BSD-3-Clause licensed by Dennis Gosnell and Grendel-Grendel-Grendel
Maintained by

Module documentation for


Build Status Hackage Stackage LTS Stackage Nightly BSD3 license

A FocusList is a sequence of elements which has one element as its Focus. It supports quick insertion and indexing by its implementation with Seq.

The fcuslist package is similar to pointed-list or list-zipper. Focuslist however is optimised for fast indexing and insertion at any point, and can be empty. For operations where linked lists perform better the other packages are likely to be superior, though for other operations focuslist is likely to be faster.


Here is a short example of using FocusList.

module Main where

import Data.FocusList
  ( Focus(Focus), FocusList, appendFL, fromListFL, getFocusItemFL, prependFL
  , singletonFL
import Data.Foldable (toList)

-- | Create a new 'FocusList' from a list.  You must set the 'Focus' of the new
-- 'FocusList'.  The 'Focus' is counting from zero, so the @goat@ element should
-- have the 'Focus'.
-- If you try to specify a 'Focus' out of range from the input list,
-- 'fromListFL' will return 'Nothing'.
myFocusList :: Maybe (FocusList String)
myFocusList = fromListFL (Focus 2) ["hello", "bye", "goat", "dog"]

-- | You can get the focused element from an existing 'FocusList'
-- If the 'FocusList' is empty, this returns 'Nothing'.
myFocusElement :: FocusList String -> Maybe String
myFocusElement focuslist = getFocusItemFL focuslist

-- | You can append to either side of a 'FocusList'.
-- 'singletonFL' creates a 'FocusList' with a single element.
-- That single element will have the 'Focus'.
-- 'myFocusListAppended' will have a value of
-- @FocusList (Focus 1) ["bye", "hello", "foobar"]@
myFocusListAppended :: FocusList String
myFocusListAppended =
  prependFL "bye" (appendFL (singletonFL "hello") "foobar")

-- | 'FocusList' is an instance of 'Functor' and 'Foldable', so you can use
-- functions like 'fmap' and 'toList' on a 'FocusList'.
fmapAndConvertToList :: FocusList Int -> [String]
fmapAndConvertToList focuslist = toList (fmap show focuslist)

main :: IO ()
main = do
  putStrLn "myFocusList:"
  print myFocusList

  putStrLn "\nmyFocusListAppended:"
  print myFocusListAppended

  putStrLn "\nmyFocusElement myFocusListAppended:"
  print (myFocusElement myFocusListAppended)

  putStrLn "\nfmap length myFocusListAppended:"
  print (fmap length myFocusListAppended)

  putStrLn "\nfmapAndConvertToList (fmap length myFocusListAppended):"
  print (fmapAndConvertToList (fmap length myFocusListAppended))



Changelog for focuslist

Unreleased changes


  • Update to allow the latest version of genvalidity (#8). Thanks George Wilson!

  • Add some lower version bounds to dependencies.


  • Enables older GHC versions for Travis


  • Initial release.
comments powered byDisqus