In the latest version of mtl, there is no State data constructor.
http://hackage.haskell.org/packages/archive/mtl/latest/doc/html/Control-Mona...
Statehttp://hackage.haskell.org/packages/archive/transformers/latest/doc/html/Con...is
defined to be a type alias to the application of Identity monad to the
StateT monad transformer.
As you've already discovered, you need to use
statehttp://hackage.haskell.org/packages/archive/transformers/latest/doc/html/Con...
instead.
HTH,
Ozgur
On 14 February 2011 18:11, Britt Anderson
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
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