[Result] Add proceed and map with helper functions (#65)

src/Psl/Fun/passthrough.php

@@ -0,0 +1,18 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Psl\Fun;
+
+/**
+ * This method creates a callback that returns the value passed as argument.
+ * It can e.g. be used as a success callback.
+ *
+ * @template T
+ * @psalm-return callable(T): T
+ * @psalm-pure
+ */
+function passthrough(): callable
+{
+    return static fn ($result) => $result;
+}

src/Psl/Fun/rethrow.php

@@ -0,0 +1,21 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Psl\Fun;
+
+use Exception;
+
+/**
+ * This method creates a callback that throws the exception passed as argument.
+ * It can e.g. be used as a failure callback.
+ *
+ * @psalm-return callable(Exception): no-return
+ * @psalm-pure
+ */
+function rethrow(): callable
+{
+    return static function (Exception $exception): void {
+        throw $exception;
+    };
+}

src/Psl/Internal/Loader.php

@@ -96,7 +96,9 @@ final class Loader
         'Psl\Arr\map_keys',
         'Psl\Arr\map_with_key',
         'Psl\Fun\after',
+        'Psl\Fun\passthrough',
         'Psl\Fun\pipe',
+        'Psl\Fun\rethrow',
         'Psl\Internal\boolean',
         'Psl\Internal\type',
         'Psl\Internal\validate_offset',

src/Psl/Result/Failure.php

@@ -65,4 +65,25 @@ public function isFailed(): bool
     {
         return true;
     }
+
+    /**
+     * Unwrapping and transforming a result can be done by using the proceed method.
+     * Since this is a failed result wrapper, the `$on_failure` callback will be triggered.
+     * The callback will receive the Exception as an argument, so that you can transform it to anything you want.
+     */
+    public function proceed(callable $on_success, callable $on_failure)
+    {
+        return $on_failure($this->exception);
+    }
+
+    /**
+     * The method can be used to transform a result into another result.
+     * Since this is a failure result wrapper, the `$on_failure` callback will be triggered.
+     * The callback will receive the Exception as an argument,
+     * so that you can use it to create a success result or rethrow the Exception.
+     */
+    public function then(callable $on_success, callable $on_failure): ResultInterface
+    {
+        return wrap(fn () => $on_failure($this->exception));
+    }
 }

src/Psl/Result/ResultInterface.php

@@ -55,4 +55,38 @@ public function isSucceeded(): bool;
      * @return bool - `true` if the operation failed; `false` otherwise
      */
     public function isFailed(): bool;
+
+    /**
+     * Unwrapping and transforming a result can be done by using the proceed method.
+     * The implementation will either run the `$on_success` or `$on_failure` callback.
+     * The callback will receive the result or Exception as an argument,
+     * so that you can transform it to anything you want.
+     *
+     * @template R
+     *
+     * @psalm-param callable(T): R $on_success
+     * @psalm-param callable(Exception): R $on_failure
+     *
+     * @psalm-return R
+     */
+    public function proceed(callable $on_success, callable $on_failure);
+
+    /**
+     * The method can be used to transform a result into another result.
+     * The implementation will either run the `$on_success` or `$on_failure` callback.
+     * The callback will receive the result value or Exception as an argument,
+     * so that you can transform use it to build a new result.
+     *
+     * This method is compatible with the `PromiseInterface::then()` function from `reactphp/promise`.
+     * You can use it in an async context as long as the package you are using is compatible with reactphp promises.
+     *
+     * @link https://github.com/reactphp/promise#promiseinterfacethen
+     *
+     * @template R
+     * @psalm-param callable(T): R $on_success
+     * @psalm-param callable(Exception): R $on_failure
+     *
+     * @psalm-return ResultInterface<R>
+     */
+    public function then(callable $on_success, callable $on_failure): ResultInterface;
 }

src/Psl/Result/Success.php

@@ -73,4 +73,24 @@ public function isFailed(): bool
     {
         return false;
     }
+
+    /**
+     * Unwrapping and transforming a result can be done by using the proceed method.
+     * Since this is a success result wrapper, the `$on_success` callback will be triggered.
+     * The callback will receive the result value as an argument, so that you can transform it to anything you want.
+     */
+    public function proceed(callable $on_success, callable $on_failure)
+    {
+        return $on_success($this->value);
+    }
+
+    /**
+     * The method can be used to transform a result into another result.
+     * Since this is a success result wrapper, the `$on_success` callback will be triggered.
+     * The callback will receive the result value as an argument, so that you can use it to create a new result.
+     */
+    public function then(callable $on_success, callable $on_failure): ResultInterface
+    {
+        return wrap(fn () => $on_success($this->value));
+    }
 }

tests/Psl/Fun/PassthroughTest.php

@@ -0,0 +1,19 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Psl\Tests\Fun;
+
+use PHPUnit\Framework\TestCase;
+use Psl\Fun;
+
+class PassthroughTest extends TestCase
+{
+    public function testPassthrough(): void
+    {
+        $expected = 'x';
+        $passthrough = Fun\passthrough();
+
+        self::assertSame($expected, $passthrough($expected));
+    }
+}

tests/Psl/Fun/RethrowTest.php

@@ -0,0 +1,20 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Psl\Tests\Fun;
+
+use PHPUnit\Framework\TestCase;
+use Psl\Fun;
+
+class RethrowTest extends TestCase
+{
+    public function testRethrow(): void
+    {
+        $exception = new \Exception('foo');
+        $rethrow   = Fun\rethrow();
+
+        $this->expectExceptionObject($exception);
+        $rethrow($exception);
+    }
+}

tests/Psl/Result/FailureTest.php

@@ -4,26 +4,29 @@
 
 namespace Psl\Tests\Result;
 
+use Exception;
 use PHPUnit\Framework\TestCase;
+use Psl\Fun;
 use Psl\Result\Failure;
+use stdClass;
 
 class FailureTest extends TestCase
 {
     public function testIsSucceeded(): void
     {
-        $wrapper = new Failure(new \Exception('foo'));
+        $wrapper = new Failure(new Exception('foo'));
         self::assertFalse($wrapper->isSucceeded());
     }
 
     public function testIsFailed(): void
     {
-        $wrapper = new Failure(new \Exception('foo'));
+        $wrapper = new Failure(new Exception('foo'));
         self::assertTrue($wrapper->isFailed());
     }
 
     public function testGetResult(): void
     {
-        $exception = new \Exception('bar');
+        $exception = new Exception('bar');
         $wrapper   = new Failure($exception);
 
         $this->expectExceptionObject($exception);
@@ -32,9 +35,51 @@ public function testGetResult(): void
 
     public function testGetException(): void
     {
-        $exception = new \Exception('bar');
+        $exception = new Exception('bar');
         $wrapper   = new Failure($exception);
         $e         = $wrapper->getException();
         self::assertSame($exception, $e);
     }
+
+    public function testProceed(): void
+    {
+        $exception = new Exception('bar');
+        $wrapper   = new Failure($exception);
+        $actual    = $wrapper->proceed(
+            static fn (string $result): int => 200,
+            static fn (Exception $exception): int => 404
+        );
+
+        self::assertSame(404, $actual);
+    }
+
+    public function testThenToSuccess(): void
+    {
+        $exception = new Exception('bar');
+        $wrapper   = new Failure($exception);
+        $actual    = $wrapper->then(
+            static function () {
+                throw new \Exception('Dont call us, we\'ll call you!');
+            },
+            static fn (Exception $exception): string => $exception->getMessage()
+        );
+
+        self::assertTrue($actual->isSucceeded());
+        self::assertSame($actual->getResult(), 'bar');
+    }
+
+    public function testThenToFailure(): void
+    {
+        $exception = new Exception('bar');
+        $wrapper   = new Failure($exception);
+        $actual    = $wrapper->then(
+            static function () {
+                throw new \Exception('Dont call us, we\'ll call you!');
+            },
+            Fun\rethrow()
+        );
+
+        self::assertFalse($actual->isSucceeded());
+        self::assertSame($actual->getException(), $exception);
+    }
 }

tests/Psl/Result/SuccessTest.php

@@ -4,9 +4,12 @@
 
 namespace Psl\Tests\Result;
 
+use Exception;
 use PHPUnit\Framework\TestCase;
+use Psl\Fun;
 use Psl\Result\Success;
 use Psl\Exception\InvariantViolationException;
+use stdClass;
 
 class SuccessTest extends TestCase
 {
@@ -37,4 +40,43 @@ public function testGetException(): void
 
         $wrapper->getException();
     }
+
+    public function testProceed(): void
+    {
+        $wrapper = new Success('hello');
+        $actual  = $wrapper->proceed(
+            static fn (string $result): int => 200,
+            static fn (Exception $exception): int => 404
+        );
+
+        self::assertSame(200, $actual);
+    }
+
+    public function testThenToSuccess(): void
+    {
+        $wrapper = new Success('hello');
+        $actual    = $wrapper->then(
+            Fun\passthrough(),
+            Fun\rethrow()
+        );
+
+        self::assertNotSame($wrapper, $actual);
+        self::assertTrue($actual->isSucceeded());
+        self::assertSame('hello', $actual->getResult());
+    }
+
+    public function testThenToFailure(): void
+    {
+        $exception = new Exception('bar');
+        $wrapper   = new Success('hello');
+        $actual    = $wrapper->then(
+            static function () use ($exception) {
+                throw $exception;
+            },
+            Fun\rethrow()
+        );
+
+        self::assertFalse($actual->isSucceeded());
+        self::assertSame($actual->getException(), $exception);
+    }
 }