It was suggested on reddit and seemed to be thought a good idea that it
would be better for the syntax to be a comment like {-# INLINE -#}, etc.
That allows backward compatibility with older versions without CPP, and
also doesn't use up syntax.
I think the problem is that we are on the doorstep of a release now and the
existing syntax has already been under use.
On Mon, Mar 24, 2014 at 8:36 AM, Mark Lentczner
Again, sorry for the 11:59 meddling....
The syntax of role annotations is very heavy weight, and requires lots of CPP where there wasn't need before. Two thoughts:
1) Couldn't we do something like use "cue" type constraints? For example, assuming the default is representational, and that phantom can just be inferred, all we need is a way to indicate nominal:
data (Nominal k) => Map k v = ...
This could then probably be easily made to compile in other compilers with some null library implementation of Nominal
2) An alternative to the above. We generally frown on constraints in a data / newtype declaration, but perhaps this is exactly the case for them, whereby we can now do away with the type role syntax: We can infer nominal if there are *any* constraints on a type parameter, *representational* if there are none, and *phantom *if there are no mentions in the right hand side:
data (Eq k) => Map k v = ...
This seems even nicer and just works with all compilers... but perhaps I'm missing something. (Yes, I imagine there are type constraints that shouldn't force nominal, but perhaps not worth worrying about?)
Mind you, this all just about syntax... the issue of what is the implication for libraries of the default being representational is still at hand.
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