CODEWITHSUNDEEP
haskell
The Show that never ends
The Show that never ends

Reputation: 160

How to use Applicative operators for the Maybe and List Monads in Haskell?

Maybe and List are monads and I can use the bind operator >>= as expected, as in:

Prelude> (Just 0) >>= (\x -> Just(succ x))
Just 1

However if try to use the <$> applicative operator, I get an error:

Prelude> (Just succ) <$> (Just 0)

<interactive>:16:6: Not in scope: ‘<$>’

How to correct my input so that this last expression evaluates to Just 1.

Upvotes: 2

Views: 184

Answers (2)

zudov
zudov

Reputation: 1056

You error arises because you didn't <$> is not is scope (just as compiler said). The <$> operator is provided by module Data.Functor and Control.Applicative. So just load the module into ghci:

:m Control.Applicative

or if you are not in ghci

import Control.Applicative

note that starting with base-4.8 (ghc 7.10), <$> is included in Prelude and thus imported by default.

However that's not your only problem. <$> (fmap) allow you to apply the function to some value inside the Functor/Monad/Applicative. For example

succ <$> Just 0
-- => Just 1
succ <$> Nothing
-- => Nothing
fmap succ (Just 0)
-- => Just 1

But if you want to apply a function which is itself inside the Functor/Monad/Applicative you need <*>

Just succ <*> Just 1
-- => Just 2
Nothing <*> Just 1
-- => Nothing
Just succ <*> Nothing
-- => Nothing

This operator is extremely useful for functions with several arguments:

(+) <$> Just 1 <*> Just 2
-- Just (+1) <*> Just 2        (intermediate step)
-- => Just 3

Upvotes: 12

Lee
Lee

Reputation: 144226

<$> and <*> are defined in Control.Applicative, so you need to import them:

import Control.Applicative ((<$>), (<*>))

<$> is an infix version of fmap, so, for your example, you need to use <*> which lifts function application over an applicative:

(Just succ) <*> (Just 0)

Upvotes: 9

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