OK -- thanks for clarifying. This all sits better with me. But, I'm still a little concerned about the "Safe Haskell" implications. My understanding is that allowing coercions when the constructor is not exported will not be considered "Safe". Here's a way forward:
Currently: a type is considered abstract when its constructors are not exported. Future proposal: a type is considered abstract when its constructors are not exported AND its type parameters are all at role Nominal. Under this new definition of "abstract", a library writer that remembers not to export a constructor but neglects to use a role annotation should consider a type *not* to be abstract. Is this what we want? Richard On Sep 7, 2013, at 1:16 PM, Simon Peyton-Jones <simo...@microsoft.com> wrote: > > | In the current code, the > | instance Coercible a b => Coercible (T a) (T b) > | is available for both data and newtypes, if T’s type argument has > | Representational role, but independent of any constructor presence. See > | the note at > | https://github.com/nomeata/ghc/compare/ntclass-clean#L9R1902 > | for a concise and complete list of the conditions for a Coercible > | instance. > > Right! I explained that badly the first time; thanks for clarifying Joachim. > > So newtype and data behave alike, except that newtype has the *additional* > property that if its constructor is available you can coerce to the > representation type. > > | If I understood Simon’s last suggestion correctly than exporting a type > | constructor with a non-Nominal role means “I am fine if you cast this > | argument”. If this is not desired (e.g. maybe Ptr a is an example here), > | then the library author has to annotate the type argument as Nominal. > > Yes, that's right. In theory someone could want the coercible instance > *plus* the nominal role, or vice versa, but I think we can jump that bridge > if we come to it. > > Simon > _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://www.haskell.org/mailman/listinfo/ghc-devs
