Instances of List and Maybe

This pages compares the Typeclassopedia classes of List and Maybe, with examples of use. You can run the examples in GHCi.

Semigroup

List	Maybe
> import Data.Semigroup > [1,2] <> [3,4] [1,2,3,4]	> import Data.Semigroup > (Just [1,2]) <> (Just [3,4]) Just [1,2,3,4]

Monoid

List	Maybe
> mempty :: [Int] [] > [1,2] <> mempty [1,2]	> mempty :: Maybe [Int] Nothing > (Just [1,2]) <> mempty Just [1,2]

Functor

List	Maybe
> fmap (+1) [2, 3] [3, 4]	> fmap (+1) (Just 2) Just 3 > fmap (+1) Nothing Nothing

Applicative

List	Maybe
> [(+1),(+2)] <*> [3,4] [4,5,5,6] > [] <*> [3,4] []	> (Just (+1)) <*> (Just 2) Just 3 > Nothing <*> (Just 2) Nothing

Alternative

List	Maybe
> import Control.Applicative > [1] <|> empty <|> [2] [1,2]	> import Control.Applicative > (Just 1) <|> (Just 2) Just 1 > Nothing <|> (Just 2) Just 2

Monad

List	Maybe
> mul xs ys = do x<-xs; y<-ys; return (x*y) > mul [1,2] [3,4,5] [3,4,5,6,8,10]	> squares x' = do x<-x'; return (x*x) > squares (Just 3) Just 9

MonadPlus

List	Maybe
> import Control.Monad > squares5 x' = do x<-x'; guard (x*x<5); return (x*x) > squares5 [1,2,3] [1,4]	> squares5 x' = do x<-x'; guard (x*x<5); return (x*x) > squares5 (Just 2) Just 4 > squares5 (Just 3) Nothing > squares5 Nothing Nothing

Foldable

List	Maybe
> foldr (+) 0 [1,2] 3	> foldr (+) 1 (Just 2) 3

Traversable

List	Maybe
> traverse (\x -> [0..x]) [0..2] [[0,0,0],[0,0,1],[0,0,2],[0,1,0],[0,1,1],[0,1,2]] > sequenceA [[1,2],[3,4]] [[1,3],[1,4],[2,3],[2,4]]	> traverse (\x -> [x, x*x]) (Just 3) [Just 3,Just 9] > sequenceA [Just 3, Just 4] Just [3,4] > sequenceA [Just 3, Nothing] Nothing > sequenceA (Just [3,4]) [Just 3,Just 4]

To do : MonadFix