Personal tools

Applicative functor

From HaskellWiki

Revision as of 08:23, 5 November 2007 by Conal (Talk | contribs)

Jump to: navigation, search
An applicative functor has more structure than a functor but less than a monad. See the Haddock docs for <div class="inline-code">
Control.Applicative
</div>
.

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.

1 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.

2 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 replace
    return
    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).