Folks, I'm having trouble creating a pickler for HLists and would appreciate a solution. The code for (HCons e HNil) works fine but I get an error trying to implement puHList for (HCons e l) where l is supposed to be (HCons e ...), i.e. another HList. Bar.hs:21:37: Couldn't match the rigid variable e' against PU e' `e' is bound by the instance declaration at Bar.hs:17:0 Expected type: HCons (PU e) l Inferred type: HCons e l In the first argument of puHList', namely l' In the second argument of pair', namely (puHList l)' Failed, modules loaded: none. ---- module Bar where import Data.Word import OOHaskell main = print "We are here!" class HList l => HLPU l where puHList :: HCons (PU e) l -> PU (HCons e l) instance HLPU HNil where puHList (HCons pe HNil) = wrap (\e -> HCons e HNil, \(HCons e HNil) -> e) pe instance HList l => HLPU (HCons e l) where puHList (HCons pe l) = wrap (\(a, b) -> HCons a b, \(HCons a b) -> (a, b)) (pair pe (puHList l)) newtype TourType = TourType TourType_ deriving (Show{-, Typeable-}) newtype AvgPot = AvgPot Word64 deriving (Show{-, Typeable-}) data TourType_ = TourNone | TourSingle | TourMulti | TourHeadsUpMulti deriving (Enum, Show{-, Typeable-}) --- Pickling data PU a = PU { appP :: (a, [Word8]) -> [Word8], appU :: [Word8] -> (a, [Word8]) } pickle :: PU a -> a -> [Word8] pickle p value = appP p (value, []) unpickle :: PU a -> [Word8] -> a unpickle p stream = fst (appU p stream) lift :: a -> PU a lift x = PU snd (\s -> (x, s)) sequ :: (b -> a) -> PU a -> (a -> PU b) -> PU b sequ f pa k = PU (\ (b, s) -> let a = f b pb = k a in appP pb (b, appP pa (a, s))) (\ s -> let (a, s') = appU pa s pb = k a in appU pb s') pair :: PU a -> PU b -> PU (a,b) pair pa pb = sequ fst pa (\ a -> sequ snd pb (\ b -> lift (a,b))) wrap :: (a -> b, b -> a) -> PU a -> PU b wrap (i, j) pa = sequ j pa (lift . i) Thanks, Joel -- http://wagerlabs.com/