Creating a `State [Int] Int`

Question

Reading through Learn You a Haskell , I'm trying to construct a Stack [Int] Int :

ghci>import Control.Monad.State

ghci> let stack = state ([1,2,3]) (1) :: State [Int] Int

<interactive>:58:20:
    Couldn't match expected type `s0 -> (State [Int] Int, s0)'
                with actual type `[t0]'
    In the first argument of `state', namely `([1, 2, 3])'
    In the expression: state ([1, 2, 3]) (1) :: State [Int] Int
    In an equation for `stack':
        stack = state ([1, 2, 3]) (1) :: State [Int] Int

How can I create a Stack [Int] Int ?

Answer 1

It depends on what you're trying to do. State sa is essentially a newtype for a certain kind of function type (specifically s -> (a, s) ), so it doesn't really make sense to make a State value from just a list. The simplified (internal) definition of State looks something like

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

Though you won't use the State constructor directly, it does illustrate the fact that a State sa value consists of a function.

You need a function that updates the state in some way (which could be considered a " State action"), then you can use runState :: State sa -> s -> (a, s) to execute the provided State action, given a certain initial state (the s argument).

It looks like you want to use [1, 2, 3] as your initial state, but you do also need to provide that update function (which is what you use to construct the State sa value itself).

In the Learn You a Haskell example, the Stack type synonym represents the actual stack data while State Stack ... represents a stateful action on the Stack data. For instance, an action of type State Stack Int uses a Stack value as its state and results in an Int when it is executed.