男装の麗人

huge.hs

import Control.Monad
import Control.Applicative
import Control.Monad.State
import Control.Monad.Writer

newtype A = A [B] deriving(Show)
newtype B = B Int deriving(Show)

type Checker a = StateT Int (WriterT [String] Maybe) a

w = lift . tell . return
f = lift . lift

checkA :: A -> Checker A
checkA (A b) = A <$> mapM checkB b

checkB :: B -> Checker B
checkB (B c) | c == 0 = B <$> (w "hoge" >> return 13)
             | c == 1 = B <$> (w "huga" >> return 13)
             | c == 2 = B <$> get
             | c == 3 = modify (*2) >> return (B c)
             | c == 4 = B <$> (f Nothing)
             | otherwise = B <$> return c

run = execWriterT $ evalStateT (checkA $ A $ map B $ [2,3,2,3,2,0,0,1,3]) 100