RE: Restrictions on polytypes with type families

Hmm, no I don't think that Flattening is a very serious problem: Firstly, we can somehow get around that if we use implication constraints and higher-order flattening variables. We have discussed that (but not implemented). For example. forall a. F [a] ~ G Int becomes: forall a. fsk a ~ G Int forall a. true => fsk a ~ F [a] Secondly: flattening under a Forall is not terribly important, unless you have type families that dispatch on polymorphic types, which is admittedly a rather rare case. We lose some completeness but that's ok. For me, a more serious problem are polymorphic RHS, which give rise to exactly the same problems for type inference as impredicative polymorphism. For instance: type instance F Int = forall a. a -> [a] g :: F Int g = undefined main = g 3 Should this type check or not? And then all our discussions about impredicativity, explicit type applications etc become relevant again. Thanks! d- From: Simon Peyton-Jones Sent: Friday, April 05, 2013 8:24 AM To: Manuel M T Chakravarty Cc: Iavor Diatchki; ghc-devs; Dimitrios Vytiniotis Subject: RE: Restrictions on polytypes with type families Manuel has an excellent point. See the Note below in TcCanonical! I have no clue how to deal with this Simon Note [Flattening under a forall] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Under a forall, we (a) MUST apply the inert subsitution (b) MUST NOT flatten type family applications Hence FMSubstOnly. For (a) consider c ~ a, a ~ T (forall b. (b, [c]) If we don't apply the c~a substitution to the second constraint we won't see the occurs-check error. For (b) consider (a ~ forall b. F a b), we don't want to flatten to (a ~ forall b.fsk, F a b ~ fsk) because now the 'b' has escaped its scope. We'd have to flatten to (a ~ forall b. fsk b, forall b. F a b ~ fsk b) and we have not begun to think about how to make that work! From: Manuel M T Chakravarty [mailto:chak@cse.unsw.edu.au] Sent: 04 April 2013 02:01 To: Simon Peyton-Jones Cc: Iavor Diatchki; ghc-devs Subject: Re: Restrictions on polytypes with type families Simon Peyton-Jones mailto:simonpj@microsoft.com>: isn't this moving directly into the territory of impredicative types? Ahem, maybe you are right. Impredicativity means that you can instantiate a type variable with a polytype So if we allow, say (Eq (forall a.a->a)) then we've instantiated Eq's type variable with a polytype. Ditto Maybe (forall a. a->a). But this is only bad from an inference point of view, especially for implicit instantiation. Eg if we had class C a where op :: Int -> a then if we have f :: C (forall a. a->a) =>... f = ...op... do we expect op to be polymorphic?? For type families maybe things are easier because there is no implicit instantiation. But I'm not sure These kinds of issues are the reason that my conclusion at the time was (as Richard put it) Or, are | there any that are restricted because someone needs to think hard before | lifting it, and no one has yet done that thinking? At the time, there were also problems with what the type equality solver was supposed to do with foralls. I know, for example, | that the unify function in types/Unify.lhs will have to be completed to | work with foralls, but this doesn't seem hard. The solver changed quite a bit since I rewrote Tom's original prototype. So, maybe it is easy now, but maybe it is more tricky than you think. The idea of rewriting complex terms into equalities at the point of each application of a type synonym family (aka type function) assumes that you can pull subterms out of a term into a separate equality, but how is this going to work if a forall is in the way? E.g., given type family F a :: * the equality Maybe (forall a. F [a]) ~ G b would need to be broken down to x ~ F [a], Maybe (forall a. x) ~ G b but you cannot do that, because you just moved 'a' out of its scope. Maybe you can move the forall out as well? Manuel