Data.DList

Difference lists: a data structure for O(1) append on lists.

Documentation

data DList a

A difference list is a function that, given a list, returns the original contents of the difference list prepended to the given list.

This structure supports O(1) append and snoc operations on lists, making it very useful for append-heavy uses (esp. left-nested uses of ++), such as logging and pretty printing.

Here is an example using DList as the state type when printing a tree with the Writer monad:

 import Control.Monad.Writer
 import Data.DList

 data Tree a = Leaf a | Branch (Tree a) (Tree a)

 flatten_writer :: Tree x -> DList x
 flatten_writer = snd . runWriter . flatten
     where
       flatten (Leaf x)     = tell (singleton x)
       flatten (Branch x y) = flatten x >> flatten y

Instances

Monad DList

Functor DList

MonadPlus DList

Applicative DList

Foldable DList

Alternative DList

Eq a => Eq (DList a)

Ord a => Ord (DList a)

Read a => Read (DList a)

Show a => Show (DList a)

IsString (DList Char)

Monoid (DList a)

NFData a => NFData (DList a)

Construction

fromList :: [a] -> DList a

Convert a list to a dlist

toList :: DList a -> [a]

Convert a dlist to a list

apply :: DList a -> [a] -> [a]

Apply a dlist to a list to get the underlying list with an extension

 apply (fromList xs) ys = xs ++ ys

Basic functions

empty :: DList a

Create a dlist containing no elements

singleton :: a -> DList a

Create dlist with a single element

cons :: a -> DList a -> DList a

O(1). Prepend a single element to a dlist

snoc :: DList a -> a -> DList a

O(1). Append a single element to a dlist

append :: DList a -> DList a -> DList a

O(1). Append dlists

concat :: [DList a] -> DList a

O(spine). Concatenate dlists

replicate :: Int -> a -> DList a

O(n). Create a dlist of the given number of elements

list :: b -> (a -> DList a -> b) -> DList a -> b

O(n). List elimination for dlists

head :: DList a -> a

O(n). Return the head of the dlist

tail :: DList a -> DList a

O(n). Return the tail of the dlist

unfoldr :: (b -> Maybe (a, b)) -> b -> DList a

O(n). Unfoldr for dlists

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

O(n). Foldr over difference lists

map :: (a -> b) -> DList a -> DList b

O(n). Map over difference lists.