Hi, Could someone please explain to me why the following error message happen: When I load the following code into GHC I get an error message: code: --------------------------------------------------------------------------------------------------- data AParser String = AP {apapply::([String]->[(String,[String])])} instance Monad AParser where return v = AP (\inp -> [(v,inp)]) (AP p) >>= f = AP (\inp -> concat [ apapply (f v) inp1 | (v,inp1) <- p inp]) --------------------------------------------------------------------------------------------------- error: --------------------------------------------------------------------------------------------------- Inferred type is less polymorphic than expected Quantified type variable `b' is unified with another quantified type variable `a' When trying to generalise the type inferred for `>>=' Signature type: forall a1 b1. AParser a1 -> (a1 -> AParser b1) -> AParser b1 Type to generalise: forall a1 b1. AParser a1 -> (a1 -> AParser b1) -> AParser b1 In the instance declaration for `Monad AParser' Failed, modules loaded: none. --------------------------------------------------------------------------------------------------- But when I create the type synonym type Stack = [String] data AParser String = AP {apapply::(Stack -> [(String,Stack)])} and use Stack instead of [String], GHC does not complain??? Thanks. Mansour.