state monads are magic

stateMonad.hs

import Control.Monad.State.Lazy

data Vec2 = Vec2 Float Float deriving (Show)
data Entity = Entity Vec2 [Char] deriving (Show)

vec2Add :: Vec2 -> Vec2 -> Vec2
vec2Add (Vec2 x1 y1) (Vec2 x2 y2) = Vec2 (x1+x2) (y1+y2)

incrementOne :: Num s => State s ()
incrementOne = do
    x <- get
    put (x + 1)

updateEntity :: State Entity ()
updateEntity = do
    e <- get
    put (changeIt e)
        where changeIt (Entity f s) = Entity (vec2Add f $ Vec2 4 8) (s ++ " yeah")

main :: IO ()
main = do
    putStrLn "Hello world, here's some state manipulation."
    a <- return $ execState updateEntity $ Entity (Vec2 1 5) "fuck yeah"
    a <- return $ execState updateEntity a
    a <- return $ execState updateEntity a
    putStrLn (show a)