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Date: Wed, 7 Apr 2021 15:58:03 +0500
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Subject: Empower Callables with Function Signature Hints
From: office.hamzaahmad@gmail.com ("Hamza Ahmad")

Hello Internals,
I want to request making possible to catch functions with wrong signature
when passed as callable. In other words, when a function or a method is
passed as callable value, PHP will first confirm whether the function is of
the write callable type that the function being executed requires. It will
help catch silent errors.
Currently, PHP does following:
1. Confirms whether a callable is valid.
2. Passes the callable value to a function, and the execution starts.
3. If the passed function receives values which's type is different than
function signature, PHP throws an error.
4. If passed function returns nothing, the variable that wanted a value from
the passed function receives null.
5. As a result, the unexpected results emerge.
To resolve this issue, I propose to introduce a mechanism that allows to
hint callable signatures. To understand the spirit of this request, look at
the similar functionality that Angel Script Project provides with.
1. One defines a function definition.
2. Use its name to hint the same signature.
3. The compiler alerts about the incorrect signature before passing this to
a function.
If I could do this in PHP, I would write like following:
`funcdef preg_replace_callback_hint(array $matches) : string;`
If I accidently pass a callable which's signature is different than desired
function call, the error is caught earlier.
This message does not suggests syntax; it only points to a problem and
suggests a solution. Any volunteers that would love to come up with syntax
and implement this?
I am curious to see the feedback.
Regards
Hamza Ahmad