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Date: Tue, 19 Mar 2024 23:20:00 +0100
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Subject: Re: [PHP-DEV] Proposal: AS assertions
To: PHP internals <internals@lists.php.net>
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From: tovilo.ilija@gmail.com (Ilija Tovilo)

Hi Rowan

On Tue, Mar 19, 2024 at 8:39=E2=80=AFPM Rowan Tommins [IMSoP]
<imsop.php@rwec.co.uk> wrote:
>
> As well pattern matching, which Ilija mentioned, another adjacent feature=
 is a richer set of casting operators. Currently, we can assert that someth=
ing is an int; or we can force it to be an int; but we can't easily say "ma=
ke this an int if safe, but throw otherwise" or "make this an int if safe, =
but substitute null/$someValue otherwise".
>
> I've been considering how we can improve that for a while, but not settle=
d on a firm proposal - there's a lot of different versions we *could* suppo=
rt, so choosing a minimal set is hard.

I've thought about this in the context of pattern matching a while
back. I was thinking about something like `$x is ~int`, where the
pattern match is successful iff `$x` is coercible to `int` without
loss of information. Given that patterns may be nested, `array<~int>`
could check that all elements of an array are coercible to `int`. The
same could work for literal patterns, e.g. `~5`, where `5`, `5.0` and
`'5'` are all accepted.

This can potentially be combined with the variable binding pattern,
`$var @ pattern`. The syntax looks a bit confusing at first, but it
basically makes sure that the matched value conforms to `pattern`, and
then binds it to `$var`. Hence, something like `$foo as Foo { $bar @
~int }` would 1. make sure `$foo` is an instance of `Foo`, 2. make
sure `$foo->bar` is coercible to `int`, and then assigned the coerced
value to `$bar`. (It gets more complicated, because the assignment
must be delayed until the entire pattern matches.) If the pattern
matching fails at any point, it throws.

This is just an idea, neither the `as` operator nor the `~` pattern
have been implemented. I don't know whether they are feasible.

Anyway, we're probably going off-topic. :)

Ilija