[Haskell-beginners] State monad

[Mihai Maruseac]

On Mon, Feb 14, 2011 at 8:18 PM, Mihai Maruseac
<mihai.maruseac at gmail.com> wrote:
> On Mon, Feb 14, 2011 at 8:11 PM, 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
>>
>>
>
> 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