RE: polymorphic type in state of state monad

| | > type State = Term a => [a] | | > data M a = M (State -> IO(State,a)) | | | | GHC yields a error message "Illegal polymorphic type". | | How to resolve this? I can tell you what it happening. If you have -fglasgow-exts on, the type for State is short for type State = forall a. Term a => [a] And then you can't use that polymorphic type in the argument of a tuple, as you are doing in the next line. Without -fglasgow-exts (i.e. in Haskell 98) the first line is plain illegal: 'a' is not in scope. But GHC may not produce as perspicuous a message as it should for that. Simon