# Module DA.Foldable¶

Class of data structures that can be folded to a summary value.
It’s a good idea to import this module qualified to avoid clashes with
functions defined in `Prelude`

. Ie.:

```
import DA.Foldable qualified as F
```

## Typeclasses¶

**class** Foldable t **where**

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

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

Right-associative fold of a structure.

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

Left-associative fold of a structure.

- foldr1
: (a -> a -> a) -> t a -> a

A variant of foldr that has no base case, and thus should only be applied to non-empty structures.

- foldl1
: (a -> a -> a) -> t a -> a

A variant of foldl that has no base case, and thus should only be applied to non-empty structures.

- toList
: t a -> [a]

List of elements of a structure, from left to right.

- null
: t a -> Bool

Test whether the structure is empty. The default implementation is optimized for structures that are similar to cons-lists, because there is no general way to do better.

- length
: t a -> Int

Returns the size/length of a finite structure as an

`Int`

. The default implementation is optimized for structures that are similar to cons-lists, because there is no general way to do better.

- product
: Multiplicative a => t a -> a

The product function computes the product of the numbers of a structure.

instanceFoldable ([])

instanceFoldable a

## Functions¶

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

Map each element of a structure to an action, evaluate these actions from left to right, and ignore the results. For a version that doesn’t ignore the results see ‘DA.Traversable.mapA’.

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

‘for_’ is ‘mapA_’ with its arguments flipped. For a version that doesn’t ignore the results see ‘DA.Traversable.forA’.

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

Evaluate each action in the structure from left to right, and ignore the results. For a version that doesn’t ignore the results see ‘DA.Traversable.sequence’.

- and
: Foldable t => t Bool -> Bool

`and`

returns the conjunction of a container of Bools. For the result to be`True`

, the container must be finite;`False`

, however, results from a`False`

value finitely far from the left end.

- or
: Foldable t => t Bool -> Bool

`or`

returns the disjunction of a container of Bools. For the result to be`False`

, the container must be finite;`True`

, however, results from a`True`

value finitely far from the left end.