I don't know what you want to do, but you may wrap the (forall a. [a] -> [a]) in an existential type:
data ListFunc = forall a. ListFunc ([a] -> [a])
class Has r Rev ListFunc => HRClass r where setHRClass :: ListFunc -> r -> r
Thanks Henning, What we're wanting to do is set the Higher-ranked function into a record type, then get it and apply it polymorphically. SPJ's example is: data HR = HR { rev :: forall a. [a] -> [a] } -- where rev is the label for the H-R function f :: HR -> ([Bool], [Char]) f r = (r.rev [True], r.rev "hello") -- where r.rev is new syntax to get the func from HR I've tried that ListFunc wrapping you suggest: data HR = HR { rev :: ListFunc } rHR1 = HR{ rev = ListFunc reverse } -- put the `reverse` function into the record type -- the setHRClass method would do this But I can't 'dig out' the H-R function and apply it (not even monomorphically): case (rev rHR1) of { (ListFunc fn) -> fn "hello" } ==> Couldn't match type `a' with `Char' `a' is a rigid type variable bound by a pattern with constructor ListFunc :: forall a. ([a] -> [a]) -> ListFunc, in a case alternative at <interactive>:0:25 Expected type: [a] Actual type: [Char] SPJ's approach (without a wrapper, but with some fancy instance constraints) can 'dig out' the function and apply it polymorphically, but he can't get the function into the record without using an explicit data constructor. What am I doing wrong? AntC -- View this message in context: http://haskell.1045720.n5.nabble.com/Re-Haskell-Higher-types-in-contexts-tp5... Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.