add test cases for constant predicate

src/Predicate/AbstractPredicate.php

@@ -20,6 +20,11 @@ abstract class AbstractPredicate implements Predicate
         return new NegatedPredicate($this);
     }
 
+    public function negate(): Predicate
+    {
+        return $this->not();
+    }
+
     public function and(...$predicate): Predicate
     {
         array_unshift($predicate, $this);

src/Predicate/ConstantPredicate.php

@@ -30,13 +30,15 @@ class ConstantPredicate extends AbstractPredicate
 
     public function not(): Predicate
     {
-        return new self(!$this->value);
+        return $this->value
+            ? self::never()
+            : self::always();
     }
 
     public function and (...$predicate): Predicate
     {
         if (!$this->value) {
-            return new self(false);
+            return self::never();
         }
 
         return all($predicate);
@@ -45,7 +47,7 @@ class ConstantPredicate extends AbstractPredicate
     public function or (...$predicate): Predicate
     {
         if ($this->value) {
-            return new self(true);
+            return self::always();
         }
 
         return any($predicate);
@@ -58,11 +60,13 @@ class ConstantPredicate extends AbstractPredicate
 
     public static function always()
     {
-        return new static(true);
+        static $always;
+        return $always ? $always : $always = new static(true);
     }
 
     public static function never()
     {
-        return new static(false);
+        static $never;
+        return $never ? $never : $never = new static(false);
     }
 }

tests/ConstantPredicateTest.php

@@ -0,0 +1,123 @@
+<?php
+/**
+ * Copyright 2018 Kacper Donat
+ *
+ * @author Kacper "Kadet" Donat <kacper@kadet.net>
+ *
+ * Full license available in separate LICENSE file
+ */
+
+use function Kadet\Functional\predicate;
+use Kadet\Functional\Predicate\ConstantPredicate;
+
+class ConstantPredicateTest extends PHPUnit\Framework\TestCase
+{
+    private function getTestPredicate()
+    {
+        return function ($arg = null) {
+            return is_bool($arg);
+        };
+    }
+
+    public function randomDataProvider()
+    {
+        return [
+            ['x'],
+            [false],
+            [true],
+            [],
+            [(object)[]],
+            [false, true],
+            [null, true],
+            [2321321]
+        ];
+    }
+
+    /**
+     * @dataProvider randomDataProvider
+     */
+    public function testNeverAndPredicate(...$args)
+    {
+        $predicate = ConstantPredicate::never()->and($this->getTestPredicate());
+
+        $this->assertFalse($predicate(...$args));
+    }
+
+    /**
+     * @dataProvider randomDataProvider
+     */
+    public function testNeverOrPredicate(...$args)
+    {
+        $predicate = predicate($this->getTestPredicate());
+        $tested    = ConstantPredicate::never()->or($predicate);
+
+        $this->assertSame($predicate(...$args), $tested(...$args));
+    }
+
+    /**
+     * @dataProvider randomDataProvider
+     */
+    public function testAlwaysOrPredicate(...$args)
+    {
+        $predicate = ConstantPredicate::always()->or($this->getTestPredicate());
+
+        $this->assertTrue($predicate(...$args));
+    }
+
+    /**
+     * @dataProvider randomDataProvider
+     */
+    public function testAlwaysAndPredicate(...$args)
+    {
+        $predicate = predicate($this->getTestPredicate());
+        $tested    = ConstantPredicate::always()->and($predicate);
+
+        $this->assertSame($predicate(...$args), $tested(...$args));
+    }
+
+    /**
+     * @dataProvider randomDataProvider
+     */
+    public function testNever(...$args)
+    {
+        $tested = ConstantPredicate::never();
+
+        $this->assertFalse($tested(...$args));
+    }
+
+    /**
+     * @dataProvider randomDataProvider
+     */
+    public function testNotAlways(...$args)
+    {
+        $tested = ConstantPredicate::always()->not();
+
+        $this->assertFalse($tested(...$args));
+    }
+
+    /**
+     * @dataProvider randomDataProvider
+     */
+    public function testNotNever(...$args)
+    {
+        $tested = ConstantPredicate::never()->not();
+
+        $this->assertTrue($tested(...$args));
+    }
+
+    /**
+     * @dataProvider randomDataProvider
+     */
+    public function testAlways(...$args)
+    {
+        $tested = ConstantPredicate::always();
+
+        $this->assertTrue($tested(...$args));
+    }
+
+    public function testAliases()
+    {
+        $this->assertSame(\Kadet\Functional\always(), ConstantPredicate::always());
+        $this->assertSame(\Kadet\Functional\never(), ConstantPredicate::never());
+    }
+}