----- Original Message ----- From: "Scott J." <[EMAIL PROTECTED]> To: "Shawn P. Garbett" <[EMAIL PROTECTED]> Sent: Monday, August 12, 2002 9:04 PM Subject: Re: Modification of State Transformer
> I 'm sorry, > > What I meant was discussion about the state transformer ST s a itself. And > how it works. What does mean the second inner forall loop and so on. I can't > find explanations of this in the Haskell library. > > Regards > > Scott > ----- Original Message ----- > From: "Shawn P. Garbett" <[EMAIL PROTECTED]> > To: "Scott J." <[EMAIL PROTECTED]> > Cc: <[EMAIL PROTECTED]> > Sent: Monday, August 12, 2002 4:19 PM > Subject: Re: Modification of State Transformer > > > > -----BEGIN PGP SIGNED MESSAGE----- > > Hash: SHA1 > > > > On Sunday 11 August 2002 07:26 pm, Scott J. wrote: > > > Hi, > > > > > > I invite you then to explain what happens with every step. > > > > > > The use of "forall" is misleading and fast to be misunderstood: I > mention > > > here the inner forall's. > > > > > > Thx > > > > > > Scott > > > > This list is great. The implementation in the ST module solves the > > > > problem and I understand how it works. > > > > > > > > Shawn > > > > Given the level of detailed explanations to date, I don't see the point. > But > > I'll go ahead and do so anyway, by summarizing what I've learned from the > > previous posts. > > > > I had read the example in Bird'd book on state transformers. The > definition > > of state however was a fixed type in the examples. Wanting to extend the > > definition and make it more general I was trying to figure out how to > modify > > the type. > > > > Bird's definition was: > > newtype St a = MkSt (State -> (a,State)) > > type State = type > > > > I had attempted to extend the type as follows > > newtype St a s = MkSt (s -> (a,s)) > > > > This died in the compiler when declaring this type as an instance of > 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 > > ghc returned the following (referencing the instance line): > > Couldn't match `*' against `* -> *' > > Expected kind: (* -> *) -> * > > Inferred kind: (* -> * -> *) -> * > > When checking kinds in `Monad St' > > In the instance declaration for `Monad St' > > > > When a type constructor has an argument it has a type of `* -> *'. > > When a type constructor has two arguments it has a type of `* -> * -> *'. > > This construction of the type can be extended to n arguments by having the > > number of `->' match the n arguments of type and the `*' be n+1. > > > > The class definition of Monad contains the following: > > class Monad m where > > return :: a -> m a > > (>>=) :: m a -> (a -> m b) -> m b > > > > > > So the class of St a s needs reduction from `* -> * -> *' to `* -> *' to > fit > > the single argument type constructor of the Monad class. By using (St a) > > which causes the type constructor to be of type `(* -> *) -> *'. Since > `(* -> > > *)' can be used as `*', by creation of another type. This because > equivalent > > to `* -> *'. > > > > The only thing left is reversing the order so that the result type is of > the > > correct form in the Monad usage. I.e, in my initial ordering the `return' > of > > the Monad would end up returning something of type `s' which is not > > particularly useful, since type `a' is the desired return type from the > > transformer. > > > > So the corrected version of State becomes: > > newtype St s a = MkSt (s -> (a, s)) > > > > instance Monad (St s) where > > ... > > > > Shawn Garbett > > - -- > > You're in a maze of twisty little statements, all alike. > > Public Key available from http://www.garbett.org/public-key > > -----BEGIN PGP SIGNATURE----- > > Version: GnuPG v1.0.7 (GNU/Linux) > > > > iD8DBQE9V8P4DtpPjAQxZ6ARAq0VAJ9toEiEm+d58vgbKEofzXBISyXrEACfasbc > > eaEg2zVi9y90vk+fXKGSrt0= > > =OrwN > > -----END PGP SIGNATURE----- > > _______________________________________________ > > Haskell-Cafe mailing list > > [EMAIL PROTECTED] > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > _______________________________________________ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
