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Date: Thu, 27 May 2021 17:52:57 -0500
To: "php internals" <internals@lists.php.net>
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Subject: Re: [PHP-DEV] Consensus Gathering: is_initialized
From: larry@garfieldtech.com ("Larry Garfield")

On Thu, May 27, 2021, at 8:23 AM, Rowan Tommins wrote:

> The combination of union types and enums actually gives a much more=20=

> expressive way of representing "valid value or explicit null or specia=
l=20
> default". To use the ORM lazy-loading example:
>=20
>=20
> enum ORMState {
>  =C2=A0=C2=A0=C2=A0 case Unloaded;
> }
>=20
> class Example {
>  =C2=A0=C2=A0=C2=A0 private int|ORMState|null $foo =3D ORMState::Unloa=
ded;
>=20
>  =C2=A0=C2=A0=C2=A0 public function getFoo(): ?int {
>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if ( $this->foo =3D=3D=3D =
ORMState::Unloaded ) {
>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 $t=
his->foo =3D $this->loadFromDatabase('foo');
>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 }
>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 return $this->foo;
>  =C2=A0=C2=A0=C2=A0 }
>=20
>  =C2=A0=C2=A0=C2=A0 private function loadFromDatabase($fieldName): ?in=
t {
>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 echo "Fetching '$fieldName=
' from database...\n";
>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 return 42;
>  =C2=A0=C2=A0=C2=A0 }
> }
>=20
> // Create object; note there's no constructor, but the property has a=20=

> default so is never uninitialized
> $obj =3D new Example;
> // On first call to method, the default value is detected and the=20
> property lazy-loaded
> var_dump($obj->getFoo());
> // Subsequent accesses return the loaded value directly, at the cost o=
f=20
> a single strict comparison, no magic function needed
> var_dump($obj->getFoo());

I had not thought of this use of Enums, but I love it.  It's not as eleg=
ant as a tagged enum for a monad, but it gets you 80% of the way there. =
 Especially if you combine it with match().

enum Result {
  case None;
}

function find_user(int $id): User|Result { ... }

$user =3D match($u =3D find_user($id)) {
  Result::None =3D> some error handling,
  default: $u,
};

--Larry Garfield