Instances of List and Maybe
Revision as of 16:52, 20 June 2018 by Pcarbonn (talk | contribs) (Pcarbonn moved page Instances of List to Instances of List and Maybe)
This pages compares the base 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) (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]
|
> import Control.Applicative
> (Just 1) <|> (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]
|
> squares x' = do x<-x'; return (x*x)
> squares (Just 3)
Just 9
|
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
|
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 [3,4])
[Just 3,Just 4]
|