What are some practical examples of Functor usage in Haskell libraries?

What are some practical examples of Functor usage in Haskell libraries?

In Haskell, Functors are widely used in libraries like 'Maybe', 'List', and 'Either'. For instance, the 'Maybe' Functor allows you to apply functions to values that might be absent, helping to avoid null pointer exceptions.

Can you show me a code example using Maybe as a Functor?

Sure! Here's a simple example:

import Data.Maybe

-- A function that adds 10 to a number, wrapped in Maybe
addTen :: Maybe Int -> Maybe Int
addTen x = fmap (+10) x

-- Usage
main = do
    print (addTen (Just 5))  -- Output: Just 15
    print (addTen Nothing)    -- Output: Nothing

What about using Functor with lists?

Great question! Lists are also Functors. You can use 'fmap' to apply a function to each element in a list:

-- A function that doubles each number in a list
doubleList :: [Int] -> [Int]
doubleList xs = fmap (*2) xs

-- Usage
main = print (doubleList [1, 2, 3])  -- Output: [2, 4, 6]

Are there any libraries that heavily utilize Functors?

Yes, libraries like 'lens' and 'transformers' make extensive use of Functors. They provide powerful abstractions for working with data structures in a functional way.