pointedlist (empty) → 0.0.1

raw patch · 4 files changed

+189/−0 lines, 4 filesdep +basesetup-changed

Dependencies added: base

Files

• Data/List/PointedList.hs +145/−0
• LICENSE +24/−0
• Setup.hs +5/−0
• pointedlist.cabal +15/−0

@@ -0,0 +1,145 @@+-- | An implementation of a zipper-like non-empty list structure that tracks+--   an index position in the list (the 'focus').+module Data.List.PointedList where++import Prelude hiding (foldl, foldr)++import Control.Applicative+import Control.Monad+import Data.Foldable+import Data.List hiding (foldl, foldr)+import Data.Maybe+import Data.Traversable++-- | The implementation of the pointed list structure which tracks the current+--   position in the list structure.+data PointedList a = PointedList [a] a [a]+ deriving (Eq, Ord)++instance (Show a) => Show (PointedList a) where+ show (PointedList ls x rs) = show (reverse ls) ++ " " ++ show x ++ " " ++ show rs++instance Functor PointedList where+ fmap f (PointedList ls x rs) = PointedList (map f ls) (f x) (map f rs)++instance Foldable PointedList where+ foldr f z (PointedList ls x rs) = foldl (flip f) (foldr f z (x:rs)) ls++instance Traversable PointedList where+ traverse f (PointedList ls x rs) = PointedList <$> (reverse <$> traverse f (reverse ls)) <*> f x <*> traverse f rs++-- | Create a @PointedList@ with a single element.+singleton :: a -> PointedList a+singleton x = PointedList [] x []++-- | Possibly create a @Just PointedList@ if the provided list has at least one+--   element; otherwise, return Nothing.+--+--   The provided list's head will be the focus of the list, and the rest of+--   list will follow on the right side.+fromList :: [a] -> Maybe (PointedList a)+fromList []     = Nothing+fromList (x:xs) = Just $ PointedList [] x xs++-- | Possibly create a @Just PointedList@ if the provided list has at least one+--   element; otherwise, return Nothing.+--+--   The provided list's last element will be the focus of the list, following+--   the rest of the list in order, to the left.+fromListEnd :: [a] -> Maybe (PointedList a)+fromListEnd [] = Nothing+fromListEnd xs = Just $ PointedList xs' x []+ where (x:xs') = reverse xs++-- | The focus element of the pointed list.+focus :: PointedList a -> a+focus (PointedList _ x _) = x++-- | Possibly move the focus to the next element in the list.+next :: PointedList a -> Maybe (PointedList a)+next (PointedList _  _ []) = Nothing+next p = (Just . tryNext) p -- GHC doesn't allow PL form here++-- | Attempt to move the focus to the next element, or 'error' if there are+--   no more elements.+tryNext :: PointedList a -> PointedList a+tryNext p@(PointedList _  _ []    ) = error "cannot move to next element"+tryNext   (PointedList ls x (r:rs)) = PointedList (x:ls) r rs++-- | Possibly move the focus to the previous element in the list.+previous :: PointedList a -> Maybe (PointedList a)+previous (PointedList [] _ _ ) = Nothing+previous p = (Just . tryPrevious) p++-- | Attempt to move the focus to the previous element, or 'error' if there are+--   no more elements.+tryPrevious :: PointedList a -> PointedList a+tryPrevious p@(PointedList []     _ _ ) =+  error "cannot move to previous element" +tryPrevious   (PointedList (l:ls) x rs) = PointedList ls l (x:rs)++-- | An alias for 'deleteRight'.+insert :: a -> PointedList a -> PointedList a+insert = insertRight++-- | Insert an element to the left of the focus, then move the focus to the new+--   element.+insertLeft :: a -> PointedList a -> PointedList a+insertLeft y (PointedList ls x rs) = PointedList ls y (x:rs)++-- | Insert an element to the right of the focus, then move the focus to the+--   new element.+insertRight :: a -> PointedList a -> PointedList a+insertRight y (PointedList ls x rs) = PointedList (x:ls) y rs++-- | An alias of 'deleteRight'.+delete :: PointedList a -> Maybe (PointedList a)+delete = deleteRight++-- | Possibly delete the element at the focus, then move the element on the+--   left to the focus. If no element is on the left, focus on the element to+--   the right. If the deletion will cause the list to be empty, return+--   @Nothing@.+deleteLeft :: PointedList a -> Maybe (PointedList a)+deleteLeft (PointedList [] _ []    ) = Nothing+deleteLeft (PointedList (l:ls) _ rs) = Just $ PointedList ls l rs+deleteLeft (PointedList [] _ (r:rs)) = Just $ PointedList [] r rs++-- | Possibly delete the element at the focus, then move the element on the+--   right to the focus. If no element is on the right, focus on the element to+--   the left. If the deletion will cause the list to be empty, return+--   @Nothing@.+deleteRight :: PointedList a -> Maybe (PointedList a)+deleteRight (PointedList [] _ []    ) = Nothing+deleteRight (PointedList ls _ (r:rs)) = Just $ PointedList ls r rs+deleteRight (PointedList (l:ls) _ []) = Just $ PointedList ls l []++-- | The length of the list.+length :: PointedList a -> Int+length = foldr (const (+1)) 0++-- | Whether the focus is the first element.+atStart :: PointedList a -> Bool+atStart (PointedList [] _ _) = True+atStart _ = False++-- | Whether the focus is the last element.+atEnd :: PointedList a -> Bool+atEnd (PointedList _ _ []) = True+atEnd _ = False++-- | Create a @PointedList@ of variations of the provided @PointedList@, in+--   which each element is focused, with the provided @PointedList@ as the+--   focus of the sets.+positions :: PointedList a -> PointedList (PointedList a)+positions p@(PointedList ls x rs) = PointedList left p right+ where left  = unfoldr (\p -> fmap (join (,)) $ previous p) p+       right = unfoldr (\p -> fmap (join (,)) $ next p) p++-- | Map over the @PointedList@s created via 'positions', such that @f@ is+--   called with each element of the list focused in the provided+--   @PointedList@. An example makes this easier to understand:+--+-- > contextMap atStart (fromJust $ fromList [1..5])+contextMap :: (PointedList a -> b) -> PointedList a -> PointedList b+contextMap f z = fmap f $ positions z

@@ -0,0 +1,24 @@+* Copyright (c) 2009, Jeff Wheeler+* All rights reserved.+*+* Redistribution and use in source and binary forms, with or without+* modification, are permitted provided that the following conditions are met:+*     * Redistributions of source code must retain the above copyright+*       notice, this list of conditions and the following disclaimer.+*     * Redistributions in binary form must reproduce the above copyright+*       notice, this list of conditions and the following disclaimer in the+*       documentation and/or other materials provided with the distribution.+*     * Neither the name of the <organization> nor the+*       names of its contributors may be used to endorse or promote products+*       derived from this software without specific prior written permission.+*+* THIS SOFTWARE IS PROVIDED BY <copyright holder> ''AS IS'' AND ANY+* EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED+* WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE+* DISCLAIMED. IN NO EVENT SHALL <copyright holder> BE LIABLE FOR ANY+* DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES+* (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;+* LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND+* ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT+* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS+* SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

@@ -0,0 +1,5 @@+#!/usr/bin/env runhaskell+import Distribution.Simple++main :: IO ()+main = defaultMain

@@ -0,0 +1,15 @@+Name:          pointedlist+Version:       0.0.1+Synopsis:      A zipper-like comonad which works as a list, tracking a position.+Category:      Data+Description:+    A PointedList tracks the position in a non-empty list which works similarly+    to a zipper. A current item is always required, and therefore the list may+    never be empty.+License:       BSD3+License-file:  LICENSE+Author:        Jeff Wheeler+Maintainer:    jeffwheeler@gmail.com+Build-type:    Simple+Build-depends: base>=4+Exposed-modules: Data.List.PointedList