Hi. I cannot make this program type check: {-# LANGUAGE TypeFamilies, FlexibleContexts #-} import qualified Data.ListLike as LL class LL.ListLike full (Item full) => ReplaceOneOf full where type Item full :: * replaceOneOf :: [Item full] -> full -> full -> full replaceOneOf from to list | LL.null list = list | x `element` from = LL.concat [to, replaceOneOf from to $ LL.dropWhile (`element` from) xs] | otherwise = LL.cons x $ replaceOneOf from to xs where x = LL.head list xs = LL.tail list element :: Item full -> [Item full] -> Bool The error message is: Line 9: 1 error(s), 0 warning(s) Could not deduce (Item full0 ~ Item full) from the context (ReplaceOneOf full) bound by the class declaration for `ReplaceOneOf' at /home/marcot/tmp/test_flymake.hs:(4,1)-(15,45) NB: `Item' is a type function, and may not be injective Expected type: [Item full0] Actual type: [Item full] In the second argument of `element', namely `from' In the expression: x `element` from I have tried using asTypeOf, but it did not work: {-# LANGUAGE TypeFamilies, FlexibleContexts, ScopedTypeVariables #-} import qualified Data.ListLike as LL class LL.ListLike full (Item full) => ReplaceOneOf full where type Item full :: * replaceOneOf :: Item full -> [Item full] -> full -> full -> full replaceOneOf xt from to list | LL.null list = list | (x `asTypeOf` xt) `element` from = LL.concat [to, replaceOneOf xt from to $ LL.dropWhile (`element` from) xs] | otherwise = LL.cons x $ replaceOneOf xt from to xs where x = LL.head list xs = LL.tail list element :: Item full -> [Item full] -> Bool Can someone point me to a solution? Greetings. -- marcot http://marcot.eti.br/