How to call a Haskell State function?

I’ve written a whole lot of exercises now on functions that use State, like

addStateAndReturnResult :: Int -> State Int Int
addStateAndReturnResult x = do
    state <- get
    put $ state + x
    return $ state + x

But I have no idea how to actually call the function and get the result out. Something like this gives me a weird type error that is not even close:

callIt :: Int
callIt = addStateAndReturnResult 3 (State 1)

How do I make function run please?

>Solution :

There are three functions for this.

The most general is runState :: State s a -> s -> (a, s). It returns a tuple of the last state and the final return value.

Then there is evalState :: State s a -> s -> a which just returns the final return value.

And there is execState :: State s a -> s -> s which just returns the last state value.

In your case, you probably want evalState:

callIt = evalState (addStateAndReturnResult 3) 1