Neither ghci nor hugs like this: g :: (?param :: a) => a g = ?param p :: Maybe ((?param :: a) => a) p = return g; 'ghci -fglasgow-exts' (5.02) says: Illegal constraint ?param :: a In the type: forall a. Maybe ({?param :: a} => a) While checking the type signature for `p' 'hugs -98' (Feb 2000) says: ERROR "Implicit.hs" (line 12): Syntax error in type expression (unexpected implicit parameter "?param") Not only that, but they both complain about this: q :: b -> ((?param :: a) => a) q b = g ...even though it can only mean this: q :: (?param :: a) => b -> a q b = g (which works fine with both). However, this _does_ work, but only in ghci: f :: ((?param :: a) => b) -> a -> b f foo a = foo with ?param=a Hugs just spins trying to do 'Type checking'. -- Ashley Yakeley, Seattle WA