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 some 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.
If you have the variables
f :: a -> b -> c a :: f a b :: f b
you can combine them in the following ways with the same result of type
pure f <*> a <*> b liftA2 f a b
But how to cope with
let and sharing in the presence of effects?
Consider the non-functorial expression:
x :: x g :: x -> y h :: y -> y -> z let y = g x in h y y
Very simple. Now we like to generalize this to
fx :: f x fg :: f (x -> y) fh :: f (y -> y -> z)
However, we note that
let fy = fg <*> fx in fh <*> fy <*> fy
runs the effect of
fy writes something to the terminal then
fh <*> fy <*> fy writes twice.
This could be intended, but how can we achieve,
that the effect is run only once and the result is used twice?
Actually, using the
liftA commands we can pull results of applicative functors
into a scope where we can talk exclusively about functor results and not about effects.
Note that functor results can also be functions.
This scope is simply a function, which contains the code that we used in the non-functorial setting.
liftA3 (\x g h -> let y = g x in h y y) fx fg fh
The order of effects is entirely determined by the order of arguments to
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 do notation 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
- The blog article The basics of applicative functors, put to practical work