Question

I’m toying around with zippers, where the current element can have a different type than the left and right lists:

data Z a b = Z ([a], b, [a])

One can navigate the zipper with moveLeft and moveRight:

moveLeft  :: (b -> a -> a) -> (a -> b -> b) -> Z a b -> Z a b
moveLeft listF currentF (Z (x:xs, c, ys)) = Z (xs, g x c, ((f c x):ys))

moveRight :: (b -> a -> a) -> (a -> b -> b) -> Z a b -> Z a b
moveRight listF currentF (Z (xs, c, y:ys)) = Z (((f c y):xs), g y c, ys)

Here, listF transforms a list element and the current one into a list element to the left or right. currentF transforms the current element and a list element into the current element.

If both the current and the list type are equal, moving is simple:

moveLeftSameType :: Z a a -> Z a a
moveLeftSameType = moveLeft const const

and everything works as expected, nice!

What I’d like to do now is to generalize the above idea, such that by only implementing the listF and currentF functions for a given Z a b (say a :: Char, b :: Int), moveLeft and moveRight do the right thing automagically. What would be the correct way to do this?

Remark

I have tried to implement a class like this:

class CPos a where
    listF :: c -> d -> d
    currentF :: d -> c -> c
    moveLeft :: a -> a
    moveRight :: a -> a

where moveLeft/Right are implemented in terms of listF and currentF, but this fails with

The class method `listF' mentions none of the type variables of the class CPos a

Remark 2

Something I don’t like about the above idea in general is the fact that be allowing arbitrary functions listF and currentF it’s impossible to guarantee

moveLeft . moveRight = id

(if the zipper is within the list, on the borders this doesn’t hold anyways). Any hints to enforce this?

Answer 1

Here is a solution to that.

class CPos a b where
    listF :: b -> a -> a
    currentF :: a -> b -> b
    moveLeft :: Z a b -> Z a b
    moveLeft (Z (x:xs, c, ys)) = Z (xs, currentF x c, ((listF c x):ys))
    moveRight :: Z a b -> Z a b
    moveRight (Z (xs, c, y:ys)) = Z (((listF c y):xs), currentF y c, ys)

I don’t think you can enforce moveLeft . moveRight = id. I mean it is undecidable to guarantee two functions are equal. The best you can do is to write quickcheck testcases to guarantee the same.