An implementation of List Zip for moving `cursor` back and forth through list.

Zip.hs

{-# OPTIONS -Wall -Werror -fwarn-tabs -fno-warn-type-defaults #-}

module Zip where

newtype Zip a = Z ([a], [a]) deriving Show

mkZip :: [a] -> Zip a
mkZip l = Z ([], l)

current :: Zip a -> a
current (Z (_, [])) = error "End of stream!"
current (Z (_, x:_)) = x

forward :: Zip a -> Zip a
forward (Z (_, [])) = error "End of stream!"
forward (Z (b, x:f)) = Z (x:b, f)

backward :: Zip a -> Zip a
backward (Z ([], _)) = error "Start of stream!"
backward (Z (x:b, f)) = Z (b, x:f)

start :: Zip a -> Bool
start (Z ([], _)) = True
start _ = False

lastToEnd :: Zip a -> Bool
lastToEnd (Z (_, [_])) = True
lastToEnd _ = False

end :: Zip a -> Bool
end (Z (_, [])) = True
end _ = False

findForward :: Eq a => a -> Zip a -> Bool
findForward a (Z (_, f)) = a `elem` f

findBackward :: Eq a => a -> Zip a -> Bool
findBackward a (Z (b, _)) = a `elem` b