state_error_example.hs

-- requires "mtl"
import Control.Monad.State
import Control.Monad.Except

import Data.Map (Map)
import qualified Data.Map as Map

data Err = Missing String | OtherErr
         deriving (Eq, Show)

type Stored = Map String Int
type StoreM = StateT Stored (Either Err)

note :: a -> Maybe b -> Either a b
note msg = maybe (Left msg) Right

save :: String -> Int -> StoreM ()
save k v = modify (Map.insert k v)

load :: String -> StoreM Int
load k = do
  store <- get
  lift $ note (Missing k) (Map.lookup k store)

stateManip :: StoreM Int
stateManip = do
  save "x" 1
  save "x" 2
  save "y" 123
  x <- load "x"
  y <- load "y"
  save "z" (x + y)
  load "z"

main = putStrLn $ show $ evalStateT stateManip Map.empty