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Date: Thu, 21 Apr 2016 13:18:31 +1000
Message-ID: <CAF1yvASAJoBp86Yr8gNq0KmgrOm21a0vC5vMQ8s2UszXBoVEJg@mail.gmail.com>
To: Rowan Collins <rowan.collins@gmail.com>
Cc: PHP internals <internals@lists.php.net>
Content-Type: multipart/alternative; boundary=f46d04289df73787b30530f62a65
Subject: Re: [PHP-DEV] [RFC:generics]
From: me@jesseschalken.com (Jesse Schalken)

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On Thu, Apr 21, 2016 at 8:56 AM, Rowan Collins <rowan.collins@gmail.com>
wrote:
>
> I'm not quite sure what bounds would mean for anything other than classes
> or interfaces. A generic type that specified that its type parameter must
> be an int seems to me to be a non-generic type, because nothing other than
> an int "is int". How is "class Foo<T is int> { public function
> __construct(T $arg) { ... } }" different from just saying "class Foo {
> public function __construct(int $arg) { ... } }"? (Unless there were some
> kind of overloading, such that you could also have Foo<T is float>, but
> please let's not go there...)
>

Sticking with your example for a moment, if the type parameter is an "int",
then the only type that can *currently* be expressed in the type language
that is a subtype of "int" is "int", so it's not very useful *right now*.
However, a possible introduction of integer literal types would change
that, especially in combination with unions. This would enable you to
express the type "1|2|3", i.e. only one of those three integers. This would
be useful to typecheck enums, which are typically an "int" or "string"
which only accepts a finite set of values. As such, the type parameter "T
is int" could be filled with "int" or "1" or "0|1|2". (Imagine a class
"Bar<T is int>" and various subclasses which each must select a single
integer as kind of tag, "Foo extends Bar<1>", "Baz extends Bar<2>" etc. I
do this in TypeScript with string literal types all the time.)

Beside from that, the introduction of any one of union types, intersection
types, generic array types or callable types would mean the type constraint
for a generic type can usefully be something besides a single
class/interface. For example, if "<T is array|Travarsable>" were written
"<T instanceof array|Traversable>" I would expect to be able to do "$t
instanceof array|Traversable", but I wouldn't be able to unless
"instanceof" is intended to and is extended to support arbitrary types.

Note that generic arrays are out of scope of the current RFC anyway, so
> it's possible that "$t instanceof array<Bar>" could be added at the same
> time as "Foo<T instanceof array<Bar>>", presumably with the meaning that
> "array<SubClassOfBar>" would be considered to pass the check.
>
> Regards,
>
> --
> Rowan Collins
> [IMSoP]
>
>
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>

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