I have ---------- import qualified Data.Map as M type Link a = (a, Int) data MChain a = Map a [Link a] deriving (Show) ------------------- and want to make a Monoid of MChain. So I have ------------------- instance Monoid (MChain a) where mempty = M.empty mappend = undefined ------------------- this won’t compile and I need M.empty to be Map a [Link a] The error is Couldn't match expected type ‘MChain a1’ with actual type ‘M.Map k0 a0’ • In the expression: M.empty :: MChain a In an equation for ‘mempty’: mempty = M.empty :: MChain a In the instance declaration for ‘Monoid (MChain a)’ This part confuses me "Couldn't match expected type ‘MChain a1 with actual type ‘M.Map k0 a0’ " How should I proceed. Many thanks Mike