haskell - Creating a `State [Int] Int` -

reading through learn haskell, i'm trying construct 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)'                 actual type `[t0]'     in first argument of `state', namely `([1, 2, 3])'     in expression: state ([1, 2, 3]) (1) :: state [int] int     in equation `stack':         stack = state ([1, 2, 3]) (1) :: state [int] int 

how can create stack [int] int?

it depends on you're trying do. state s a newtype kind of function type (specifically s -> (a, s)), doesn't make sense make state value list. simplified (internal) definition of state looks like

newtype state s = state { runstate :: s -> (a, s) } 

though won't use state constructor directly, illustrate fact state s a value consists of function.

you need function updates state in way (which considered "state action"), can use runstate :: state s -> s -> (a, s) execute provided state action, given initial state (the s argument).

it looks want use [1, 2, 3] initial state, need provide update function (which use construct state s a value itself).

in learn haskell example, stack type synonym represents actual stack data while state stack ... represents stateful action on stack data. instance, action of type state stack int uses stack value state , results in int when executed.