IO.hh

gistfile1.php

<?hh // strict
namespace traitorous;

use traitorous\algebraic\Applicative;
use traitorous\algebraic\Functor;
use traitorous\algebraic\Monad;
use traitorous\algebraic\MonadOps;

final class IO<T> implements Functor<T>, Applicative<T>, Monad<T> {

    public function __construct(private (function(): T) $_f) { }

    public static function of($x): IO<T> {
        return new IO(() ==> $x);
    }

    public function map<Tb>((function(T): Tb) $f): IO<Tb> {
        return $this->flatMap(($a) ==> IO::of($f($a)));
    }

    public function ap<Tb, Tc>(Applicative<Tb> $next): Applicative<Tc> {
        return $this->flatMap(($f) ==> {
            invariant($f instanceof (function(Tb): Tc), "Expects callable");
            return $next->map($f);
        });
    }

    public function flatMap<Tb>((function(T): Monad<Tb>) $f): IO<Tb> {
        return new IO(() ==> $f($this->unsafePerform())->unsafePerform());
    }

    public function unsafePerform(): T {
        $redeemer = $this->_f;
        return $redeemer();
    }

}