Hi, Instead of the following piece of code : data State s a = S(s -> (a,s)) instance Monad (State s) where return a = S(\s -> (a,s)) (S f ) >>= g = S(\s -> let (a,s1) = f s S fun = g a (b,s2) = fun s1 in (b,s2) ) I thought to write it in the following form using "where": data State s a = S(s -> (a,s)) instance Monad (State s) where return a = S(\s -> (a,s)) (S f ) >>= g = S(\s -> (b,s2) where (a,s1) = f s S fun = g a (b,s2) = fun s1 ) However the compiler does not accept this use of "where". What is the reason? Thanks Jan