Issue #1: Add PSALM type declaration.

.editorconfig

@@ -6,13 +6,10 @@
 
 root = true
 
-[*.{php,inc,module}]
+[*]
 charset = utf-8
 end_of_line = lf
 insert_final_newline = true
 trim_trailing_whitespace = true
 indent_style = space
 indent_size = 4
-
-[*.{json,json.dist,yml,yml.dist}]
-indent_size = 4

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

grumphp.yml.dist

@@ -4,6 +4,7 @@ imports:
 parameters:
   process_timeout: 600
   extra_tasks:
+    psalm: ~
     phpspec:
       verbose: true
       metadata:

psalm.xml

@@ -0,0 +1,55 @@
+<?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>
+
+    <issueHandlers>
+        <LessSpecificReturnType errorLevel="info" />
+
+        <!-- level 3 issues - slightly lazy code writing, but provably low false-negatives -->
+
+        <DeprecatedMethod errorLevel="info" />
+        <DeprecatedProperty errorLevel="info" />
+        <DeprecatedClass errorLevel="info" />
+        <DeprecatedConstant errorLevel="info" />
+        <DeprecatedFunction errorLevel="info" />
+        <DeprecatedInterface errorLevel="info" />
+        <DeprecatedTrait errorLevel="info" />
+
+        <InternalMethod errorLevel="info" />
+        <InternalProperty errorLevel="info" />
+        <InternalClass errorLevel="info" />
+
+        <MissingClosureReturnType errorLevel="info" />
+        <MissingReturnType errorLevel="info" />
+        <MissingPropertyType errorLevel="info" />
+        <InvalidDocblock errorLevel="info" />
+        <MisplacedRequiredParam errorLevel="info" />
+
+        <PropertyNotSetInConstructor errorLevel="info" />
+        <MissingConstructor errorLevel="info" />
+        <MissingClosureParamType errorLevel="info" />
+        <MissingParamType errorLevel="info" />
+
+        <RedundantCondition errorLevel="info" />
+
+        <DocblockTypeContradiction errorLevel="info" />
+        <RedundantConditionGivenDocblockType errorLevel="info" />
+
+        <UnresolvableInclude errorLevel="info" />
+
+        <RawObjectIteration errorLevel="info" />
+
+        <InvalidStringClass errorLevel="info" />
+    </issueHandlers>
+</psalm>

spec/loophp/combinator/Combinator/ASpec.php

@@ -23,7 +23,7 @@ public function it_is_initializable()
             ->shouldReturnAnInstanceOf(Closure::class);
 
         $this
-            ->closure()(...$arguments)
+            ->closure()(...$arguments)()
             ->shouldReturn('FOO');
     }
 }

spec/loophp/combinator/Combinator/BSpec.php

@@ -31,7 +31,7 @@ public function it_is_initializable()
             ->shouldReturnAnInstanceOf(Closure::class);
 
         $this
-            ->closure()(...$arguments)
+            ->closure()(...$arguments)()
             ->shouldReturn(17);
     }
 }

spec/loophp/combinator/Combinator/CSpec.php

@@ -29,7 +29,7 @@ public function it_is_initializable()
             ->shouldReturnAnInstanceOf(Closure::class);
 
         $this
-            ->closure()(...$arguments)
+            ->closure()(...$arguments)()
             ->shouldReturn('ba');
     }
 }

spec/loophp/combinator/Combinator/DSpec.php

@@ -35,7 +35,7 @@ public function it_is_initializable()
             ->shouldReturnAnInstanceOf(Closure::class);
 
         $this
-            ->closure()(...$arguments)
+            ->closure()(...$arguments)()
             ->shouldReturn('f(x)(g(y))');
     }
 }

spec/loophp/combinator/Combinator/ESpec.php

@@ -35,7 +35,7 @@ public function it_is_initializable()
             ->shouldReturnAnInstanceOf(Closure::class);
 
         $this
-            ->closure()(...$arguments)
+            ->closure()(...$arguments)()
             ->shouldReturn((float) 10);
     }
 }

spec/loophp/combinator/Combinator/FSpec.php

@@ -29,7 +29,7 @@ public function it_is_initializable()
             ->shouldReturnAnInstanceOf(Closure::class);
 
         $this
-            ->closure()(...$arguments)
+            ->closure()(...$arguments)()
             ->shouldReturn((float) 9);
     }
 }

spec/loophp/combinator/Combinator/GSpec.php

@@ -33,7 +33,7 @@ public function it_is_initializable()
             ->shouldReturnAnInstanceOf(Closure::class);
 
         $this
-            ->closure()(...$arguments)
+            ->closure()(...$arguments)()
             ->shouldReturn((float) 9);
     }
 }

spec/loophp/combinator/Combinator/HSpec.php

@@ -31,7 +31,7 @@ public function it_is_initializable()
             ->shouldReturnAnInstanceOf(Closure::class);
 
         $this
-            ->closure()(...$arguments)
+            ->closure()(...$arguments)()
             ->shouldReturn(17);
     }
 }

spec/loophp/combinator/Combinator/ISpec.php

@@ -23,7 +23,7 @@ public function it_is_initializable()
             ->shouldReturnAnInstanceOf(Closure::class);
 
         $this
-            ->closure()(...$arguments)
+            ->closure()(...$arguments)()
             ->shouldReturn('foo');
     }
 }

spec/loophp/combinator/Combinator/JSpec.php

@@ -29,7 +29,7 @@ public function it_is_initializable()
             ->shouldReturnAnInstanceOf(Closure::class);
 
         $this
-            ->closure()(...$arguments)
+            ->closure()(...$arguments)()
             ->shouldReturn(6);
     }
 }

spec/loophp/combinator/Combinator/KSpec.php

@@ -23,7 +23,7 @@ public function it_is_initializable()
             ->shouldReturnAnInstanceOf(Closure::class);
 
         $this
-            ->closure()(...$arguments)
+            ->closure()(...$arguments)()
             ->shouldReturn('bar');
     }
 }

spec/loophp/combinator/Combinator/LSpec.php

@@ -37,7 +37,7 @@ public function it_is_initializable()
             ->shouldReturnAnInstanceOf(Closure::class);
 
         $this
-            ->closure()(...$arguments)
+            ->closure()(...$arguments)()
             ->shouldReturn(36);
     }
 }

spec/loophp/combinator/Combinator/MSpec.php

@@ -31,7 +31,7 @@ public function it_is_initializable()
             ->shouldReturnAnInstanceOf(Closure::class);
 
         $this
-            ->closure()(...$arguments)
+            ->closure()(...$arguments)()
             ->shouldReturn('argnull');
     }
 }

spec/loophp/combinator/Combinator/OSpec.php

@@ -35,7 +35,7 @@ public function it_is_initializable()
             ->shouldReturnAnInstanceOf(Closure::class);
 
         $this
-            ->closure()(...$arguments)
+            ->closure()(...$arguments)()
             ->shouldReturn('g(f(g()))');
     }
 }

spec/loophp/combinator/Combinator/PsiSpec.php

@@ -31,7 +31,7 @@ public function it_is_initializable()
             ->shouldReturnAnInstanceOf(Closure::class);
 
         $this
-            ->closure()(...$arguments)
+            ->closure()(...$arguments)()
             ->shouldReturn(68);
     }
 }

spec/loophp/combinator/Combinator/QSpec.php

@@ -35,7 +35,7 @@ public function it_is_initializable()
             ->shouldReturnAnInstanceOf(Closure::class);
 
         $this
-            ->closure()(...$arguments)
+            ->closure()(...$arguments)()
             ->shouldReturn('g(f(a))');
     }
 }

spec/loophp/combinator/Combinator/RSpec.php

@@ -29,7 +29,7 @@ public function it_is_initializable()
             ->shouldReturnAnInstanceOf(Closure::class);
 
         $this
-            ->closure()(...$arguments)
+            ->closure()(...$arguments)()
             ->shouldReturn('f(y)(x)');
     }
 }

spec/loophp/combinator/Combinator/SSpec.php

@@ -33,7 +33,7 @@ public function it_is_initializable()
             ->shouldReturnAnInstanceOf(Closure::class);
 
         $this
-            ->closure()(...$arguments)
+            ->closure()(...$arguments)()
             ->shouldReturn((float) 21);
     }
 }

spec/loophp/combinator/Combinator/TSpec.php

@@ -23,7 +23,7 @@ public function it_is_initializable()
             ->shouldReturnAnInstanceOf(Closure::class);
 
         $this
-            ->closure()(...$arguments)
+            ->closure()(...$arguments)()
             ->shouldReturn('FOO');
     }
 }

spec/loophp/combinator/Combinator/USpec.php

@@ -43,7 +43,7 @@ public function it_is_initializable()
             ->shouldReturnAnInstanceOf(Closure::class);
 
         $this
-            ->closure()(...$arguments)
+            ->closure()(...$arguments)()
             ->shouldReturn('g(f(f)(g))');
     }
 }

spec/loophp/combinator/Combinator/VSpec.php

@@ -29,7 +29,7 @@ public function it_is_initializable()
             ->shouldReturnAnInstanceOf(Closure::class);
 
         $this
-            ->closure()(...$arguments)
+            ->closure()(...$arguments)()
             ->shouldReturn('f(x)(y)');
     }
 }

spec/loophp/combinator/Combinator/WSpec.php

@@ -29,7 +29,7 @@ public function it_is_initializable()
             ->shouldReturnAnInstanceOf(Closure::class);
 
         $this
-            ->closure()(...$arguments)
+            ->closure()(...$arguments)()
             ->shouldReturn(11);
     }
 }

spec/loophp/combinator/Combinator/YSpec.php

@@ -28,10 +28,9 @@ public function it_is_initializable()
             ->closure()
             ->shouldReturnAnInstanceOf(Closure::class);
 
-        $closure = $this
-            ->closure();
+        $closure = $this->closure();
 
-        $closure($f)(6)
+        $closure($f)()(6)
             ->shouldReturn(720);
     }
 }

src/Combinator/A.php

@@ -4,15 +4,19 @@
 
 namespace loophp\combinator\Combinator;
 
+use Closure;
 use loophp\combinator\Combinator;
 
 /**
  * Class A.
+ *
+ * @psalm-template XType
+ * @psalm-template ResultType
  */
 final class A extends Combinator
 {
     /**
-     * @var callable
+     * @var Closure
      */
     private $f;
 
@@ -24,16 +28,21 @@ final class A extends Combinator
     /**
      * A constructor.
      *
+     * @psalm-param callable(XType) : ResultType $f
+     * @psalm-param XType $x
+     *
      * @param callable $f
      * @param mixed $x
      */
     public function __construct(callable $f, $x)
     {
-        $this->f = $f;
+        $this->f = Closure::fromCallable($f);
         $this->x = $x;
     }
 
     /**
+     * @psalm-return ResultType
+     *
      * @return mixed
      */
     public function __invoke()
@@ -42,12 +51,14 @@ public function __invoke()
     }
 
     /**
-     * @return callable
+     * @psalm-return \Closure(callable, mixed) : mixed
+     *
+     * @return Closure
      */
-    public static function closure(): callable
+    public static function closure(): Closure
     {
-        return static function (callable $f, $x) {
-            return (new self($f, $x))();
+        return static function (callable $f, $x): Closure {
+            return Closure::fromCallable([(new self($f, $x)), '__invoke']);
         };
     }
 }