(custom monad class)
Revision as of 17:02, 25 December 2008
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 whereever possible.It is too tempting to get rid of IO by
but we want to present some clean techniques to avoid IO.
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")
in case of failure.
Since you have now an expression for the complete result as string, you have a simple object that can be re-used in other contexts.E.g. you can also easily compute the length of the written string using
without bothering the file system, again.
2 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(
can be defined via IO
randomDist :: (Random a, Num a) => (a -> a) -> IO a randomDist distInv = liftM distInv (randomRIO (0,1))
but there is no need to do so. You don't need the state of the whole world just for remembering the state of a random number generator. What about
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)
3 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
and you can define new operations in ST, but then you need to resort to unsafe operations. You can escape from ST to non-monadic code in a safe, and in many cases efficient, way.
4 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
5 Last resortThe method of last resort is
When you apply it, think about how to reduce its use and how you can encapsulate it in a library with a well chosen interface.You may define new operations in the