Monads/Writer monad: Difference between revisions

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=={{header|PHP}}==

<lang php>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());</lang>

{{out}}

<pre>

The Golden Ratio is: 1.6180339887499

Initial value: 5

square root: 2.2360679774998

add one: 3.2360679774998

half: 1.6180339887499