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 disguise the use of I/O with unsafePerformIO, but we want to present 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")

Similarly, 

do
  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.

Writer

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")

  do
    logText "Before operation A"
    opA
    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.)

State

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)

ST

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 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, but then you need to resort to unsafe operations by using the unsafeIOtoST operation. You can escape from ST to non-monadic code in a safe, and in many cases efficient, way.

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. This specific way of using the abstract values ensures referential transparency is preserved even in the presence of the effects they carry.

For Haskell, external effects typically implies the monadic IO type, so there will usually be at least one I/O definition to build the initial structured value, and the abstract values therein. An example is the simple unique-value supply from State in Haskell by John Launchbury and Simon Peyton Jones. While it relies on the also-derided unsafeInterleaveIO:

  • its use is confined to the I/O operation defining the structured value: newUniqueSupply;
  • the presence of other I/O actions (e.g. the atomicModifyIORef call) is still reflected in the type of the defining operation: IO UniqueSupply.

From pages 39-40 of Launchbury and Peyton-Jones's paper, using more-contemporary syntax: 

 -- unique-supply ADT
 --
newUniqueSupply   :: IO UniqueSupply
splitUniqueSupply :: UniqueSupply -> (UniqueSupply, UniqueSupply)
getUnique         :: UniqueSupply -> Unique

data UniqueSupply =  US Unique UniqueSupply UniqueSupply

 -- ...and implementation
 --
type Unique       =  Int

newUniqueSupply   =  do uvar <- newIORef 0
                        let incr   :: Int -> (Int, Unique)
                            incr u =  (u+1, u) 

                            next   :: IO Unique
                            next   =  unsafeInterleaveIO $
                                      atomicModifyIORef uvar incr

                            supply :: IO UniqueSupply
                            supply =  unsafeInterleaveIO $
                                      liftM3 US next supply supply

                        supply

splitUniqueSupply (US _ s1 s2) =  (s1, s2)
getUnique (US u _ _) =  u

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. 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. Since unsafePerformIO makes I/O operations look like non-I/O functions, they should also behave like non-I/O functions e.g. file access must not be hidden by using unsafePerformIO, whereas careful memory manipulation may be safe – see for instance the Data.ByteString module.

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