Write You a Haskell Stephen Diehl

evalStack :: Stack a -> IO [Int]
evalStack m = execWriterT (evalStateT (unStack m) 0)
StateT
e state monad allows functions within a stateful monadic context to access and modify shared state.
put :: s -> State s () -- set the state value
get :: State s s -- get the state
gets :: (s -> a) -> State s a -- apply a function over the state, and return the result
modify :: (s -> s) -> State s () -- set the state, using a modifier function
Evaluation functions often follow the naming convention of using the prefixes run, eval, and exec:
execState :: State s a -> s -> s
-- yield the state
evalState :: State s a -> s -> a
-- yield the return value
runState :: State s a -> s -> (a, s) -- yield the state and return value
For example:
import Control.Monad.State
test :: State Int Int
test = do
put 3
modify (+1)
get
main :: IO ()
main = print $ execState test 0
ReaderT
e Reader monad allows a fixed value to be passed around inside the monadic context.
ask :: Reader r r -- get the value
asks :: (r -> a) -> Reader r a -- apply a function to the value, and return the res
local :: (r -> r) -> Reader r a -> Reader r a -- run a monadic action, with the value modified by
For example:
import Control.Monad.Reader
data MyContext = MyContext
{ foo :: String
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