Difference between revisions of "Instances of List and Maybe"

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(compare List and Maybe)
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This pages lists all class instances of List in base, with examples of use. You can run the examples in GHCi.
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This pages compares the base classes of List and Maybe, with examples of use. You can run the examples in GHCi.
−  
−
==Semigroup==
  
   
  +
{| class="wikitable"
  +
|+ Semigroup
  +
|-
  +
! List
  +
! Maybe
  +
|-
  +
|
 
<haskell>
  
<haskell>
 
> import Data.Semigroup
  
> import Data.Semigroup
Line 8: Line 13:
 
[1,2,3,4]
  
[1,2,3,4]
 
</haskell>
  
</haskell>
  +
|
  +
<haskell>
  +
> import Data.Semigroup
  +
> (Just [1,2]) <> (Just [3,4])
  +
Just [1,2,3,4]
  +
</haskell>
  +
|}
   
−
==Monoid==
  
   
  +
{| class="wikitable"
  +
|+ Monoid
  +
|-
  +
! List
  +
! Maybe
  +
|-
  +
|
 
<haskell>
  
<haskell>
−
> [] == mempty
 +
> mempty :: [Int]
  +
[]
−
True
  
  +
> [1,2] <> mempty
  +
[1,2]
 
</haskell>
  
</haskell>
  +
|
  +
<haskell>
  +
> mempty :: Maybe [Int]
  +
Nothing
  +
> (Just [1,2]) <> mempty
  +
Just [1,2]
  +
</haskell>
  +
  +
|}
   
−
==Functor==
  
   
  +
{| class="wikitable"
  +
|+ Functor
  +
|-
  +
! List
  +
! Maybe
  +
|-
  +
|
 
<haskell>
  
<haskell>
 
> fmap (+1) [2, 3]
  
> fmap (+1) [2, 3]
 
[3, 4]
  
[3, 4]
 
</haskell>
  
</haskell>
  +
|
  +
<haskell>
  +
> fmap (+1) (Just 2)
  +
Just 3
  +
> fmap (+1) Nothing
  +
Nothing
  +
</haskell>
  +
  +
|}
   
−
==Applicative==
  
   
  +
{| class="wikitable"
  +
|+ Applicative
  +
|-
  +
! List
  +
! Maybe
  +
|-
  +
|
 
<haskell>
  
<haskell>
 
> [(+1),(+2)] <*> [3,4]
  
> [(+1),(+2)] <*> [3,4]
 
[4,5,5,6]
  
[4,5,5,6]
  +
> [] <*> [3,4]
  +
[]
 
</haskell>
  
</haskell>
  +
|
  +
<haskell>
  +
> (Just (+1)) <*> (Just 2)
  +
Just 3
  +
> Nothing <*> (Just 2)
  +
Nothing
  +
</haskell>
  +
|}
  +
   
−
==Alternative==
  
   
  +
{| class="wikitable"
  +
|+ Alternative
  +
|-
  +
! List
  +
! Maybe
  +
|-
  +
|
 
<haskell>
  
<haskell>
 
> import Control.Applicative
  
> import Control.Applicative
Line 37: Line 104:
 
[1,2]
  
[1,2]
 
</haskell>
  
</haskell>
  +
|
  +
<haskell>
  +
> import Control.Applicative
  +
> (Just 1) <|> (Just 2)
  +
Just 1
  +
> Nothing <|> (Just 2)
  +
Just 2
  +
</haskell>
  +
|}
  +
   
−
==Monad==
  
   
  +
{| class="wikitable"
  +
|+ Monad
  +
|-
  +
! List
  +
! Maybe
  +
|-
  +
|
 
<haskell>
  
<haskell>
−
> (\ lst1 lst2 -> do x<-lst1; y<-lst2; return (x*y)) [1,2] [3,4]
 +
> mul xs ys = do x<-xs; y<-ys; return (x*y)
−
[3,4,6,8]
 +
> mul [1,2] [3,4,5]
  +
[3,4,5,6,8,10]
 
</haskell>
  
</haskell>
  +
|
  +
<haskell>
  +
> squares x' = do x<-x'; return (x*x)
  +
> squares (Just 3)
  +
Just 9
  +
</haskell>
  +
|}
  +
   
−
==MonadPlus==
  
   
  +
{| class="wikitable"
  +
|+ MonadPlus
  +
|-
  +
! List
  +
! Maybe
  +
|-
  +
|
 
<haskell>
  
<haskell>
 
> import Control.Monad
  
> import Control.Monad
−
> (\lst -> do x<-lst; guard (x*x<5); return (x*x)) [1,2,3]
 +
> squares5 x' = do x<-x'; guard (x*x<5); return (x*x)
  +
> squares5 [1,2,3]
 
[1,4]
  
[1,4]
 
</haskell>
  
</haskell>
  +
|
  +
<haskell>
  +
> squares5 x' = do x<-x'; guard (x*x<5); return (x*x)
  +
> squares5 (Just 2)
  +
Just 4
  +
> squares5 (Just 3)
  +
Nothing
  +
> squares5 Nothing
  +
Nothing
  +
</haskell>
  +
|}
   
−
==Foldable==
  
   
  +
  +
{| class="wikitable"
  +
|+ MonadPlus
  +
|-
  +
! List
  +
! Maybe
  +
|-
  +
|
 
<haskell>
  
<haskell>
 
> foldr (+) 0 [1,2]
  
> foldr (+) 0 [1,2]
 
3
  
3
 
</haskell>
  
</haskell>
  +
|
  +
<haskell>
  +
> foldr (+) 1 (Just 2)
  +
3
  +
</haskell>
  +
|}
   
−
==Traversable==
  
   
  +
  +
{| class="wikitable"
  +
|+ Traversable
  +
|-
  +
! List
  +
! Maybe
  +
|-
  +
|
 
<haskell>
  
<haskell>
 
> traverse (\x -> [0..x]) [0..2]
  
> traverse (\x -> [0..x]) [0..2]
Line 68: Line 198:
 
[[1,3],[1,4],[2,3],[2,4]]
  
[[1,3],[1,4],[2,3],[2,4]]
 
</haskell>
  
</haskell>
  +
|
  +
<haskell>
  +
> 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]
  +
</haskell>
  +
|}

Revision as of 16:50, 20 June 2018

This pages compares the base 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


MonadPlus
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 [3,4])
[Just 3,Just 4]
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