Haskell Cookbook

How it works...

The Applicative type class is defined as follows: 

    class Functor f => Applicative (f :: * -> *) where      pure :: a -> f a      (<*>) :: f (a -> b) -> f a -> f b      (*>) :: f a -> f b -> f b      (<*) :: f a -> f b -> f a      {-# MINIMAL pure, (<*>) #-}

The minimal definition of an Applicative instance requires at least pure and <*> to be defined. The definition also implies that we can define an instance of an Applicative for f only if f is also an instance of a Functor.

The pure function takes a value and creates a data type. For example, in the context of List, Maybe, and Either, the pure function will fetch the following values:

    pure 10 :: [Int] = [10]  -- in the context of List    pure 10 :: Maybe Int = Just 10 -- in the context of Maybe pure ...