override '.' member access

[spir]

On 01/26/2011 01:06 AM, Simen kjaeraas wrote:
> spir <denis.spir at gmail.com> wrote:
>
>> On 01/25/2011 10:29 PM, Simen kjaeraas wrote:
>>> spir <denis.spir at gmail.com> wrote:
>>>
>>>> Hello,
>>>>
>>>> Cannot find corresponding opSomething method, if any. (opDispatch seems to
>>>> specialise for method call.)
>>>> Else, how to catch obj.member?
>>>
>>> opDispatch is likely what you want. with the @property annotation, it
>>> will readily support obj.member; and obj.member = foo; syntax.
>>
>> Thank you, Simen, i'll try using opDispatch with @property. But I'm not sure
>> how to write that concretely. My use case is of a type holding a
>> string[AnyThing] AA called symbols. Then, I wish to map
>> obj.id
>> to
>> obj.symbols["id"]
>> (hope I'm clear)
>
> I just found that it is, in fact, unpossible. That is, you can support
> either a = foo.id; or foo.id = a; - not both. This is caused by bug 620:
> http://d.puremagic.com/issues/show_bug.cgi?id=620

I do not really understand the subtleties of this bug, but indeed, I found no 
way to use opDispatch to set data members.
In other words, I could implement python's __getattr__ or Lua's __index,
not  python's __setattr__ or Lua's __newindex.

EDIT: Could do this as a workaround.

alias int[string] Symbols;

class T {
     int i;
     Symbols symbols;
     this (int i, Symbols symbols) {
         this.i = i;
         this.symbols = symbols;
     }
     auto opDispatch (string name) () {
         writeln("get");
         auto p = (name in this.symbols);
         if (p)
             return *p;
         throw new Exception("No symbol called "~name~".");
     }
     auto opDispatch (string name, Value) (Value value) {
         writeln("set ");
         auto p = (name in this.symbols);
         if (p) {
             *p = value;
             return;
         }
         throw new Exception("No symbol called '"~name~"'.");
     }
}
unittest {
     auto t = new T(1, ["j":2, "k":3]);
     writefln("<%s %s %s>", t.i, t.j,t.k);
     t.i = 11;
     t.j(22);
     t.k(33);
     writefln("<%s %s %s>", t.i, t.j,t.k);
//~     writeln(t.l);   // throws as expected
//~     t.l(44);   // throws as expected
     t.j = 222;  // Error: function
                 // __trials__.T.opDispatch!("j").opDispatch ()
                 // is not callable using argument types (int)
}

The last example shows how normal member-set syntax fails. I would like to know 
into what
	obj.name = val
is rewritten.

Denis
-- 
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