Improve options type declarations and add andThen method

src/Psl/Option/Option.php

@@ -98,9 +98,11 @@ public function unwrap(): mixed
      * @note:   Arguments passed to `Option::unwrapOr()` are eagerly evaluated;
      *          if you are passing the result of a function call, it is recommended to use `Option::unwrapOrElse()`, which is lazily evaluated.
      *
-     * @param T $default
+     * @template O
      *
-     * @return T
+     * @param O $default
+     *
+     * @return T|O
      */
     public function unwrapOr(mixed $default): mixed
     {
@@ -114,9 +116,11 @@ public function unwrapOr(mixed $default): mixed
     /**
      * Returns the contained some value or computes it from a closure.
      *
-     * @param (Closure(): T) $default
+     * @template O
      *
-     * @return T
+     * @param (Closure(): O) $default
+     *
+     * @return T|O
      */
     public function unwrapOrElse(Closure $default): mixed
     {
@@ -215,6 +219,25 @@ public function map(Closure $closure): Option
         return $this;
     }
 
+    /**
+     * Maps an `Option<T>` to `Option<Tu>` by applying a function to a contained value that returns an Option<Tu>.
+     *
+     * @template Tu
+     *
+     * @param (Closure(T): Option<Tu>) $closure
+     *
+     * @return Option<Tu>
+     */
+    public function andThen(Closure $closure): Option
+    {
+        if ($this->option !== null) {
+            return $closure($this->option[0]);
+        }
+
+        /** @var Option<Tu> */
+        return $this;
+    }
+
     /**
      * Applies a function to the contained value (if some),
      * or returns `Option<Tu>::some()` with the provided `$default` value (if none).

tests/static-analysis/Option/unwrap.php

@@ -0,0 +1,25 @@
+<?php
+
+declare(strict_types=1);
+
+use Psl\Option;
+
+function test_some_unwrap_or(): ?string
+{
+    return Option\some('string')->unwrapOr(null);
+}
+
+function test_none_unwrap_or(): ?string
+{
+    return Option\none()->unwrapOr(null);
+}
+
+function test_some_unwrap_or_else(): ?string
+{
+    return Option\some('string')->unwrapOrElse(static fn () => null);
+}
+
+function test_none_unwrap_or_else(): ?string
+{
+    return Option\none()->unwrapOrElse(static fn() => null);
+}

tests/unit/Option/NoneTest.php

@@ -101,4 +101,11 @@ public function testMapOrElse(): void
 
         static::assertSame(4, $option->mapOrElse(static fn($i) => $i + 1, static fn() => 4)->unwrap());
     }
+
+    public function testAndThen(): void
+    {
+        $option = Option\none();
+
+        static::assertNull($option->andThen(static fn($i) => Option\some($i + 1))->unwrapOr(null));
+    }
 }

tests/unit/Option/SomeTest.php

@@ -98,4 +98,11 @@ public function testMapOrElse(): void
 
         static::assertSame(3, $option->mapOrElse(static fn($i) => $i + 1, static fn() => 4)->unwrap());
     }
+
+    public function testAndThen(): void
+    {
+        $option = Option\some(2);
+
+        static::assertSame(3, $option->andThen(static fn($i) => Option\some($i + 1))->unwrapOr(null));
+    }
 }