 final class H extends Combinator
 {
@@ -17,7 +20,7 @@ final class H extends Combinator
     private $f;
 
     /**
-     * @var callable
+     * @var mixed
      */
     private $x;
 
@@ -29,6 +32,10 @@ final class H extends Combinator
     /**
      * H constructor.
      *
+     * @psalm-param callable(XType) : callable $f
+     * @psalm-param XType $x
+     * @psalm-param XType $y
+     *
      * @param callable $f
      * @param mixed $x
      * @param mixed $y
@@ -41,7 +48,7 @@ public function __construct(callable $f, $x, $y)
     }
 
     /**
-     * @return mixed
+     * @psalm-return ResultType
      */
     public function __invoke()
     {
diff --git a/src/Combinator/I.php b/src/Combinator/I.php
index f551bb6..e571710 100644
--- a/src/Combinator/I.php
+++ b/src/Combinator/I.php
@@ -8,6 +8,9 @@
 
 /**
  * Class I.
+ *
+ * @psalm-template XType
+ * @psalm-template ResultType
  */
 final class I extends Combinator
 {
@@ -19,6 +22,8 @@ final class I extends Combinator
     /**
      * I constructor.
      *
+     * @psalm-param XType $x
+     *
      * @param mixed $x
      */
     public function __construct($x)
@@ -27,7 +32,7 @@ public function __construct($x)
     }
 
     /**
-     * @return mixed
+     * @psalm-return XType
      */
     public function __invoke()
     {
diff --git a/src/Combinator/J.php b/src/Combinator/J.php
index 8dfca5a..fb23f19 100644
--- a/src/Combinator/J.php
+++ b/src/Combinator/J.php
@@ -8,6 +8,9 @@
 
 /**
  * Class J.
+ *
+ * @psalm-template XType
+ * @psalm-template ResultType
  */
 final class J extends Combinator
 {
@@ -17,7 +20,7 @@ final class J extends Combinator
     private $f;
 
     /**
-     * @var callable
+     * @var mixed
      */
     private $x;
 
@@ -34,6 +37,11 @@ final class J extends Combinator
     /**
      * J constructor.
      *
+     * @psalm-param callable(XType) : callable $f
+     * @psalm-param XType $x
+     * @psalm-param XType $y
+     * @psalm-param XType $z
+     *
      * @param callable $f
      * @param mixed $x
      * @param mixed $y
@@ -48,7 +56,7 @@ public function __construct(callable $f, $x, $y, $z)
     }
 
     /**
-     * @return mixed
+     * @psalm-return ResultType
      */
     public function __invoke()
     {
diff --git a/src/Combinator/K.php b/src/Combinator/K.php
index a14af13..6f067cf 100644
--- a/src/Combinator/K.php
+++ b/src/Combinator/K.php
@@ -8,6 +8,9 @@
 
 /**
  * Class K.
+ *
+ * @psalm-template XType
+ * @psalm-template ResultType
  */
 final class K extends Combinator
 {
@@ -24,6 +27,9 @@ final class K extends Combinator
     /**
      * K constructor.
      *
+     * @psalm-param XType $x
+     * @psalm-param XType $y
+     *
      * @param mixed $x
      * @param mixed $y
      */
@@ -34,7 +40,7 @@ public function __construct($x, $y)
     }
 
     /**
-     * @return mixed
+     * @psalm-return XType
      */
     public function __invoke()
     {
diff --git a/src/Combinator/L.php b/src/Combinator/L.php
index f1f40fc..a2d42da 100644
--- a/src/Combinator/L.php
+++ b/src/Combinator/L.php
@@ -8,6 +8,8 @@
 
 /**
  * Class L.
+ *
+ * @psalm-template ResultType
  */
 final class L extends Combinator
 {
@@ -34,6 +36,8 @@ public function __construct(callable $f, callable $g)
     }
 
     /**
+     * @psalm-return ResultType
+     *
      * @return mixed
      */
     public function __invoke()
diff --git a/src/Combinator/M.php b/src/Combinator/M.php
index 23b6bf1..ff6cbf4 100644
--- a/src/Combinator/M.php
+++ b/src/Combinator/M.php
@@ -8,6 +8,8 @@
 
 /**
  * Class M.
+ *
+ * @psalm-template ResultType
  */
 final class M extends Combinator
 {
@@ -19,6 +21,8 @@ final class M extends Combinator
     /**
      * M constructor.
      *
+     * @psalm-param callable $f
+     *
      * @param callable $f
      */
     public function __construct(callable $f)
@@ -27,7 +31,7 @@ public function __construct(callable $f)
     }
 
     /**
-     * @return mixed
+     * @psalm-return ResultType
      */
     public function __invoke()
     {
diff --git a/src/Combinator/O.php b/src/Combinator/O.php
index 84146f4..6d6bc1b 100644
--- a/src/Combinator/O.php
+++ b/src/Combinator/O.php
@@ -8,6 +8,8 @@
 
 /**
  * Class O.
+ *
+ * @psalm-template ResultType
  */
 final class O extends Combinator
 {
@@ -34,7 +36,7 @@ public function __construct(callable $f, callable $g)
     }
 
     /**
-     * @return mixed
+     * @psalm-return ResultType
      */
     public function __invoke()
     {
diff --git a/src/Combinator/Q.php b/src/Combinator/Q.php
index 5265114..4eb0f66 100644
--- a/src/Combinator/Q.php
+++ b/src/Combinator/Q.php
@@ -8,6 +8,9 @@
 
 /**
  * Class Q.
+ *
+ * @psalm-template XType
+ * @psalm-template ResultType
  */
 final class Q extends Combinator
 {
@@ -29,6 +32,10 @@ final class Q extends Combinator
     /**
      * Q constructor.
      *
+     * @psalm-param callable(XType) : ResultType $f
+     * @psalm-param callable(XType) : ResultType $g
+     * @psalm-param XType $x
+     *
      * @param callable $f
      * @param callable $g
      * @param mixed $x
@@ -41,7 +48,7 @@ public function __construct(callable $f, callable $g, $x)
     }
 
     /**
-     * @return mixed
+     * @psalm-return ResultType
      */
     public function __invoke()
     {
diff --git a/src/Combinator/R.php b/src/Combinator/R.php
index f776d62..0bee21a 100644
--- a/src/Combinator/R.php
+++ b/src/Combinator/R.php
@@ -8,6 +8,9 @@
 
 /**
  * Class R.
+ *
+ * @psalm-template XType
+ * @psalm-template ResultType
  */
 final class R extends Combinator
 {
@@ -29,6 +32,10 @@ final class R extends Combinator
     /**
      * R constructor.
      *
+     * @psalm-param XType $x
+     * @psalm-param callable(XType) : callable $f
+     * @psalm-param XType $y
+     *
      * @param mixed $x
      * @param callable $f
      * @param mixed $y
@@ -41,7 +48,7 @@ public function __construct($x, callable $f, $y)
     }
 
     /**
-     * @return mixed
+     * @psalm-return ResultType
      */
     public function __invoke()
     {
diff --git a/src/Combinator/S.php b/src/Combinator/S.php
index 2f5f15d..3f9e42a 100644
--- a/src/Combinator/S.php
+++ b/src/Combinator/S.php
@@ -8,6 +8,9 @@
 
 /**
  * Class S.
+ *
+ * @psalm-template XType
+ * @psalm-template ResultType
  */
 final class S extends Combinator
 {
@@ -29,6 +32,10 @@ final class S extends Combinator
     /**
      * S constructor.
      *
+     * @psalm-param callable(XType) : callable $f
+     * @psalm-param callable(XType) : ResultType $g
+     * @psalm-param XType $x
+     *
      * @param callable $f
      * @param callable $g
      * @param mixed $x
@@ -41,7 +48,7 @@ public function __construct(callable $f, callable $g, $x)
     }
 
     /**
-     * @return mixed
+     * @psalm-return ResultType
      */
     public function __invoke()
     {
diff --git a/src/Combinator/T.php b/src/Combinator/T.php
index 0545be1..97cbd38 100644
--- a/src/Combinator/T.php
+++ b/src/Combinator/T.php
@@ -8,6 +8,9 @@
 
 /**
  * Class T.
+ *
+ * @psalm-template XType
+ * @psalm-template ResultType
  */
 final class T extends Combinator
 {
@@ -24,6 +27,9 @@ final class T extends Combinator
     /**
      * T constructor.
      *
+     * @psalm-param callable(XType) : ResultType $f
+     * @psalm-param XType $x
+     *
      * @param callable $f
      * @param mixed $x
      */
@@ -34,7 +40,7 @@ public function __construct($x, callable $f)
     }
 
     /**
-     * @return mixed
+     * @psalm-return ResultType
      */
     public function __invoke()
     {
diff --git a/src/Combinator/U.php b/src/Combinator/U.php
index 5d2e7c3..5a52574 100644
--- a/src/Combinator/U.php
+++ b/src/Combinator/U.php
@@ -8,6 +8,8 @@
 
 /**
  * Class U.
+ *
+ * @psalm-template ResultType
  */
 final class U extends Combinator
 {
@@ -24,6 +26,9 @@ final class U extends Combinator
     /**
      * U constructor.
      *
+     * @psalm-param callable $f
+     * @psalm-param callable $g
+     *
      * @param callable $f
      * @param callable $g
      */
@@ -34,7 +39,7 @@ public function __construct(callable $f, callable $g)
     }
 
     /**
-     * @return mixed
+     * @psalm-return ResultType
      */
     public function __invoke()
     {
diff --git a/src/Combinator/V.php b/src/Combinator/V.php
index 5ad8b32..8377c91 100644
--- a/src/Combinator/V.php
+++ b/src/Combinator/V.php
@@ -8,6 +8,9 @@
 
 /**
  * Class V.
+ *
+ * @psalm-template XType
+ * @psalm-template ResultType
  */
 final class V extends Combinator
 {
@@ -29,6 +32,10 @@ final class V extends Combinator
     /**
      * V constructor.
      *
+     * @psalm-param XType $x
+     * @psalm-param XType $y
+     * @psalm-param callable(XType) : callable $f
+     *
      * @param mixed $x
      * @param mixed $y
      * @param callable $f
@@ -41,7 +48,7 @@ public function __construct($x, $y, callable $f)
     }
 
     /**
-     * @return mixed
+     * @psalm-return ResultType
      */
     public function __invoke()
     {
diff --git a/src/Combinator/W.php b/src/Combinator/W.php
index 0334737..fccb43e 100644
--- a/src/Combinator/W.php
+++ b/src/Combinator/W.php
@@ -8,6 +8,9 @@
 
 /**
  * Class W.
+ *
+ * @psalm-template XType
+ * @psalm-template ResultType
  */
 final class W extends Combinator
 {
@@ -24,6 +27,9 @@ final class W extends Combinator
     /**
      * W constructor.
      *
+     * @psalm-param callable(XType) : callable $f
+     * @psalm-param XType $x
+     *
      * @param callable $f
      * @param mixed $x
      */
@@ -34,7 +40,7 @@ public function __construct(callable $f, $x)
     }
 
     /**
-     * @return mixed
+     * @psalm-return ResultType
      */
     public function __invoke()
     {
diff --git a/src/Combinator/Y.php b/src/Combinator/Y.php
index 9ff3d78..7aeb6f1 100644
--- a/src/Combinator/Y.php
+++ b/src/Combinator/Y.php
@@ -4,10 +4,13 @@
 
 namespace loophp\combinator\Combinator;
 
+use Closure;
 use loophp\combinator\Combinator;
 
 /**
  * Class Y.
+ *
+ * @psalm-template ResultType
  */
 final class Y extends Combinator
 {
@@ -19,6 +22,8 @@ final class Y extends Combinator
     /**
      * Y constructor.
      *
+     * @psalm-param callable $f
+     *
      * @param callable $f
      */
     public function __construct(callable $f)
@@ -27,14 +32,26 @@ public function __construct(callable $f)
     }
 
     /**
-     * @return mixed
+     * @psalm-return Closure(Closure(callable): mixed): mixed
      */
     public function __invoke()
     {
         $callable = $this->f;
 
-        $f = static function (callable $f) use ($callable): callable {
+        /**
+         * @psalm-suppress MissingClosureReturnType
+         *
+         * @param callable $f
+         *
+         * @return mixed
+         */
+        $f = static function (callable $f) use ($callable) {
             return $callable(
+                /**
+                 * @psalm-suppress MissingClosureParamType
+                 *
+                 * @param array $arguments
+                 */
                 static function (...$arguments) use ($f) {
                     return $f($f)(...$arguments);
                 }