Fully dynamic d by opDotExp overloading

[Steven Schveighoffer]

On Fri, 17 Apr 2009 23:43:22 -0400, Steven Schveighoffer  
<schveiguy at yahoo.com> wrote:

> On Fri, 17 Apr 2009 21:54:52 -0400, Steven Schveighoffer  
> <schveiguy at yahoo.com> wrote:
>
>> Andrei wrote:
>>> We are discussing a language extension. That language extension will  
>>> allow a type to choose flexibility in defining methods dynamically,  
>>> while being otherwise integrated syntactically with the current  
>>> values. This has advantages, but also alters the expectations.
>>
>> As long as it identifies what can be dynamic and what cannot.  I can't  
>> imagine Walter will go for this with his strict view of hijacking.
>
> Let me add that if there was a way for syntax to easily allow for  
> unintentional calls to be translated to compile-time errors, I think  
> this would be a workable solution.
>
> For example, I don't have any problem with your Pascalize example,  
> because you have not removed any static typing from the code (i.e. no  
> unexpected noops or exceptions are built in).  If there were some way to  
> enforce this, then I think it would be a usable idea.  For instance, if  
> you only allow CTFE to specify a function that is called when certain  
> strings are passed in, I don't have a problem with that, because you are  
> simply dispatching the data to strongly typed functions at compile time,  
> which provide compile-time errors when you mess up.

I gave this a lot of thought, and I think here is a possible solution:

the main reason I'm hesitant on this idea is because of code like this:

class X
{
   auto opDotExp(string fname, T...)(T args)
   {
      if(fname == "blah")
        return foo(args);
      else if(fname == "blither")
        return bar(args);
      // else, nothing happens
   }
}

Which leaves code open to lots of compiled code that doesn't do the right  
thing (or throws some runtime exception).  What would be nice is if the  
default behavior is what statically bound functions do, that is, compile  
error, and only let the cases be handled which the author expects to  
handle.

For that, I think if we make the following rule, we will see much less  
code that is poorly written, and I think dynamic functions will be  
feasible:

If the compiler can determine during compilation of an opDotExp instance  
that the resulting function is empty, then it is a compiler error, just  
like if you tried to call a function that doesn't exist.  This behavior  
can be overridden by putting a return statement in an otherwise empty  
function.

So for example, the above can be statically determined to compile to  
nothing if fname is not "blah" or "blither", and therefore would be a  
compiler error.  Of course, if you call functions that are not statically  
evaluable, then you are back to the danger of truly dynamic bindings, but  
that would make sense for things that cannot be evaluated at compile time.

What do you think?

-Steve