Re: When are scala macros expanded?

Hi Andrew,

I understand that this is an edge case, and probably not a good idea to 
begin with.  However, it's still an interesting question: if a method calls 
implicitly[Something], but that method is never called, can you consider 
the implicit Something to be required?  Should compilation then fail if it 
isn't provided?  The compiler has no problem checking for the implicit - 
the issue (if you can call it that) is that the compiler checks it eagerly 
during compilation of the class, not at the call site of the method which 
calls implicitly[Something].

On Wednesday, June 17, 2015 at 8:10:44 PM UTC-7, Andrew Phillips wrote:
>
> Hi Jeremy
>
> Without having spent much time trying to come up with a creative approach, 
> I would be surprised if what you're trying to do were possible.
>
> If I understand your problem statement correctly, you're trying to add a 
> condition to a method defined on an interface "after the fact:" an 
> additional (implicit) parameter, in this case. Yet this condition is only 
> supposed to be checked for by the compiler if the implementation class of 
> the interface happens to be *your* specific class. Which would seem to 
> require that the compiler know up front what the *runtime* class of the 
> implementation will be.
>
> If I write:
>
> val myObj: SomeInterface = ...
> myObj.foo(...)
>
> I would imagine that the compiler can *only* verify that my attempt to 
> call foo is compatible with the definition of foo in SomeInterface, 
> i.e. the static type of myObj. I don't see how, at compile time, we would 
> be able to say: "Ah, if myObj in this case happens to be an instance of 
> SpecialImplOfSomeInterface, we need to check for an extra 
> implicit/parameter/parameter list/whatever."
>
> Curious to see what other participants in this thread will come up with!
>
> Regards
>
> ap
>

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