Hi, the recent discussion about roles and abstraction lets me wonder whether we should re-evaluate the question of where the Coercible instances come from. Currently, upon use of "coerce", GHC will create Coercible instances out of thin air, as long as the roles agree, and checking some constructor-in-scope invariants. This is troublesome WRT. abstraction. One alternative that was discussed before is explicit Coercible instances. That is, if I write a datatype datatype MyList a = Cons a (MyList a) | MyNil then I have to explicitly write either deriving Coercible to get instance Coercible a b => Coercible (MyList a) (MyList b) or – especially if I want to control which parameters are coercible – have to use standalone deriving and write deriving instance Coercible a b => Coercible (MyList a) (MyList b) For something like Set, the user would not use “deriving Coercible”, but would use deriving instance Coercible a b => Coercible (Set k a) (Set k b) Manual instances of Coercible would be prohibited. The instances are checked for things like: * do the roles allow the behaviour? * are the constructors in scope (for standalone deriving)? * are the arguments of the constructors coercible? The last check ensures that the user cannot break abstraction by wrapping an abstract datatype in a datatype of its own. But note that these instances (between the datacon argument types) are only checked, never actually used! Therefore we must sure that only sound instances exists, and that the user cannot bogus instances (e.g. one with all fields undefined). This would imply a few differences: * Already for syntactical reasons, there will be no instances for coercing constraints. As we have seen, this is a good thing. * Would be no way to use a coercion within the abstraction boundaries, but not outside, because you either have the instance, or you don’t. And assuming GND would simply implement its classes by passing each method through "coerce", this would hopefully also yield a sensible design for GND. Greetings, Joachim -- Joachim “nomeata” Breitner mail@joachim-breitner.de • http://www.joachim-breitner.de/ Jabber: nomeata@joachim-breitner.de • GPG-Key: 0x4743206C Debian Developer: nomeata@debian.org