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Date: Wed, 5 Oct 2022 13:34:43 +0700
Cc: Eugene Sidelnyk <zsidelnik@gmail.com>,
 PHP Internals <internals@lists.php.net>
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 <6F41F6B1-E27B-4BF4-BDB0-66D63DA4F130@woofle.net>
To: Dusk <dusk@woofle.net>
Subject: Re: [PHP-DEV] Union type casts
From: php-lists@koalephant.com (Stephen Reay)



> On 5 Oct 2022, at 12:41, Dusk <dusk@woofle.net> wrote:
>=20
> On Oct 4, 2022, at 21:46, Eugene Sidelnyk <zsidelnik@gmail.com> wrote:
>>   $foo =3D (int|float)$bar;
>=20
> As written, I wouldn't know what to expect this to do with a string =
value -- would it cast it to int or float?
>=20
> Based on the behavior of your second example, the answer appears to be =
"float", so this syntax seems to be equivalent to:
>=20
>    $foo =3D \is_int($bar) ? $bar : (float) $bar;
>=20
> Or, even more concisely:
>=20
>    $foo =3D 0+$bar;
>=20
> I'd be even less sure what to expect when casting to other union =
types. What would the expected result of casting a string to =
(bool|array|object) be, for example? I'm not sure there are many =
meaningful operations which could be constructed here.
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> PHP Internals - PHP Runtime Development Mailing List
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>=20


Hi,

I think the intention is that it would follow precisely the same casting =
rules as function parameters do.

So e.g:

```
function foo(int|float $var) {
	return $var;=20
}

$f1 =3D foo(=E2=80=981=E2=80=99);
$c1 =3D (int|float) =E2=80=981=E2=80=99;


$f2 =3D foo(=E2=80=981.1=E2=80=99);
$c2 =3D (int|float) =E2=80=981.1=E2=80=99;

```

In both scenarios the $f1/$c1 and $f2/$c2 would result in the same type.

Your first example isn=E2=80=99t quite the same though; Passing =E2=80=981=
=E2=80=99 to a parameter typed as `int|float` will cast it to an =
integer, your example casts such a value to a float.

The `0+$value` thing is interesting, but also definitely seems less =
intuitive to my eye.


Given your last comment I think its worth reiterating Eugene=E2=80=99s =
point is simply about exposing the **existing** cast behaviour that =
happens with typed parameters, to be usable on variables.


I can see some benefit in the proposal, but I think I=E2=80=99d be more =
interested in the ability to define local variables as being a given =
type (or union), and then have the engine cast (or error) when assigning =
to them - essentially how typed object properties work, but for regular =
variables.