I'm not sure if it's possible (and I have an alternate method of doing this so it isn't strictly speaking _necessary_, but would make the code cleaner if I can), but I would like to convert a "value" of kind (k -> Constraint) into one of kind (Proxy k -> Constraint). I can achieve this using a type family (note: I've also made this polymorphic in terms of the Proxy type used, but that isn't necessary):
type family LiftConstraint_ c pa :: Constraint where LiftConstraint_ c (proxy a) = c a LiftConstraint_ c pa = () ~ Bool -- A nonsensical failing Constraint value
which has the problem that it can't be partially applied (which I also need). This can be wrapped up in a typeclass:
class (LiftConstraint_ c pa) => LiftConstraint c pa instance (LiftConstraint_ c pa) => LiftConstraint c pa
The problem with this is that if I try and use it somewhere that - having already a type of kind (c :: * -> Constraint) from the environment - that expects a function of type (forall a. (c a) => a -> b) with a fixed "b" value, then trying to use something like this function prevents it from working (for the purposes of argument, assume the old specification of Num that had an Eq superclass; this is the way I can produce the smallest example):
fn :: forall pn. (LiftConstraint Num (Proxy n)) => Proxy n -> Bool fn = join (==) . asProxyTypeOf 0
The resulting error message is: "Could not deduce (a ~ Proxy n0) ..." This occurs even if I make the type of fn polymorphic in terms of the proxy type or if I make LiftConstraint_ specific. Note that the error goes away if I make the definition to be "fn = const True"; it's only if I try and _use_ the proxy value. I've tried defining and using an IsProxy type family + class but that fails to the same basic problem: it can't tell that the provided value can be pulled apart to get what's in the Proxy. Any ideas? -- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com http://IvanMiljenovic.wordpress.com