Re: [PHP-DEV] [Discussion] Revisiting typed closures

Hi Osama,

> The syntax collides at the lexer. `Closure(int): array` can't be tokenized cleanly, because `(int)` is a cast token (same for `(string)`, `(array)`, and so on).
> It's been that way for years, and the one attempt to fix it (PR https://github.com/php/php-src/pull/1667) was rejected as a token-stream BC break.
> So the natural spelling is, unfortunately, the engine-hostile one.

This is fixable, it's not a real limitation.

> There's a single `Closure` class (Ilija's point here: https://externals.io/message/120083#120099), so `Closure(int, string): array` isn't a subtype you can instanceof...
> it has to be checked another way at the boundary, which is where the usual per-boundary runtime-cost concern comes in.

I agree on this, I think the `fn` keyword should be used here.