Portability | portable |
---|---|

Stability | experimental |

Maintainer | libraries@haskell.org |

Class of data structures that can be folded to a summary value.

Many of these functions generalize Prelude, Control.Monad and
Data.List functions of the same names from lists to any `Foldable`

functor. To avoid ambiguity, either import those modules hiding
these names or qualify uses of these function names with an alias
for this module.

- class Foldable t where
- foldr' :: Foldable t => (a -> b -> b) -> b -> t a -> b
- foldl' :: Foldable t => (a -> b -> a) -> a -> t b -> a
- foldrM :: (Foldable t, Monad m) => (a -> b -> m b) -> b -> t a -> m b
- foldlM :: (Foldable t, Monad m) => (a -> b -> m a) -> a -> t b -> m a
- traverse_ :: (Foldable t, Applicative f) => (a -> f b) -> t a -> f ()
- for_ :: (Foldable t, Applicative f) => t a -> (a -> f b) -> f ()
- sequenceA_ :: (Foldable t, Applicative f) => t (f a) -> f ()
- asum :: (Foldable t, Alternative f) => t (f a) -> f a
- mapM_ :: (Foldable t, Monad m) => (a -> m b) -> t a -> m ()
- forM_ :: (Foldable t, Monad m) => t a -> (a -> m b) -> m ()
- sequence_ :: (Foldable t, Monad m) => t (m a) -> m ()
- msum :: (Foldable t, MonadPlus m) => t (m a) -> m a
- toList :: Foldable t => t a -> [a]
- concat :: Foldable t => t [a] -> [a]
- concatMap :: Foldable t => (a -> [b]) -> t a -> [b]
- and :: Foldable t => t Bool -> Bool
- or :: Foldable t => t Bool -> Bool
- any :: Foldable t => (a -> Bool) -> t a -> Bool
- all :: Foldable t => (a -> Bool) -> t a -> Bool
- sum :: (Foldable t, Num a) => t a -> a
- product :: (Foldable t, Num a) => t a -> a
- maximum :: (Foldable t, Ord a) => t a -> a
- maximumBy :: Foldable t => (a -> a -> Ordering) -> t a -> a
- minimum :: (Foldable t, Ord a) => t a -> a
- minimumBy :: Foldable t => (a -> a -> Ordering) -> t a -> a
- elem :: (Foldable t, Eq a) => a -> t a -> Bool
- notElem :: (Foldable t, Eq a) => a -> t a -> Bool
- find :: Foldable t => (a -> Bool) -> t a -> Maybe a

# Folds

Data structures that can be folded.

Minimal complete definition: `foldMap`

or `foldr`

.

For example, given a data type

data Tree a = Empty | Leaf a | Node (Tree a) a (Tree a)

a suitable instance would be

instance Foldable Tree where foldMap f Empty = mempty foldMap f (Leaf x) = f x foldMap f (Node l k r) = foldMap f l `mappend` f k `mappend` foldMap f r

This is suitable even for abstract types, as the monoid is assumed
to satisfy the monoid laws. Alternatively, one could define `foldr`

:

instance Foldable Tree where foldr f z Empty = z foldr f z (Leaf x) = f x z foldr f z (Node l k r) = foldr f (f k (foldr f z r)) l

fold :: Monoid m => t m -> mSource

Combine the elements of a structure using a monoid.

foldMap :: Monoid m => (a -> m) -> t a -> mSource

Map each element of the structure to a monoid, and combine the results.

foldr :: (a -> b -> b) -> b -> t a -> bSource

foldl :: (a -> b -> a) -> a -> t b -> aSource

## Special biased folds

foldr' :: Foldable t => (a -> b -> b) -> b -> t a -> bSource

Fold over the elements of a structure, associating to the right, but strictly.

foldl' :: Foldable t => (a -> b -> a) -> a -> t b -> aSource

Fold over the elements of a structure, associating to the left, but strictly.

foldrM :: (Foldable t, Monad m) => (a -> b -> m b) -> b -> t a -> m bSource

Monadic fold over the elements of a structure, associating to the right, i.e. from right to left.

foldlM :: (Foldable t, Monad m) => (a -> b -> m a) -> a -> t b -> m aSource

Monadic fold over the elements of a structure, associating to the left, i.e. from left to right.

## Folding actions

### Applicative actions

traverse_ :: (Foldable t, Applicative f) => (a -> f b) -> t a -> f ()Source

Map each element of a structure to an action, evaluate these actions from left to right, and ignore the results.

for_ :: (Foldable t, Applicative f) => t a -> (a -> f b) -> f ()Source

sequenceA_ :: (Foldable t, Applicative f) => t (f a) -> f ()Source

Evaluate each action in the structure from left to right, and ignore the results.

asum :: (Foldable t, Alternative f) => t (f a) -> f aSource

The sum of a collection of actions, generalizing `concat`

.

### Monadic actions

mapM_ :: (Foldable t, Monad m) => (a -> m b) -> t a -> m ()Source

Map each element of a structure to a monadic action, evaluate these actions from left to right, and ignore the results.

sequence_ :: (Foldable t, Monad m) => t (m a) -> m ()Source

Evaluate each monadic action in the structure from left to right, and ignore the results.

msum :: (Foldable t, MonadPlus m) => t (m a) -> m aSource

The sum of a collection of actions, generalizing `concat`

.

## Specialized folds

concat :: Foldable t => t [a] -> [a]Source

The concatenation of all the elements of a container of lists.

concatMap :: Foldable t => (a -> [b]) -> t a -> [b]Source

Map a function over all the elements of a container and concatenate the resulting lists.

any :: Foldable t => (a -> Bool) -> t a -> BoolSource

Determines whether any element of the structure satisfies the predicate.

all :: Foldable t => (a -> Bool) -> t a -> BoolSource

Determines whether all elements of the structure satisfy the predicate.

sum :: (Foldable t, Num a) => t a -> aSource

The `sum`

function computes the sum of the numbers of a structure.

product :: (Foldable t, Num a) => t a -> aSource

The `product`

function computes the product of the numbers of a structure.

maximumBy :: Foldable t => (a -> a -> Ordering) -> t a -> aSource

The largest element of a non-empty structure with respect to the given comparison function.

minimumBy :: Foldable t => (a -> a -> Ordering) -> t a -> aSource

The least element of a non-empty structure with respect to the given comparison function.