dynamic classes and duck typing

[Andrei Alexandrescu]

Steven Schveighoffer wrote:
> On Tue, 01 Dec 2009 08:49:58 -0500, Denis Koroskin <2korden at gmail.com> 
> wrote:
> 
>> On Tue, 01 Dec 2009 16:46:25 +0300, Steven Schveighoffer 
>> <schveiguy at yahoo.com> wrote:
>>
>>> On Mon, 30 Nov 2009 23:32:21 -0500, Bill Baxter <wbaxter at gmail.com> 
>>> wrote:
>>>
>>>> On Mon, Nov 30, 2009 at 7:12 PM, Walter Bright
>>>> <newshound1 at digitalmars.com> wrote:
>>>>> Bill Baxter wrote:
>>>>>>
>>>>>> So we can overload on @property-ness?
>>>>>
>>>>> No.
>>>>>
>>>>>> I.e. this works
>>>>>>
>>>>>> struct S
>>>>>> {
>>>>>> @property
>>>>>> float x() { return 1.0f; }
>>>>>> float x() { return 2.0f; }
>>>>>> }
>>>>>>
>>>>>> void main()
>>>>>> {
>>>>>>    S  s;
>>>>>>    writefln("%s", s.x); // writes 1.0
>>>>>>    writefln("%s", s.x()); // writes 2.0
>>>>>> }
>>>>>
>>>>> That just looks wrong.
>>>>>
>>>>
>>>> Ok, so you can't have both dynamic properties and dynamic methods with
>>>> this.  One or the other, your pick.
>>>> Seems like an unfortunate limitation.
>>>
>>>
>>> what a minute, can't you use template conditionals to distinguish?  
>>> i.e. I would expect this to work:
>>>
>>> struct S
>>> {
>>>    @property float opDispatch(string s)() if (s == "x") {return 1.0f;}
>>>    float opDispatch(string s)() { return 2.0f;}
>>> }
>>>
>>> void main()
>>> {
>>>    S s;
>>>    writefln("%s", s.x); // 1.0
>>>    writefln("%s", s.y()); // 2.0
>>> }
>>>
>>> Overloading opDispatch based on the called symbol name should always 
>>> be possible, and overloading on parameter types is always possible.
>>>
>>> -Steve
>>
>> What if you don't know argument names a-priori? Consider a generic 
>> Dynamic class that has nothing but a single opDispatch method.
> 
> although opDispatch allows some dynamic function definitions, the 
> *usage* of opDispatch is always static.  The question is, if you are for 
> example wrapping another type, can you introspect the attributes of its 
> methods?
> 
> For example, I'd expect something like this should be possible in the 
> future:
> 
> struct Wrapper(T)
> {
>    T t;
>    @property auto opDispatch(string s)() if(isProperty!T(s) ) 
> {mixin("return t." ~ s ~ ";");} // getters
>    @property auto opDispatch(string s, A)(A arg) if(isProperty!T(s) ) 
> {mixin("return (t." ~ s ~ " = arg);"); } // setters
>    auto opDispatch(string s, A...)(A args) { mixin("return t." ~ s ~ 
> "(args);");}
> }
> 
> Now, given the function attributes that are possible (this does not 
> include const and immutable, which are overloaded via parameter types), 
> this is going to get pretty ugly quickly.  Unfortunately, the attributes 
> are not decided by the caller, but by the callee, so you have to use 
> template conditionals.  It would be nice if there was a way to say "copy 
> the attributes from function x" when defining template functions in a 
> way that doesn't involve conditionals, but even then, you would have a 
> hard time defining such usage because you don't know what function you 
> want until you evaluate the template string.
> 
> -Steve

Yes, we need to implement that. Essentially the pipe dream is to add 
opDispatch to Variant and have it accept any call that would go through 
the contained type.

These are heady days for D!


Andrei