Write You a Haskell Stephen Diehl

e IO monad is wired into the runtime with compiler support. It is a special case and most monads in
Haskell have nothing to do with effects in this sense.
putStrLn :: String -> IO ()
print :: Show a => a -> IO ()
e type of main is always IO ().
main :: IO ()
main = do
putStrLn ”Enter a number greater than 3: ”
x 3)
e essence of monadic IO in Haskell is that effects are reified as first class values in the language and reflected
in the type system. is is one of foundational ideas of Haskell, although it is not unique to Haskell.
Monad Transformers
Monads can be combined together to form composite monads. Each of the composite monads consists
of layers of different monad functionality. For example, we can combine an error-reporting monad with
a state monad to encapsulate a certain set of computations that need both functionalities. e use of
monad transformers, while not always necessary, is often one of the primary ways to structure modern
Haskell programs.
class MonadTrans t where
lift :: Monad m => m a -> t m a
e implementation of monad transformers is comprised of two different complementary libraries,
transformers and mtl. e transformers library provides the monad transformer layers and mtl
extends this functionality to allow implicit lifting between several layers.
To use transformers, we simply import the Trans variants of each of the layers we want to compose and
then wrap them in a newtype.
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
import Control.Monad.Trans
import Control.Monad.Trans.State
import Control.Monad.Trans.Writer
newtype Stack a = Stack { unStack :: StateT Int (WriterT [Int] IO) a }
deriving (Monad)
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