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Date: Thu, 21 Apr 2016 12:24:14 +0100
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Subject: Re: [PHP-DEV] [RFC:generics]
From: rowan.collins@gmail.com (Rowan Collins)

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Jesse Schalken wrote on 21/04/2016 04:18:
>
> 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.)

I'm struggling to follow this example; it seems like you are using the 
type parameter to supply actual data to the implementation, which goes 
rather beyond generics, and into full-blown template metaprogramming. 
I'm not sure why you'd ever need the sub-class to compile in a value 
that way rather than just overriding a method somewhere, and would need 
some convincing that PHP needed this kind of complexity.


> 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.

This is a stronger argument, although I think extending the instanceof 
operator might be a good idea in that case anyway - otherwise, we would 
have a constraint that you can assert (produce a TypeError at a function 
boundary) but not easily test for. To avoid replicating the engine's 
entire checking routine, you'd end up doing this:

function ugly_instance_of<T>($value): boolean {
     try {
         return (function(T $checked) { return true; })($value);
     }
     catch ( TypeError $e ) {
         return false;
     }
}

if ( ugly_instance_of<array<Foo>>($x) ) // if ( $x instanceof array<Foo> )

*shudder* ;)

Regards,
-- 
Rowan Collins
[IMSoP]

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