[Fun] add when function (#85)

src/Psl/Fun/when.php

@@ -0,0 +1,36 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Psl\Fun;
+
+/**
+ * Creates a callback that can run a left or right function based on a condition.
+ *
+ * Example:
+ *
+ *      $greet = Fun\when(
+ *          static fn(string $name): bool => $name === 'Jos',
+ *          static fn(string $name): string => 'Bonjour Jos!',
+ *          static fn(string $name): string => 'Hello ' . $name . '!'
+ *      );
+ *
+ *      $greet('World')
+ *      => Str('Hello World!');
+ *
+ *      $greet('Jos')
+ *      => Str('Bonjour Jos!');
+ *
+ * @template Ti
+ * @template To
+ *
+ * @psalm-param (callable(Ti): bool)   $condition
+ * @psalm-param (callable(Ti): To) $then
+ * @psalm-param (callable(Ti): To) $else
+ *
+ * @psalm-return (callable(Ti): To)
+ */
+function when(callable $condition, callable $then, callable $else): callable
+{
+    return fn ($value) =>  $condition($value) ? $then($value) : $else($value);
+}

src/Psl/Internal/Loader.php

@@ -99,6 +99,7 @@ final class Loader
         'Psl\Fun\passthrough',
         'Psl\Fun\pipe',
         'Psl\Fun\rethrow',
+        'Psl\Fun\when',
         'Psl\Internal\boolean',
         'Psl\Internal\type',
         'Psl\Internal\validate_offset',

tests/Psl/Fun/WhenTest.php

@@ -0,0 +1,33 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Psl\Tests\Fun;
+
+use PHPUnit\Framework\TestCase;
+use Psl\Fun;
+
+final class WhenTest extends TestCase
+{
+    public function testItRunsLeftFunction(): void
+    {
+        $greet = Fun\when(
+            static fn(string $name): bool => $name === 'Jos',
+            static fn(string $name): string => 'Bonjour ' . $name . '!',
+            static fn(string $name): string => 'Hello ' . $name . '!'
+        );
+
+        self::assertSame('Bonjour Jos!', $greet('Jos'));
+    }
+
+    public function testItRunsRightfunction(): void
+    {
+        $greet = Fun\when(
+            static fn(string $name): bool => $name === 'Jos',
+            static fn(string $name): string => 'Bonjour ' . $name . '!',
+            static fn(string $name): string => 'Hello ' . $name . '!'
+        );
+
+        self::assertSame('Hello World!', $greet('World'));
+    }
+}