Data commit

Task/Monads-Writer-monad/PHP/monads-writer-monad.php

@@ -0,0 +1,44 @@
+class WriterMonad {
+
+	/** @var mixed */
+	private $value;
+	/** @var string[] */
+	private $logs;
+
+	private function __construct($value, array $logs = []) {
+		$this->value = $value;
+		$this->logs = $logs;
+	}
+
+	public static function unit($value, string $log): WriterMonad {
+		return new WriterMonad($value, ["{$log}: {$value}"]);
+	}
+
+	public function bind(callable $mapper): WriterMonad  {
+		$mapped = $mapper($this->value);
+		assert($mapped instanceof WriterMonad);
+		return new WriterMonad($mapped->value, [...$this->logs, ...$mapped->logs]);
+	}
+
+	public function value() {
+		return $this->value;
+	}
+
+	public function logs(): array {
+		return $this->logs;
+	}
+}
+
+$root = fn(float $i): float => sqrt($i);
+$addOne = fn(float $i): float => $i + 1;
+$half = fn(float $i): float => $i / 2;
+
+$m = fn (callable $callback, string $log): callable => fn ($value): WriterMonad => WriterMonad::unit($callback($value), $log);
+
+$result = WriterMonad::unit(5, "Initial value")
+	->bind($m($root, "square root"))
+	->bind($m($addOne, "add one"))
+	->bind($m($half, "half"));
+
+print "The Golden Ratio is: {$result->value()}\n";
+print join("\n", $result->logs());