Applicative functor

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An applicative functor has more structure than a functor but less than a monad. See the Haddock docs for Control.Applicative.

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 is some cases, but in other cases it is disadvantageous.

Some advantages of applicative functors

  • Code that uses only on the Applicative interface are more general than ones uses the Monad interface, because there are more applicative functors than monads.
  • Programming with Applicative has 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 liftM, liftM2, etc or ap where you can, in place of do/(>>=).
  • When you notice you're only using those monad methods, then import Control.Applicative and replacereturn with pure, liftM with (<$>) (or fmap or liftA), liftM2 with liftA2, etc, and ap with (<*>). If your function signature was Monad m => ..., change to Applicative m => ... (and maybe rename m to f or whatever).