Data.Lens.Zipper

Documentation

We provide a simple, heterogenous, fully type-checked, generic zipper implementation. This flavor of zipper doesn't use "left" and "right" for navigation, and instead relies on lenses to indicate a child type to "move to".

Zipper type

data Zipper st b

Our zipper type, parameterized by a focus and "history stack", supporting completely type-checked zipper operations.

Zipper history

data Top a

Instances

~ * a b => Hist Top a b

data (st :> b) c

Instances

Hist st a b => Hist (:> st b) a c

class Hist st a c

Instances

~ * a b => Hist Top a b

Hist st a b => Hist (:> st b) a c

Zipper operations

zipper :: a -> Zipper Top a

"enter" a data type. Move the focus with move and moveUp. Exit the zipper with close.

 zipper = Zipper Top

close :: Hist st a b => Zipper st b -> a

exit the zipper, rebuilding the structure a:

 close (Zipper st b) = runHist st b

Motions

move :: Monad m => LensM m b c -> Zipper st b -> m (Zipper (st :> b) c)

navigate to a child element indicated by the passed lens, returning the new Zipper in the monad m. This will be Maybe when the standard (:~>) Lens is used. For pure lenses, use moveP.

moveP :: (b :-> c) -> Zipper st b -> Zipper (st :> b) c

navigate to a child element indicated by the passed pure lens

 moveP l = runIdentity . move l

moveUp :: Zipper (st :> b) c -> Zipper st b

navigate up a level in a zipper not already at Top

 moveUp (Zipper (Snoc st cont) c) = Zipper st $ cont c

Focus

focus :: Zipper st b :-> b

A lens on the focus of the zipper.

setf :: Zipper st b -> b -> Zipper st b

Set the zipper focus

 setf = set focus

modf :: (b -> b) -> Zipper st b -> Zipper st b

Modify the zipper focus

 modf = modify focus