[Haskell-beginners] State monad

[Ozgur Akgun]

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

http://hackage.haskell.org/packages/archive/mtl/latest/doc/html/Control-Monad-State-Lazy.html#g:2

State<http://hackage.haskell.org/packages/archive/transformers/latest/doc/html/Control-Monad-Trans-State-Lazy.html#t:State>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
state<http://hackage.haskell.org/packages/archive/transformers/latest/doc/html/Control-Monad-Trans-State-Lazy.html#v:state>
 instead.

HTH,

Ozgur

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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