Safe Haskell  Trustworthy 

Language  Haskell98 
This module provides efficient and streaming left folds that you can combine
using Applicative
style.
Import this module qualified to avoid clashing with the Prelude:
>>>
import qualified Control.Foldl as L
Use fold
to apply a Fold
to a list:
>>>
L.fold L.sum [1..100]
5050
Fold
s are Applicative
s, so you can combine them using Applicative
combinators:
>>>
import Control.Applicative
>>>
let average = (/) <$> L.sum <*> L.genericLength
Taking the sum, the sum of squares, ..., upto the sum of x^5
>>>
import Data.Traversable
>>>
let powerSums = sequenceA [L.premap (^n) L.sum  n < [1..5]]
>>>
L.fold powerSums [1..10]
[55,385,3025,25333,220825]
These combined folds will still traverse the list only once, streaming efficiently over the list in constant space without space leaks:
>>>
L.fold average [1..10000000]
5000000.5>>>
L.fold ((,) <$> L.minimum <*> L.maximum) [1..10000000]
(Just 1,Just 10000000)
 data Fold a b = forall x . Fold (x > a > x) x (x > b)
 data FoldM m a b = forall x . FoldM (x > a > m x) (m x) (x > m b)
 fold :: Foldable f => Fold a b > f a > b
 foldM :: (Foldable f, Monad m) => FoldM m a b > f a > m b
 scan :: Fold a b > [a] > [b]
 mconcat :: Monoid a => Fold a a
 foldMap :: Monoid w => (a > w) > (w > b) > Fold a b
 head :: Fold a (Maybe a)
 last :: Fold a (Maybe a)
 lastDef :: a > Fold a a
 lastN :: Int > Fold a [a]
 null :: Fold a Bool
 length :: Fold a Int
 and :: Fold Bool Bool
 or :: Fold Bool Bool
 all :: (a > Bool) > Fold a Bool
 any :: (a > Bool) > Fold a Bool
 sum :: Num a => Fold a a
 product :: Num a => Fold a a
 maximum :: Ord a => Fold a (Maybe a)
 maximumBy :: (a > a > Ordering) > Fold a (Maybe a)
 minimum :: Ord a => Fold a (Maybe a)
 minimumBy :: (a > a > Ordering) > Fold a (Maybe a)
 elem :: Eq a => a > Fold a Bool
 notElem :: Eq a => a > Fold a Bool
 find :: (a > Bool) > Fold a (Maybe a)
 index :: Int > Fold a (Maybe a)
 elemIndex :: Eq a => a > Fold a (Maybe Int)
 findIndex :: (a > Bool) > Fold a (Maybe Int)
 random :: FoldM IO a (Maybe a)
 randomN :: Vector v a => Int > FoldM IO a (Maybe (v a))
 mapM_ :: Monad m => (a > m ()) > FoldM m a ()
 sink :: (Monoid w, Monad m) => (a > m w) > FoldM m a w
 genericLength :: Num b => Fold a b
 genericIndex :: Integral i => i > Fold a (Maybe a)
 list :: Fold a [a]
 revList :: Fold a [a]
 nub :: Ord a => Fold a [a]
 eqNub :: Eq a => Fold a [a]
 set :: Ord a => Fold a (Set a)
 vector :: (PrimMonad m, Vector v a) => FoldM m a (v a)
 purely :: (forall x. (x > a > x) > x > (x > b) > r) > Fold a b > r
 purely_ :: (forall x. (x > a > x) > x > x) > Fold a b > b
 impurely :: Monad m => (forall x. (x > a > m x) > m x > (x > m b) > r) > FoldM m a b > r
 impurely_ :: Monad m => (forall x. (x > a > m x) > m x > m x) > FoldM m a b > m b
 generalize :: Monad m => Fold a b > FoldM m a b
 simplify :: FoldM Identity a b > Fold a b
 hoists :: Monad m => (forall x. m x > n x) > FoldM m a b > FoldM n a b
 duplicateM :: Applicative m => FoldM m a b > FoldM m a (FoldM m a b)
 _Fold1 :: (a > a > a) > Fold a (Maybe a)
 premap :: (a > b) > Fold b r > Fold a r
 premapM :: (a > b) > FoldM m b r > FoldM m a r
 type Handler a b = forall x. (b > Const (Dual (Endo x)) b) > a > Const (Dual (Endo x)) a
 handles :: Handler a b > Fold b r > Fold a r
 newtype EndoM m a = EndoM {
 appEndoM :: a > m a
 type HandlerM m a b = forall x. (b > Const (Dual (EndoM m x)) b) > a > Const (Dual (EndoM m x)) a
 handlesM :: Monad m => HandlerM m a b > FoldM m b r > FoldM m a r
 folded :: (Contravariant f, Applicative f, Foldable t) => (a > f a) > t a > f (t a)
 filtered :: Monoid m => (a > Bool) > (a > m) > a > m
 module Control.Monad.Primitive
 module Data.Foldable
 module Data.Vector.Generic
Fold Types
Efficient representation of a left fold that preserves the fold's step function, initial accumulator, and extraction function
This allows the Applicative
instance to assemble derived folds that
traverse the container only once
A 'Fold
a b' processes elements of type a and results in a
value of type b.
forall x . Fold (x > a > x) x (x > b) 

Like Fold
, but monadic.
A 'FoldM
m a b' processes elements of type a and
results in a monadic value of type m b.
forall x . FoldM (x > a > m x) (m x) (x > m b) 

Monad m => Profunctor (FoldM m) Source  
Monad m => Functor (FoldM m a) Source  
Monad m => Applicative (FoldM m a) Source  
(Monad m, Floating b) => Floating (FoldM m a b) Source  
(Monad m, Fractional b) => Fractional (FoldM m a b) Source  
(Monad m, Num b) => Num (FoldM m a b) Source  
(Monoid b, Monad m) => Monoid (FoldM m a b) Source 
Folding
Folds
head :: Fold a (Maybe a) Source
Get the first element of a container or return Nothing
if the container is
empty
last :: Fold a (Maybe a) Source
Get the last element of a container or return Nothing
if the container is
empty
lastDef :: a > Fold a a Source
Get the last element of a container or return a default value if the container is empty
maximumBy :: (a > a > Ordering) > Fold a (Maybe a) Source
Computes the maximum element with respect to the given comparison function
minimumBy :: (a > a > Ordering) > Fold a (Maybe a) Source
Computes the minimum element with respect to the given comparison function
find :: (a > Bool) > Fold a (Maybe a) Source
(find predicate)
returns the first element that satisfies the predicate or
Nothing
if no element satisfies the predicate
index :: Int > Fold a (Maybe a) Source
(index n)
returns the n
th element of the container, or Nothing
if the
container has an insufficient number of elements
elemIndex :: Eq a => a > Fold a (Maybe Int) Source
(elemIndex a)
returns the index of the first element that equals a
, or
Nothing
if no element matches
findIndex :: (a > Bool) > Fold a (Maybe Int) Source
(findIndex predicate)
returns the index of the first element that
satisfies the predicate, or Nothing
if no element satisfies the predicate
randomN :: Vector v a => Int > FoldM IO a (Maybe (v a)) Source
Pick several random elements, using reservoir sampling
mapM_ :: Monad m => (a > m ()) > FoldM m a () Source
Converts an effectful function to a fold. Specialized version of sink
.
sink :: (Monoid w, Monad m) => (a > m w) > FoldM m a w Source
Converts an effectful function to a fold
sink (f <> g) = sink f <> sink g  if `(<>)` is commutative sink mempty = mempty
Generic Folds
genericIndex :: Integral i => i > Fold a (Maybe a) Source
Container folds
nub :: Ord a => Fold a [a] Source
O(n log n). Fold values into a list with duplicates removed, while preserving their first occurrences
eqNub :: Eq a => Fold a [a] Source
O(n^2). Fold values into a list with duplicates removed, while preserving their first occurrences
Utilities
purely
and impurely
allow you to write folds compatible with the foldl
library without incurring a foldl
dependency. Write your fold to accept
three parameters corresponding to the step function, initial
accumulator, and extraction function and then users can upgrade your
function to accept a Fold
or FoldM
using the purely
or impurely
combinators.
For example, the pipes
library implements fold
and foldM
functions in
Pipes.Prelude
with the following type:
Pipes.Prelude.fold :: Monad m > (x > a > x) > x > (x > b) > Producer a m () > m b Pipes.Prelude.foldM :: Monad m => (x > a > m x) > m x > (x > m b) > Producer a m () > m b
Both fold
and foldM
is set up so that you can wrap them with either
purely
or impurely
to accept a Fold
or FoldM
, respectively:
purely Pipes.Prelude.fold :: Monad m => Fold a b > Producer a m () > m b impurely Pipes.Prelude.foldM :: Monad m => FoldM m a b > Producer a m () > m b
Similarly the ofoldlUnwrap
and ofoldMUnwrap
functions from the
monotraversable
package are written to interoperate with this library:
ofoldlUnwrap :: MonoFoldable mono => (x > Element mono > x) > x > (x > b) > mono > b ofoldMUnwrap :: (Monad m, MonoFoldable mono) => (x > Element mono > m x) > m x > (x > m b) > mono > m b
You can wrap these to accept Fold
or FoldM
, too:
purely ofoldlUnwrap :: MonoFoldable mono => Fold (Element mono) b > mono > b impurely ofoldMUnwrap :: MonoFoldable mono => FoldM m (Element mono) b > mono > m b
purely :: (forall x. (x > a > x) > x > (x > b) > r) > Fold a b > r Source
Upgrade a fold to accept the Fold
type
purely_ :: (forall x. (x > a > x) > x > x) > Fold a b > b Source
Upgrade a more traditional fold to accept the Fold
type
impurely :: Monad m => (forall x. (x > a > m x) > m x > (x > m b) > r) > FoldM m a b > r Source
Upgrade a monadic fold to accept the FoldM
type
impurely_ :: Monad m => (forall x. (x > a > m x) > m x > m x) > FoldM m a b > m b Source
Upgrade a more traditional monadic fold to accept the FoldM
type
generalize :: Monad m => Fold a b > FoldM m a b Source
duplicateM :: Applicative m => FoldM m a b > FoldM m a (FoldM m a b) Source
premap :: (a > b) > Fold b r > Fold a r Source
(premap f folder)
returns a new Fold
where f is applied at each step
fold (premap f folder) list = fold folder (map f list)
>>>
fold (premap Sum mconcat) [1..10]
Sum {getSum = 55}
>>>
fold mconcat (map Sum [1..10])
Sum {getSum = 55}
premap id = id premap (f . g) = premap g . premap f
premap k (pure r) = pure r premap k (f <*> x) = premap k f <*> premap k x
premapM :: (a > b) > FoldM m b r > FoldM m a r Source
(premapM f folder)
returns a new FoldM
where f is applied to each input
element
foldM (premapM f folder) list = foldM folder (map f list)
premapM id = id premapM (f . g) = premap g . premap f
premapM k (pure r) = pure r premapM k (f <*> x) = premapM k f <*> premapM k x
handles :: Handler a b > Fold b r > Fold a r Source
(handles t folder)
transforms the input of a Fold
using a lens,
traversal, or prism:
handles _1 :: Fold a r > Fold (a, b) r handles _Left :: Fold a r > Fold (Either a b) r handles traverse :: Traversable t => Fold a r > Fold (t a) r handles folded :: Foldable t => Fold a r > Fold (t a) r
>>>
fold (handles traverse sum) [[1..5],[6..10]]
55
>>>
fold (handles (traverse.traverse) sum) [[Nothing, Just 2, Just 7],[Just 13, Nothing, Just 20]]
42
>>>
fold (handles (filtered even) sum) [1..10]
30
>>>
fold (handles _2 mconcat) [(1,"Hello "),(2,"World"),(3,"!")]
"Hello World!"
handles id = id handles (f . g) = handles f . handles g
handles t (pure r) = pure r handles t (f <*> x) = handles t f <*> handles t x
instance Monad m => Monoid (EndoM m a) where mempty = EndoM return mappend (EndoM f) (EndoM g) = EndoM (f <=< g)
type HandlerM m a b = forall x. (b > Const (Dual (EndoM m x)) b) > a > Const (Dual (EndoM m x)) a Source
handlesM :: Monad m => HandlerM m a b > FoldM m b r > FoldM m a r Source
(handlesM t folder)
transforms the input of a FoldM
using a lens,
traversal, or prism:
handlesM _1 :: FoldM m a r > FoldM (a, b) r handlesM _Left :: FoldM m a r > FoldM (Either a b) r handlesM traverse :: Traversable t => FoldM m a r > FoldM m (t a) r handlesM folded :: Foldable t => FoldM m a r > FoldM m (t a) r
handlesM
obeys these laws:
handlesM id = id handlesM (f . g) = handlesM f . handlesM g
handlesM t (pure r) = pure r handlesM t (f <*> x) = handlesM t f <*> handlesM t x
folded :: (Contravariant f, Applicative f, Foldable t) => (a > f a) > t a > f (t a) Source
folded :: Foldable t => Fold (t a) a handles folded :: Foldable t => Fold a r > Fold (t a) r
filtered :: Monoid m => (a > Bool) > (a > m) > a > m Source
>>>
fold (handles (filtered even) sum) [1..10]
30
>>>
foldM (handlesM (filtered even) (mapM_ print)) [1..10]
2 4 6 8 10
Reexports
Control.Monad.Primitive
reexports the PrimMonad
type class
Data.Foldable
reexports the Foldable
type class
Data.Vector.Generic
reexports the Vector
type class
module Control.Monad.Primitive
module Data.Foldable
module Data.Vector.Generic