Avoiding IO

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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 definitions 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 I/O wherever possible. It is too tempting to unsafely disguise the use of I/O, so here instead are some clean techniques to avoid I/O.

Lazy definition of structured data

You can avoid a series of output functions by constructing a complex data structure with non-I/O code and output it with one output definition.

Instead of

-- import Control.Monad (replicateM_)
replicateM_ 10 (putStr "foo")

you can also create the complete string and output it with one call of putStr:

putStr (concat $ replicate 10 "foo")


  h <- openFile "foo" WriteMode
  replicateM_ 10 (hPutStr h "bar")
  hClose h

can be shortened to

writeFile "foo" (concat $ replicate 10 "bar")

which also ensures proper closing of the handle h 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. For example, you can also easily compute the length of the written string using length without bothering the file system, again.

Use simpler monadic types

It may be possible to use a simpler, more specific type than IO for certain tasks.


If the only reason that you need I/O 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")

    logText "Before operation A"
    logText "After operation A"

(This is "inefficient", because String means [Char], tell "writes" to the "end" of the log using mappend, and code for lists (i.e. (++)) is O(n), where n is the length of the left-hand list (i.e. the log). In other words, the bigger the log gets, the slower logging becomes. To avoid this, you should generally use a type that has O(1) mappend, such as Data.Sequence, and fold the complete log (using Foldable) afterwards if you need to.)


If you want to maintain a running state, it is tempting to use IORef. But this is not necessary, since there is the comfortable State monad and its transformer counterpart.

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 definition which computes a random value with respect to a custom distribution (distInv is the inverse of the distribution function) can be defined using I/O

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, 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)))

You can get actual values by running the State as follows:

evalState (randomDist distInv) (mkStdGen an_arbitrary_seed)


In some cases a state monad is simply not efficient enough. Let's 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 ST was invented.

It provides STRef as replacement for IORef, STArray as replacement for IOArray, STUArray as replacement for IOUArray, and you can define new operations in ST. You can escape from ST to non-monadic code in a safe, and in many cases efficient, way e.g. by using runST.

Applicative functor style

Say you have written the operation

translate :: String -> IO String
translate word =
   do dict <- readDictionary "english-german.dict"
      return (Map.findWithDefault word word dict)

You can only use this operation within the I/O monad, and it is not very efficient either, since for every translation the dictionary must be read from disk. You can rewrite this operation 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"]))

I call this Applicative Functor style because you can use the application operator from Control.Applicative:

makeTranslator <*> getLine

Custom monad-based type class

If you only use a small set of I/O operations in otherwise non-I/O code you may define a custom monad-based type class which implements just these operations. You can then implement them based on I/O for the application and without I/O for the test suite.

As an example consider the operation

localeTextIO :: String -> IO String

which converts an English phrase to the currently configured user language of the system. You can abstract the IO type away using

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 Map and use this for translation.

newtype Interpreter a = Interpreter (Reader (Map String String) a)

instance Locale Interpreter where
   localeText text = Interpreter $ fmap (Map.findWithDefault text text) ask

Pseudodata: structured data with embedded effects

If the set of required effects is small, F. Warren Burton's pseudodata technique can be used to access them via abstract values in a larger structured value e.g. a (theoretically) infinite tree. Pure selector functions can then be used to:

  • access new parts of the structured value;
  • retrieve the abstract, effect-bearing values from individual parts.

Using Burton's terminology, the effects only occur when the abstract values are initially used by special functions. Once used, the abstract values remain constant - reusing them has no further effect. Burton briefly explains how using the abstract values in this way ensures referential transparency is preserved, even in the presence of the effects they carry.

In Haskell, external effects typically implies the monadic IO type, so there will usually be at least one I/O operation to build the initial structured value, and the abstract values therein. One example is Iavor Diatchki's value-supply, inspired by the functional pearl On generating unique names by Lennart Augustsson, Mikael Rittri and Dan Synek.

For more information on, and other examples of Burton's technique, see Nondeterminism with Referential Transparency in Functional Programming Languages.

The last resort

The method of last resort is unsafePerformIO. If you're writing something as complex as e.g. an operating system or a webserver, sometimes it's necessary. But for any other task, your safest option is to avoid using it altogether.

So if you absolutely have to use unsafePerformIO, think very carefully about how to minimise its use and how you can encapsulate it in a library with a well-chosen interface. Because Haskell's non-strict semantics makes the direct use of observable effects thoroughly impractical, you must only use it with IO actions that behave like pure Haskell functions. For example, it must not be used to hide file-access operations, whereas careful memory manipulation (as used in the Data.ByteString module) may be safe. But always remember: your Haskell implementation cannot check if you're using it correctly - that is your responsibility.

The same also applies when using other unsafe entities, such as unsafeInterleaveIO (used in the aforementioned name-supply package) or unsafeIOtoST in ST actions.

Using the FFI instead

Depending on the code being called, changing the type used in a FFI declaration can be another option:

foreign import ccall "noObservableEffects" shouldBeSafe :: Int -> Int

This should only be used for foreign code which behaves like pure Haskell functions - it merely avoids needing to use unsafePerformIO:

foreign import ccall "noObservableEffects" shouldBeSafeButinIO :: Int -> IO Int

shouldBeSafe :: Int -> Int
shouldBeSafe n = unsafePerformIO (shouldBeSafeButinIO n)

It will not make the foreign call "safe".