Wang Meng writes: | Hi All, | | Any one of your have the experience of defining a state of a state monad | as a polymorphic type? | I want to have: | | > 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? If you only want to vary the state type between separate monadic computations, all you need to do is parameterise your datatype over the state type:
data M s a = M (s -> IO (s, a))
This is available in some libraries, such as http://www.haskell.org/ghc/docs/latest/html/base/Control.Monad.State.html You could use it like this:
import Control.Monad.State type M s = StateT s IO
If, on the other hand, you want to vary the state type *during* a single monadic computation, it gets messy. You could try one of the following. - Declare a datatype with a branch for each instance of Term, and use this as the state type. > data State = TermType1 TermType1 > | TermType2 TermType2 > | ... > | TermTypeN TermTypeN - Do something similar with an existential type. > data State = forall a . Term a => State a - Don't use monadic state at all, but instead pass the state around explicitly. Regards, Tom