# Modification of State Transformer

**Shawn P. Garbett
**
listman@garbett.org

*Thu, 8 Aug 2002 14:11:54 -0500*

I'm trying to modify Richard Bird's state transformer. The example in his
book (_Introduction_to_Functional_Programming_using_Haskell_) has State
defined as a explicit type.
I.e. Here's the relevant snippet:
-- State transformer definition
newtype St a = MkSt (State -> (a, State))
type State = Int
-- State transformer applied to state
apply :: St a -> State -> (a, State)
apply (MkSt f) s = f s
-- State monad
instance Monad St where
return x = MkSt f where f s = (x,s)
p >>= q = MkSt f where f s = apply (q x) s'
where (x, s') = apply p s
-----------------------------------------
What I want is something like this, so that the state transformer has a
generic state type:
newtype St a s = MkSt (s -> (a, s))
apply :: St a s -> s -> (a, s)
apply (MkSt f) s = f s
instance Monad St where
return x = MkSt f where f s = (x,s)
p >>= q = MkSt f where f s = apply (q x) s'
where (x, s') = apply p s
-----------------------------------------------------------
The trouble occurs on the instance line
Couldn't match `*' against `* -> *'
Expected kind: (* -> *) -> *
Inferred kind: (* -> * -> *) -> *
When checking kinds in `Monad St'
In the instance declaration for `Monad St'
Failed, modules loaded: none.