PHP Maybe Monad

To deal with computations that may fail. A port of Haskell's Data.Maybe module for PHP > 5.4.

Install

You can install via Composer.

composer require haskellcamargo/php-maybe-monad

Example

use HaskellCamargo\Maybe;

Maybe\Maybe(@$_GET["username"])->bind(function($user)) {
  echo "Welcome, $user. You're logged in!";
});

$userAge = Maybe\Maybe(null)->fromMaybe(0); // => 0
$userAge = Maybe\Maybe(19)->fromMaybe(0); // => 19

Documentation

A Maybe type encapsulates an optional value. A value of type Maybe a either contains a value of type a (represented as Just a), or it is empty (represented as Nothing). Using Maybe is a good way to deal with errors or exceptional cases without resorting to drastic measures such as Exception. The Maybe type is also a monad. It is a simple kind of error monad, where all errors are represented by Nothing. A richer error monad can be built using the Either type.

bind :: (Maybe a, callable) -> Maybe b

Equivalent to Haskell's >>= operator. Its first argument is a value in a monadic type, its second argument is a function that maps from the underlying type of the first argument to another monadic type, and its results is in that other monadic type.

$age = Maybe\Maybe(null)->bind(function($x) {
  return 10;
}); // => Nothing

$age = Maybe\Maybe(10)
->bind(function($x) {
  return $x + 10; // => Just(20);
})
->bind(function($x) {
  return $x + 20; // => Just(40);
})->fromJust(); // => 40

fromJust :: Maybe a -> a

Extracts the element out of a Just and returns an error if its argument is Nothing.

Maybe\Maybe("Foo")->fromJust(); // => "Foo"
Maybe\Maybe(null)->fromJust(); // => Exception: Cannot cal fromJust() on Nothing

fromMaybe :: (Maybe a, a) -> a

Takes a Maybe value and a default value. If the Maybe is Nothing, it returns the default values; otherwise, it returns the value contained in the Maybe.

Maybe\Maybe(10)->fromMaybe(5); // => 10
Maybe\Maybe(null)->fromMaybe(5); // => 5

isJust :: Maybe a -> boolean

Returns true if its argument is of the form Just _.

Maybe\Maybe(10)->isJust(); // => true
Maybe\Maybe(null)->isJust(); // => false

isNothing :: Maybe a -> boolean

Returns true if its argument is of the form Nothing.

Maybe\Maybe(10)->isNothing(); // => false
Maybe\Maybe(null)->isNothing(); // => true

maybe :: (Maybe a, b, callable) -> b

Takes a default value, a function and, of course, a Maybe value. If the Maybe value is Nothing, the function returns the default value. Otherwise, it applies the function to the value inside the Just and returns the result.

$just = Maybe\Maybe(10);
$nothing = Maybe\Maybe(null);

$just->maybe(40, function($num) {
        return $num + 15;
}); // => 25

$nothing->maybe(40, function($num) {
  return $num + 15;
}); // => 40

toList :: Maybe a -> array

Returns an empty list when given Nothing or a singleton list when not given Nothing.

Maybe\Maybe(10)->toList(); // => [10]
Maybe\Maybe(null)->toList(); // => []

Made with ❤️ by Marcelo Camargo and Reinaldo Rauch

License

MIT