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Date: Sun, 11 Jul 2021 18:19:20 -0500
To: "php internals" <internals@lists.php.net>
Content-Type: text/plain
Subject: Re: [PHP-DEV] Type casting syntax
From: larry@garfieldtech.com ("Larry Garfield")

On Sun, Jul 11, 2021, at 5:30 PM, Levi Morrison via internals wrote:
> I like Rust's From and TryFrom traits, which allow types to define
> conversions into or from other types. The From trait always succeeds,
> and TryFrom can fail. However, it's not a "cast" -- the user calls an
> `into`/`try_into` or `from`/`try_from` methods.
> 
> I would be supportive of defining official ways to convert types from
> one to another, not necessarily casts, though. I think these qualities
> are important:
> 
>  1. Would have failable and non-failable variants. It's nice to know
> that a particular conversion cannot or can fail.
>  2. It should be amenable to static analysis. Tools should be able to
> warn about missing paths for handling failable conversions, or that
> conversion from A to B is possible but A to C is not.
> 
> I'm not sure how to technically achieve this for PHP. I don't think
> any casting proposal I've seen meets these characteristics, so I would
> personally vote no on them.

What are the use cases for integrated object to object (ie, class to class) conversion in PHP?  Rust's type system is a very different beast, so its casting logic makes more sense there.  I'm not entirely clear on what the use case is in PHP.  When would that be superior to "just write an asFoo() method and move on with life"?

Scalar conversion I can see, but every time someone suggests adding siblings to __toString(), there's major pushback.

--Larry Garfield