On Sep 6, 2013, at 12:47 PM, Simon Peyton-Jones <simo...@microsoft.com> wrote:
> > So > > * for newtypes > newtype N a = MkN <rep-ty> > the coercion is between (N a) and its representation type <rep_ty>. > The coercion is allowed if the data constructor MkN is in scope > > * for data types (T a), the coercion is between (T a) and (T b), > The coercion is allowed if the roles allow it. > > The two are handled quite differently. This makes me say "Yikes!" I've always lived under the impression that changing the word "newtype" to "data" should have exactly 0 effect on the compile-time behavior of Haskell programs. Yet, you're proposing that > module Foo (T) where -- MkT is *not* exported > > newtype T a = MkT [a] is different from the same with "newtype" replaced with "data". To wit, in the "data" version, the instance (Coercible a b => Coercible (T a) (T b)) would exist, but this would not be derivable in the "newtype" version. Right? A related issue is that, even with roles, I don't think GeneralizedNewtypeDeriving (GND) is in the Safe Haskell subset because it can break abstraction barriers -- you can use GND even when a newtype's constructor is not in scope. The above proposal for coercions will have the same problem. What to do? Map should certainly not export its constructor(s). Yet, we want (Coercible a b => Coercible (Map x a) (Map x b)). It seems that the writer of Map would have to explicitly export this instance. This goes at odds with the idea of "there aren't any instances of Coercible, really", but otherwise I have a hard time seeing how this would all work. Am I missing something here? Richard _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://www.haskell.org/mailman/listinfo/ghc-devs
