m (→ST monad)
Latest revision as of 11:41, 19 December 2010
Haskell requires an explicit type for operations involving input and output. This way it makes a problem explicit, that exists in every language: Input and output functions can have so many effects, that the type signature says more or less that almost everything must be expected. It is hard to test them, because they can in principle depend on every state of the real world. Thus in order to maintain modularity you should avoid IO wherever possible.It is too tempting to get rid of IO by
 1 Lazy construction
You can avoid a series of output functions by constructing a complex data structure with non-IO code and output it with one output function.
-- import Control.Monad (replicateM_) replicateM_ 10 (putStr "foo")
putStr (concat $ replicate 10 "foo")
do h <- openFile "foo" WriteMode replicateM_ 10 (hPutStr h "bar") hClose h
can be shortened to
writeFile "foo" (concat $ replicate 10 "bar")
 2 Writer monad
If the only reason that you need IO is to output information (e.g. logging, collecting statistics), a Writer monad might do the job. This technique works just fine with lazy construction, especially if the lazy object that you need to create is a Monoid.
An inefficient example of logging:
logText :: (MonadWriter String m) => String -> m () logText text = tell (text ++ "\n") do logText "Before operation A" opA logText "After operation A"
 3 State monadIf you want to maintain a running state, it is tempting to use
Another example is random number generation. In cases where no real random numbers are required, but only arbitrary numbers, you do not need access to the outside world. You can simply use a pseudo random number generator with an explicit state. This state can be hidden in a State monad.Example: A function which computes a random value with respect to a custom distribution (
randomDist :: (Random a, Num a) => (a -> a) -> IO a randomDist distInv = liftM distInv (randomRIO (0,1))
You don't need the state of the whole world just for remembering the state of a random number generator, instead you can use something similar to this:
randomDist :: (RandomGen g, Random a, Num a) => (a -> a) -> State g a randomDist distInv = liftM distInv (State (randomR (0,1)))
evalState (randomDist distInv) (mkStdGen an_arbitrary_seed)
 4 ST monad
In some cases a state monad is simply not efficient enough. Say the state is an array and the update operations are modification of single array elements.For this kind of application the State Thread monad
You can escape from ST to non-monadic code in a safe, and in many cases efficient, way.
 5 Applicative functor style
Say you have written the function
translate :: String -> IO String translate word = do dict <- readDictionary "english-german.dict" return (Map.findWithDefault word word dict)
You can only call this function within the IO monad, and it is not very efficient either, since for every translation the dictionary must be read from disk. You can rewrite this function in a way that it generates a non-monadic function that can be used anywhere.
makeTranslator :: IO (String -> String) makeTranslator = do dict <- readDictionary "english-german.dict" return (\word -> Map.findWithDefault word word dict) main :: IO () main = do translate <- makeTranslator putStr (unlines (map translate ["foo", "bar"]))
makeTranslator <*> getLine
 6 Custom monad type class
If you only use a small set of IO operations in otherwise non-IO code you may define a custom monad type class which implements just these functions. You can then implement these functions based on IO for the application and without IO for the test suite.
As an example consider the function
localeTextIO :: String -> IO String
which converts an English phrase to the currently configured user language of the system.You can abstract the
class Monad m => Locale m where localeText :: String -> m String instance Locale IO where localeText = localeTextIO instance Locale Identity where localeText = Identity
where the first instance can be used for the application and the second one for "dry" tests.For more sophisticated tests, you may load a dictionary into a
newtype Interpreter a = Interpreter (Reader (Map String String) a) instance Locale Interpreter where localeText text = Interpreter $ fmap (Map.findWithDefault text text) ask