Difference between revisions of "Instances of List and Maybe"

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(+ [Typeclassopedia])
(align List and Maybe examples)
 
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> fmap (+1) [2, 3]
  
> fmap (+1) [2, 3]
 
[3, 4]
  
[3, 4]
  +
> fmap (+1) []
  +
[]
 
</haskell>
  
</haskell>
 
|
  
|
Line 103: Line 105:
 
> [1] <|> empty <|> [2]
  
> [1] <|> empty <|> [2]
 
[1,2]
  
[1,2]
  +
> empty <|> [2]
  +
[2]
 
</haskell>
  
</haskell>
 
|
  
|
 
<haskell>
  
<haskell>
 
> import Control.Applicative
  
> import Control.Applicative
−
> (Just 1) <|> (Just 2)
 +
> (Just 1) <|> empty <|> (Just 2)
 
Just 1
  
Just 1
 
> Nothing <|> (Just 2)
  
> Nothing <|> (Just 2)
Line 130: Line 134:
 
|
  
|
 
<haskell>
  
<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>
  
</haskell>
 
|}
  
|}
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> squares5 [1,2,3]
  
> squares5 [1,2,3]
 
[1,4]
  
[1,4]
  +
> squares5 []
  +
[]
 
</haskell>
  
</haskell>
 
|
  
|
 
<haskell>
  
<haskell>
  +
> import Control.Monad
 
> squares5 x' = do x<-x'; guard (x*x<5); return (x*x)
  
> squares5 x' = do x<-x'; guard (x*x<5); return (x*x)
 
> squares5 (Just 2)
  
> squares5 (Just 2)
Line 166: Line 173:
   
 
{| class="wikitable"
  
{| class="wikitable"
−
|+ MonadPlus
 +
|+ Foldable
 
|- style="text-align: center"
  
|- style="text-align: center"
 
! List
 
! List
Line 204: Line 211:
 
> 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]

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

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