[Haskell-beginners] State monad

Ozgur Akgun ozgurakgun at gmail.com
Mon Feb 14 19:44:32 CET 2011

In the latest version of mtl, there is no State data constructor.


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



On 14 February 2011 18:11, Britt Anderson <britt.uwaterloo at 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
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Ozgur Akgun
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