An applicative functor has more structure than a functor but less than a monad. See the Haddock docs for
It has turned out that many applications do not require monad functionality but only those of applicative functors. Monads allow you to run actions depending on the outcomes of earlier actions.
do text <- getLine if null text then putStrLn "You refuse to enter something?" else putStrLn ("You entered " ++ text)
This is obviously necessary in some cases, but in other cases it is disadvantageous.
Consider an extended IO monad which handles automated closing of allocated resources. This is possible with a monad.
openDialog, openWindow :: String -> CleanIO () liftToCleanup :: IO a -> CleanIO a runAndCleanup :: CleanIO a -> IO a runAndCleanup $ do text <- liftToCleanup getLine if null text then openDialog "You refuse to enter something?" else openWindow ("You entered " ++ text)
The (fictive) functions
could not only open dialogs and windows but could also register come cleanup routine in the
runAndCleanup would first run the opening actions and afterwards the required cleanup actions.
I.e. if the dialog was opened, the dialog must be closed, but not the window.
That is, the cleanup procedure depends on the outcomes of earlier actions.
Now consider the slightly different task, where functions shall register initialization routines
that shall be run before the actual action takes place.
(See the original discussion started by Michael T. Richter in Haskell-Cafe:
Practical Haskell Question)
This is impossible in the monadic framework.
Consider the example above where the choice between
depends on the outcome of
You cannot run initialization code for either
because you do not know which one will be called before executing
If you eliminate this dependency, you end up in an applicative functor
and there you can do the initialization trick.
You could write
initializeAndRun $ liftA2 (liftToInit getLine) (writeToWindow "You requested to open a window")
writeToWindow registers an initialization routine which opens the window.
Some advantages of applicative functors
- Code that uses only on the
Applicativeinterface are more general than ones uses the
Monadinterface, because there are more applicative functors than monads.
- Programming with
Applicativehas a more applicative/functional feel. Especially for newbies, it may encourage functional style even when programming with effects. Monad programming with
donotation encourages a more sequential & imperative style.
How to switch from monads
- Start using
liftM2, etc or
apwhere you can, in place of
- When you notice you're only using those monad methods, then import
liftA2, etc, and
(<*>). If your function signature was
Monad m => ..., change to
Applicative m => ...(and maybe rename