[Haskell-beginners] state monad

[Lorenzo Bolla]

I believe that the type constructor for State is now lowercase: "state"
not "State".


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)

play = do
        push 1
        push 2
        push 3
        pop 

main = print $ runState play []


L.


On Sun, Apr 22, 2012 at 01:40:17AM -0700, Paul Higham wrote:
> I am trying to come to grips with the state monad and as a starting point I copied the some code directly from the book Learn You a Haskell for Great Good then added the type synonym for Stack
> 
> 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)  
> 
> 
> However, attempting to load this into ghci produces the errors
> 
> stackPlay.hs:6:7: Not in scope: data constructor `State'
> 
> stackPlay.hs:9:10: Not in scope: data constructor `State'
> 
> 
> Please excuse the cognitive deficit, but I cannot figure out why this should not work as advertised.
> 
> ::paul
> 
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-- 
Lorenzo Bolla
http://lbolla.info
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