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Instances of List and Maybe

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> sequenceA [Just 3, Just 4]
 
> sequenceA [Just 3, Just 4]
 
Just [3,4]
 
Just [3,4]
 +
> sequenceA [Just 3, Nothing]
 +
Nothing
 
> sequenceA (Just [3,4])
 
> sequenceA (Just [3,4])
 
[Just 3,Just 4]
 
[Just 3,Just 4]

Revision as of 17:37, 20 June 2018

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