Hannes Magnusson wrote on 04/02/2015 23:58:
On Wed, Feb 4, 2015 at 10:49 AM, Nikita Popov <nikita....@gmail.com> wrote:
Hi internals!
Currently we do not allow [1] removing a typehint during inheritance. For
example the following code is not valid:
interface A {
public function method(Typehint $param);
}
class B implements A {
public function method($param);
}
// Fatal error: Declaration of B::method() must be compatible with
A::method(Typehint $param)
The above code does *not* constitute an LSP violation, because B::method()
accepts more inputs than A::method(). However we still forbid it.
So what it supports "more inputs"?
It does constitute an LSP violation. "more inputs" is not what the
guarantee is at all, if that is what you want you'd typehint on a
interface.
Have a look at the Wikipedia article on covariance and contravariance;
the idea is that if you know you have a container which can contain
Cats, a custom implementation that also happens to accept Dogs is fine,
but an implementation that only accepts Kittens is not, because the
object is contractually required to accept any Cat that you give it.
Return types are the reverse, because the object is contractually
required to return you nothing but Cats, so returning a Dog would be a
violation; but constraining itself to only return Kittens doesn't affect
the contract you have with it.
The special case proposed here is that the lack of type hint means "any
type", as though you were type hinting the base of all possible types.
Thus removing a typehint on a parameter does not allow the object to
violate the parent's contract, and nor does adding a return typehint.
However, the fact that this is tricky to understand is probably a big
problem for it. The Wikipedia article notes that few languages implement
parameter contravariance, mostly because they'd have to mix it with
type-based function overloading, making real-life examples hard to find.
Regards,
--
Rowan Collins
[IMSoP]
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