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.
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] |
List | Maybe |
---|---|
> mempty :: [Int]
[]
> [1,2] <> mempty
[1,2] |
> mempty :: Maybe [Int]
Nothing
> (Just [1,2]) <> mempty
Just [1,2] |
List | Maybe |
---|---|
> fmap (+1) [2, 3]
[3, 4]
> fmap (+1) []
[] |
> fmap (+1) (Just 2)
Just 3
> fmap (+1) Nothing
Nothing |
List | Maybe |
---|---|
> [(+1),(+2)] <*> [3,4]
[4,5,5,6]
> [] <*> [3,4]
[] |
> (Just (+1)) <*> (Just 2)
Just 3
> Nothing <*> (Just 2)
Nothing |
List | Maybe |
---|---|
> import Control.Applicative
> [1] <|> empty <|> [2]
[1,2]
> empty <|> [2]
[2] |
> import Control.Applicative
> (Just 1) <|> empty <|> (Just 2)
Just 1
> Nothing <|> (Just 2)
Just 2 |
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] |
> mul x' y' = do x<-x'; y<-y' return (x*y)
> mul (Just 2) (Just 5)
Just 10 |
List | Maybe |
---|---|
> import Control.Monad
> squares5 x' = do x<-x'; guard (x*x<5); return (x*x)
> squares5 [1,2,3]
[1,4]
> squares5 []
[] |
> import Control.Monad
> squares5 x' = do x<-x'; guard (x*x<5); return (x*x)
> squares5 (Just 2)
Just 4
> squares5 (Just 3)
Nothing
> squares5 Nothing
Nothing |
List | Maybe |
---|---|
> foldr (+) 0 [1,2]
3 |
> foldr (+) 1 (Just 2)
3 |
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