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
| 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