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

On Wed, Jul 1, 2026, 12:11 PM Matheus Martins <[email protected]> wrote:

> > I would rather wait for generics and possibly make Closure a generic
> type.
>
> Fair point, but I am worried that's misleading, and I would keep the two
> separate.
>
> Generics are a much larger and more contentious topic; typed closure is a
> much
> smaller topic: a runtime shape check on a Closure value at the boundary,
> nothing
> more.
>
> The concrete benefit is typing the callback a higher-order method takes,
> so a mismatched map/filter callback is caught at the call site rather than
> deep
> inside the operation (or only by static analysis):
>
> ```php
> // expected signature: (int) => bool
> public function filter(Closure(int): bool $pred): static;
>
> // ok
> $nums->filter(fn(int $n): bool => $n % 2 === 0);
>
> // TypeError: string not expected
> $nums->filter(fn(string $s): bool => $s !== '');
> ```
>
> Typed closures neither need generics nor blocks them. In fact, eventually
> typed
> closures will support generics. But I would rather keep these topics
> separate.
>
> On its own, is a runtime-checked closure signature something you would find
> useful?
>

I would find this very useful. That said, restricting it to closures and
not any callable is very limiting, particularly now that we have
first-class callable syntax.

Perhaps something more along the line of :

    callable(int):bool

This would give more flexibility, and provide us contract guarantees. Right
now, SA can validate these, but only via annotations.

-- 
Matthew Weier O'Phinney
[email protected]
https://mwop.net/
he/him

>