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Update fold section

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@ -1,17 +1,16 @@
2013-2020 Stephen Diehl
Copyright © 2009-2020 Stephen Diehl
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rights, to the extent allowed by law.
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tutorial.md
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@ -6970,6 +6970,26 @@ of any data structure which is parameterized over its element type ( List, Map,
Set, Maybe, ...). These two classes are used everywhere in modern Haskell
and are extremely important.
```haskell
class Foldable t where
fold :: Monoid m => t m -> m
foldMap :: Monoid m => (a -> m) -> t a -> m
foldr :: (a -> b -> b) -> b -> t a -> b
foldr' :: (a -> b -> b) -> b -> t a -> b
foldl :: (b -> a -> b) -> b -> t a -> b
foldl' :: (b -> a -> b) -> b -> t a -> b
foldr1 :: (a -> a -> a) -> t a -> a
foldl1 :: (a -> a -> a) -> t a -> a
toList :: t a -> [a]
null :: t a -> Bool
length :: t a -> Int
elem :: Eq a => a -> t a -> Bool
maximum :: Ord a => t a -> a
minimum :: Ord a => t a -> a
sum :: Num a => t a -> a
product :: Num a => t a -> a
```
A foldable instance allows us to apply functions to data types of monoidal
values that collapse the structure using some logic over ``mappend``.
@ -7027,21 +7047,6 @@ instance Monoid (Endo a) where
Endo f `mappend` Endo g = Endo (f . g)
```
```haskell
class Foldable t where
fold :: Monoid m => t m -> m
foldMap :: Monoid m => (a -> m) -> t a -> m
foldr :: (a -> b -> b) -> b -> t a -> b
foldr' :: (a -> b -> b) -> b -> t a -> b
foldl :: (b -> a -> b) -> b -> t a -> b
foldl' :: (b -> a -> b) -> b -> t a -> b
foldr1 :: (a -> a -> a) -> t a -> a
foldl1 :: (a -> a -> a) -> t a -> a
```
For example:
```haskell