Add: Monad interface and Option types.

src/Monad.php

@@ -0,0 +1,27 @@
+<?php
+
+declare(strict_types=1);
+
+namespace EndouMame\PhpMonad;
+
+use Closure;
+
+/**
+ * @template T
+ */
+interface Monad
+{
+    /**
+     * @template T2
+     * @param  Closure(T):static<T2> $f
+     * @return static<T2>
+     */
+    public function bind(Closure $f): self;
+
+    /**
+     * @template TValue
+     * @param  TValue         $value
+     * @return static<TValue>
+     */
+    public static function unit(mixed $value): self;
+}

src/Option.php

@@ -0,0 +1,24 @@
+<?php
+
+declare(strict_types=1);
+
+namespace EndouMame\PhpMonad;
+
+use EndouMame\PhpMonad\Option\Some;
+
+/**
+ * @template T
+ * @implements Monad<T>
+ */
+abstract class Option implements Monad
+{
+    /**
+     * @template TValue
+     * @param  TValue         $value
+     * @return Option<TValue>
+     */
+    public static function unit(mixed $value): self
+    {
+        return new Some($value);
+    }
+}

src/Option/None.php

@@ -0,0 +1,48 @@
+<?php
+
+declare(strict_types=1);
+
+namespace EndouMame\PhpMonad\Option;
+
+use Closure;
+use EndouMame\PhpMonad\Option;
+use Override;
+
+/**
+ * @template T
+ * @extends Option<T>
+ */
+final class None extends Option
+{
+    /**
+     * @param T $value
+     */
+    public function __construct(protected mixed $value)
+    {
+    }
+
+    /**
+     * @template T2
+     * @param  Closure(T): static<T2> $f
+     * @return static<T2>
+     */
+    #[Override]
+    public function bind(Closure $f): self
+    {
+        /** @var T2 */
+        $v = null;
+
+        return self::unit($v);
+    }
+
+    /**
+     * @template TValue
+     * @param  TValue       $value
+     * @return None<TValue>
+     */
+    #[Override]
+    public static function unit(mixed $value): self
+    {
+        return new self($value);
+    }
+}

src/Option/Some.php

@@ -0,0 +1,33 @@
+<?php
+
+declare(strict_types=1);
+
+namespace EndouMame\PhpMonad\Option;
+
+use Closure;
+use EndouMame\PhpMonad\Option;
+use Override;
+
+/**
+ * @template T
+ * @extends Option<T>
+ */
+final class Some extends Option
+{
+    /**
+     * @param T $value
+     */
+    public function __construct(protected mixed $value)
+    {
+    }
+
+    /**
+     * @param  Closure(T): static<T> $f
+     * @return static<T>
+     */
+    #[Override]
+    public function bind(Closure $f): self
+    {
+        return $f($this->value);
+    }
+}