dynamic classes and duck typing

[Denis Koroskin]

On Tue, 01 Dec 2009 15:47:43 +0300, Ary Borenszweig <ary at esperanto.org.ar>  
wrote:

> Denis Koroskin wrote:
>> On Tue, 01 Dec 2009 15:05:16 +0300, Ary Borenszweig  
>> <ary at esperanto.org.ar> wrote:
>>
>>> Ary Borenszweig wrote:
>>>> retard wrote:
>>>>> Tue, 01 Dec 2009 03:16:47 -0800, Walter Bright wrote:
>>>>>
>>>>>> Ary Borenszweig wrote:
>>>>>>> Can you show examples of points 2, 3 and 4?
>>>>>> Have opDispatch look up the string in an associative array that  
>>>>>> returns
>>>>>> an associated delegate, then call the delegate.
>>>>>>
>>>>>> The dynamic part will be loading up the associative array at run  
>>>>>> time.
>>>>>
>>>>> This is not exactly what everyone of us expected. I'd like to have  
>>>>> something like
>>>>>
>>>>> void foo(Object o) {
>>>>>   o.duckMethod();
>>>>> }
>>>>>
>>>>> foo(new Object() { void duckMethod() {} });
>>>>>
>>>>> The feature isn't very dynamic since the dispatch rules are defined  
>>>>> statically. The only thing you can do is rewire the associative  
>>>>> array when forwarding statically precalculated dispatching.
>>>>  Exactly! That's the kind of example I was looking for, thanks.
>>>
>>> Actuall, just the first part of the example:
>>>
>>> void foo(Object o) {
>>>     o.duckMethod();
>>> }
>>>
>>> Can't do that because even if the real instance of Object has an  
>>> opDispatch method, it'll give a compile-time error because Object does  
>>> not defines duckMethod.
>>>
>>> That's why this is something useful in scripting languages (or ruby,  
>>> python, etc.): if the method is not defined at runtime it's an error  
>>> unless you define the magic function that catches all. Can't do that  
>>> in D because the lookup is done at runtime.
>>>
>>> Basically:
>>>
>>> Dynanic d = ...;
>>> d.something(1, 2, 3);
>>>
>>> is just a shortcut for doing
>>>
>>> d.opDispatch!("something")(1, 2, 3);
>>>
>>> (and it's actually what the compiler does) but it's a standarized way  
>>> of doing that. What's the fun in that?
>>  The fun is that you can call d.foo and d.bar() even though there is no  
>> such method/property.
>>  In ActionScript (and JavaScript, too, I assume), foo.bar is  
>> auto-magically rewritten as foo["bar"]. What's fun in that?
>
> The fun is that in Javascript I can do:
>
> ---
> function yourMagicFunction(d) {
>    d.foo();
> }
>
> var something = fromSomewhere();
> yourMagicFunction(something);
> ---
>
> and it'll work in Javascript because there's no type-checking at  
> compile-time (well, because there's no compile-time :P)
>
> Let's translate this to D:
>
> ---
> void yourMagicFunction(WhatTypeToPutHere d) {
>    d.foo();
> }
>
> auto something = fromSomewhere();
> yourMagicFunction(something);
> ---
>

I believe there will soon be a library type that would allow that.