dynamic classes and duck typing

[Denis Koroskin]

On Sun, 29 Nov 2009 16:02:24 +0300, Lutger <lutger.blijdestijn at gmail.com>  
wrote:

> biozic wrote:
>
>> Le 29/11/09 13:16, Michel Fortin a écrit :
>>> On 2009-11-29 06:14:21 -0500, "Simen kjaeraas" <simen.kjaras at gmail.com>
>>> said:
>>>
>>>> That is because your opDispatch is instantiated no matter what the  
>>>> name
>>>> is, but only does something sensible if it's foo. Try this:
>>>>
>>>> string opDispatch( string name )( ) {
>>>> static if ( name == "foo" ) {
>>>> return "foo";
>>>> } else {
>>>> static assert( false, "Invalid member name." );
>>>> }
>>>> }
>>>
>>> Wouldn't this be even better?
>>>
>>> string opDispatch(string name)() if (name == "foo") {
>>> return "foo";
>>> }
>>>
>>> I haven't tested that it works though.
>>>
>>
>> It doesn't improve the error message, but it works. It's been a long
>> time since I used D: I didn't know this syntax!
>
> Don has made a patch to improve these kind of error messages in  
> templates,
> but that will probably come after D2 is finalized (doesn't affect the
> language).
>
> If you want to resolve the symbol at runtime I think you can get a better
> error message for throwing an exception or assertion. I don't have the  
> svn
> dmd, so this isn't tested:
>
> void opDispatch(string name)(string file = __FILE__, int line = __LINE__)
> {
>   if ( !dynamicDispatch(name) )
>   {
>     // line and file are default initialized from the call site:
>     throw new MethodMissingException(name, file, line);
>   }
> }
>
>

IIRC, this trick only works when __FILE__ and __LINE__ are both template  
arguments.