Question

I was reading Learn You a Haskell’s guide on the state monad, but I had trouble understanding it since the stack example couldn’t compile. In the guide, he used the following piece of code:

import Control.Monad.State  

type Stack = [Int]

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

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

While I understand what it’s supposed to do, it won’t compile. I have no idea why. The error message is:

Stack.hs:6:7: Not in scope: data constructor `State'

Stack.hs:9:10: Not in scope: data constructor `State'

This makes no sense to me, since “State” is, to my knowledge, in fact a data constructor, defined as

newtype State s a = State { runState :: s -> (a,s) }

Is the guide “wrong”, and if so, how do I fix it?

Answer 1

As I mentioned in comments, you ought to use state instead of State.

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The problem is that State is not standalone data type (or rather newtype), but it is the StateT transformer applied to Identity monad. Actually, it’s defined as

type State s = StateT s Indentity

and because it’s just type synonym, it cannot have State constructor. That’s why Control.Monad.State uses state.