Documentation

A class for list types. Every list has an underlying monad.

Convert a list to a MonadPlus

> fromList [] :: Maybe Int
Nothing
> fromList [5] :: Maybe Int
Just 5

filter for any MonadPlus

> filter (> 5) (Just 3)
Nothing

Generalized concat

For List l => l (l a) -> l a use join

Genereralized concatMap

For List l => (a -> l b) -> l a -> l b use =<< (monadic bind)

foldr for Lists. the result and 'right side' values are monadic actions.

An action to do foldl for Lists

An action to transform a List to a list

> runIdentity $ toList "hello!"
"hello!"

Consume a list (execute its actions) and return its length

> runIdentity $ lengthL [1,2,3]
3

Consume all items and return the last one

> runIdentity $ lastL "hello"
'o'

Merge two lists sorted by a criteria given the criteria

> merge2On id "01568" "239"
"01235689"

Merge many lists sorted by a criteria given the criteria

> mergeOn length [["hi", "hey", "hello"], ["cat", "falcon"], ["banana", "cucumber"]]
["hi","cat","hey","hello","banana","falcon","cucumber"]

Execute the monadic actions in a List

Transform a list of actions to a list of their results

> joinM [Identity 4, Identity 7]
[4,7]

Monadic version of iterate. Can be used to produce trees given a children of node function.

import Data.List.Tree (bfsLayers)
take 3 $ bfsLayers (iterateM (\i -> [i*2, i*2+1]) [1] :: ListT [] Int)
[[1],[2,3],[4,5,6,7]]

Monadic variant of splitAt. Consumes x items from the list and return them with the remaining monadic list.

Monadic variant of break. Consumes items from the list until a condition holds.

Transform the underlying monad of a list given a way to transform the monad

> import Data.List.Tree (bfs)
> bfs (transformListMonad (\(Identity x) -> [x, x]) "hey" :: ListT [] Char)
"hheeeeyyyyyyyy"

listStateJoin can transform a ListT (StateT s m) a to a StateT s m (ListT m a).

When iterating a list, a state is already maintained and passed along in the form of the location along the list. This joins the inner StateT s into the list. The list will fork the state given to it and won't share its changes.