Confused about State constructor

[Adam Miezianko]

I'm working through Learn You a Haskell for Great Good [1] and getting
a compiler error while playing around with some of the code. I have
this:

-- BEGIN state.hs
import Control.Monad.State

type Stack = [Int]

pop :: State Stack Int
pop = State $ \(x:xs) -> (x,xs)

push :: Int -> State Stack ()
push x = State $ \xs -> ((), x:xs)

stackManip :: State Stack Int
stackManip = do
push 3
pop
pop
-- END state.hs

But when I try to load it into ghci I get the following errors:

Prelude> :load "/home/admi/.pe/state.hs"
[1 of 1] Compiling Main ( /home/admi/.pe/state.hs,
interpreted )

/home/admi/.pe/state.hs:6:6: Not in scope: data constructor `State'

/home/admi/.pe/state.hs:9:9: Not in scope: data constructor `State'
Failed, modules loaded: none.
Prelude>

Now, I'm not exactly sure how to read the documentation for
Control.Monad.State [2] but it seems that newtype State s a = State
{...} defines a constructor, or am I wrong on that point too? So,
what am I missing here? In case it matters, I am using mtl-2.0.1.0
and ghci 6.12.3.

[1] http://learnyouahaskell.com/for-a-few-monads-more
[2]
http://cvs.haskell.org/Hugs/pages/libraries/mtl/Control-Monad-State.html

--
Adam Miezianko

[Albert Y. C. Lai]

On 10-11-23 09:36 PM, Adam Miezianko wrote:
> import Control.Monad.State
>
> type Stack = [Int]
>
> pop :: State Stack Int
> pop = State $ \(x:xs) -> (x,xs)

> /home/admi/.pe/state.hs:6:6: Not in scope: data constructor `State'

> In case it matters, I am using mtl-2.0.1.0
> and ghci 6.12.3.

Learn You A Haskell was written when mtl-1.x was around. mtl-2.x has
incompatible changes, and this is one of them. State Stack Int is now an
alias for StateT Stack Identity Int, which in turn goes like

newtype StateT s m a = StateT{ runStateT :: s -> m (a,s) }

and you set m to Identity:

newtype Identity a = Identity{ runIdentity :: a }

So for example you have to rewrite pop as:

pop = StateT $ \(x:xs) -> Identity (x,xs)

Alternatively, hide away mtl-2.x and install mtl-1.x.