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Message-ID: <57179F54.7020302@gmail.com>
Date: Wed, 20 Apr 2016 16:25:08 +0100
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Subject: Re: [PHP-DEV] [RFC:generics]
From: rowan.collins@gmail.com (Rowan Collins)

Larry Garfield wrote on 20/04/2016 16:12:
> On 4/20/16 10:01 AM, Rowan Collins wrote:
>> guilhermeblanco@gmail.com wrote on 20/04/2016 03:54:
>>> 1- Even though mentioned, I'd still use "extends" or "implements" 
>>> instead
>>> of "is" (which would be a new pseudo-reserved keyword) to enforce 
>>> data type
>>> consistency and prevent developers to potentially referring to one 
>>> thing
>>> while consider another.
>>
>> Perhaps "instanceof" would make more sense here? "class 
>> FileProcessor<T extends FileHandle>" seems to exclude instantiating 
>> FileProcessor<FileHandle>, because "FileHandle extends FileHandle" 
>> doesn't make any sense.
>>
>> You could read "class Box<T instanceof Boxable>" as an assertion that 
>> the class passes the constraint, as in "public function __construct(T 
>> $t) { assert($t instanceof Boxable); }"
>>
>> Regards,
>
> I'm not an expert in generics by any means, but how is that different 
> than just
>
> public function __construct(Boxable $t) {}
>
> I thought the point of generics was for cases like "these two 
> variables must be the same type, but they can be any same type."
>

The is/extends/instanceof clause adds to the rule: "...they can be any 
same type, *as long as that type meets this constraint*"

So using an unbounded generic, you could manually check:

class FileProcessor<T>
{
public function __construct(T $t) {
assert($t instanceof FileHandle);
}
}
$f = new FileProcessor<ZipArchiveEntryHandle>($zip->getEntryHandle(1)); 
// "T" is substituted for "ZipArchiveEntryHandle"
$f = new FileProcessor<int>(42); // assertion fails!


A bounded generic, using "is" as the keyword as in the current RFC, 
performs the assertion for you:

class FileProcessor<T is FileHandle>
{
public function __construct(T $t) {
// no need for an assertion
}
}
$f = new FileProcessor<int>(42); // throws a TypeError


My suggestion was that the constraint is like an "instanceof" assertion, 
so we could use that keyword:

class FileProcessor<T instanceof FileHandle>
{
public function __construct(T $t) {
// no need for an assertion
}
}
$f = new FileProcessor<int>(42); // throws a TypeError


I hope that makes sense.

Regards,
-- 
Rowan Collins
[IMSoP]