DIP66 1.2 (Multiple) alias this. Continuation of work.

Steven Schveighoffer via Digitalmars-d digitalmars-d at puremagic.com
Tue Mar 31 13:20:54 PDT 2015

On 3/31/15 4:01 PM, Andrei Alexandrescu wrote:
> On 3/31/15 7:28 AM, IgorStepanov wrote:
>> On Monday, 30 March 2015 at 18:33:17 UTC, Andrei Alexandrescu wrote:
>>> On 3/30/15 8:04 AM, Steven Schveighoffer wrote:
>>>> On 3/29/15 1:34 PM, IgorStepanov wrote:
>>>>> 1. We should reject types which use opDispatch and alias this at the
>>>>> same time.
>>>> Why? Alias this has no filter. opDispatch can use template constraints.
>>>> It makes perfect sense to prefer opDispatch, unless it doesn't have a
>>>> valid match, and then use alias this instead.
>>>> For example, if I wanted to wrap a type so I can instrument calls to
>>>> 'foo', I could do something like this:
>>>> struct FooWrapper(T)
>>>> {
>>>>    T t;
>>>>    alias t this;
>>>>    auto opDispatch(string s, A...)(A args) if(s == "foo") {
>>>> writeln("calling foo"); return t.foo(args); }
>>>> }
>>>> Why is this a bad use case?
>>> The idea is to start restrictive and define interaction meaningfully
>>> later based on compelling use cases. -- Andrei
>> Andrei, do you approve those changes? Can we move to work on my github
>> PR?
> I made a few editorial passes, no major changes. I think there's still a
> fly in the ointment. The resolution algorithm goes:
> 1. If xyz is a symbol (member, method, enum etc) defined inside
> typeof(obj) then lookup is done.
> 2. Otherwise, if xyz is a symbol introduced in the base class (where
> applicable), then lookup is done.
> 3. Otherwise, if opDispatch!"xyz" exists, then lookup is done.
> 4. Otherwise, alias this is attempted transitively, and if xyz is found,
> then lookup is done.
> 5. Otherwise an UFCS rewrite is effected.

swap 2 and 3.

> This puts opDispatch in between inheritance and subtyping, which I think
> we discussed is inappropriate - alias this should be effectively subtyping.

I don't understand this statement. What is the difference between 
inheritance and subtyping?

To me, opDispatch is equivalent to adding a member function (with 
specific members overriding opDispatch), alias this is equivalent to 
inheriting from another type (with inherited members overriding alias this).

And I still think that alias this + opDispatch conflicts should defer to 
opDispatch. It makes no sense to do it any other way.


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