module ListZipper ( ListZipper(..)
, focus
, left
, right
, reset
, fromList
, toList
, next
, atEnd
, at
)
where
import Data.Maybe
import Data.Monoid
import Data.Foldable (Foldable,foldMap)
data ListZipper a = LZ [a] (Maybe a) [a] deriving (Show,Eq,Ord)
left (LZ xs _ _) = xs
focus (LZ _ x _) = x
right (LZ _ _ ys) = ys
fromList [] = LZ [] Nothing []
fromList (x:xs) = LZ [] (Just x) xs
toList (LZ xs (Just x) ys) = reverse xs ++ [x] ++ ys
toList (LZ xs Nothing []) = reverse xs
next :: ListZipper a -> ListZipper a
next = nextWith id id
nextWith f g (LZ lxs (Just x) rxs) =
case rxs of
y:ys -> LZ (f x:lxs) (Just (g y)) ys
[] -> LZ (f x:lxs) Nothing []
atEnd = isNothing . focus
reset (LZ [] (Just y) ys) = LZ [] (Just y) ys
reset (LZ [] Nothing []) = LZ [] Nothing []
reset (LZ xs (Just y) ys) = LZ [] (Just z) (zs++[y]++ys)
where z:zs = reverse xs
reset (LZ xs Nothing []) = LZ [] (Just z) zs
where z:zs = reverse xs
from :: (Monoid m) => Maybe m -> m
from Nothing = mempty
from (Just x) = x
index 1 (x:xs) = Just x
index i [] = Nothing
index i (x:xs) = index (i1) xs
at :: (Monoid m) => ListZipper m -> Int -> m
at z 0 = from . focus $ z
at z i | i < 0 = from . index (negate i) . left $ z
at z i = from . index i . right $ z
instance Functor ListZipper where
fmap f (LZ ls x rs) = LZ (fmap f ls) (fmap f x) (fmap f rs)
instance Foldable ListZipper where
foldMap f (LZ ls x rs) = mconcat $ fmap f (reverse ls)
++ [from $ fmap f x] ++ fmap f rs