On Mon, Feb 14, 2011 at 8:18 PM, Mihai Maruseac <mihai.maruseac@gmail.com> wrote:

On Mon, Feb 14, 2011 at 8:11 PM, Britt Anderson <britt.uwaterloo@gmail.com> wrote:

...

Many of the tutorials on the state monad seem to suggest that you can use State as a data constructor, but I can't get this to work. E.g. in ghci after :m Control.Monad.State I can

let f = (\x -> (x,x)) let y = state f

but if I,

let z = State f

I get an error message: Not in scope: data constructor `State'.

Can someone please explain?

Thanks, Britt

GHCi doesn't load all Haskell modules by default. It only loads Prelude. You'll have to load them by hand. Use

:m +Control.Monad.State

Also, see if you have the mtl library installed (or Transformers). -- Mihai

On Monday 14 February 2011 19:23:43, Britt Anderson wrote:

Actually, I can load the module and I have mtl. I just don't understand why I can use State as a data constructor, but the inverse of runState, i.e. state works just fine. Has this library/package been reconfigured from when most people were writing tutorials?

Exactly that. As of mtl-2.*, mtl is now a wrapper around transformers and no longer an independent library. One change which broke a lot of tutorials (and some code) is that State is no longer a separate datatype. (State s) is now a type synonym for (StateT s Identity), similarly for Reader, Writer. Those parts of the tutorials whcih don't use the data-constructor should work unchanged, though.