Instances of List and Maybe: Difference between revisions

Latest revision as of 15:53, 21 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) [] []	> 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] > empty <\|> [2] [2]	> import Control.Applicative > (Just 1) <\|> empty <\|> (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]	> mul x' y' = do x<-x'; y<-y' return (x*y) > mul (Just 2) (Just 5) Just 10

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 []
[]

> 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

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

@@ Line 56: / Line 56: @@
 > fmap (+1) [2, 3]
 [3, 4]
+> fmap (+1) []
+[]
 </haskell>
 |
@@ Line 103: / Line 105: @@
 > [1] <|> empty <|> [2]
 [1,2]
+> empty <|> [2]
+[2]
 </haskell>
 |
 <haskell>
 > import Control.Applicative
-> (Just 1) <|> (Just 2)
+> (Just 1) <|> empty <|> (Just 2)
 Just 1
 > Nothing <|> (Just 2)
@@ Line 130: / Line 134: @@
 |
 <haskell>
-> squares x' = do x<-x'; return (x*x)
+> mul x' y' = do x<-x'; y<-y' return (x*y)
-> squares (Just 3)
+> mul (Just 2) (Just 5)
-Just 9
+Just 10
 </haskell>
 |}
@@ Line 150: / Line 154: @@
 > squares5 [1,2,3]
 [1,4]
+> squares5 []
+[]
 </haskell>
 |
 <haskell>
+> import Control.Monad
 > squares5 x' = do x<-x'; guard (x*x<5); return (x*x)
 > squares5 (Just 2)