Control.Lens.Zipper

This module provides a Zipper with fairly strong type checking guarantees.

The code here is inspired by Brandon Simmons' zippo package, but uses a slightly different approach to represent the Zipper that makes the whole thing look like his breadcrumb trail, and can move side-to-side through traversals.

Some examples types:

Top :> a

represents a trivial Zipper with its focus at the root.

Top :> Tree a :> a

represents a Zipper that starts with a Tree and descends in a single step to values of type a.

Top :> Tree a :> Tree a :> Tree a

represents a Zipper into a Tree with an intermediate bookmarked Tree, focusing in yet another Tree.

Since individual levels of a Zipper are managed by an arbitrary Traversal, you can move left and right through the Traversal selecting neighboring elements.

>>> zipper ("hello","world") & downward _1 & fromWithin traverse & focus .~ 'J' & rightmost & focus .~ 'y' & rezip
("Jelly","world")

This is particularly powerful when compiled with plate, uniplate or biplate for walking down into self-similar children in syntax trees and other structures.

Zippers

data Top

This is used to represent the Top of the Zipper.

Every Zipper starts with Top.

e.g. Top :> a is the type of the trivial Zipper.

Instances

Zipping Top a

data h :> a

This is the type of a Zipper. It visually resembles a "breadcrumb trail" as used in website navigation. Each breadcrumb in the trail represents a level you can move up to.

This type operator associates to the left, so you can use a type like

Top :> (String,Double) :> String :> Char

to represent a zipper from (String,Double) down to Char that has an intermediate crumb for the String containing the Char.

You can construct a zipper into *any* data structure with zipper.

You can repackage up the contents of a zipper with rezip.

>>> rezip $ zipper 42
42

The combinators in this module provide lot of things you can do to the zipper while you have it open.

Note that a value of type h :> s :> a doesn't actually contain a value of type h :> s -- as we descend into a level, the previous level is unpacked and stored in Coil form. Only one value of type _ :> _ exists at any particular time for any particular Zipper.

Instances

Zipping h s => Zipping (:> h s) a

type Zipper = :>

This is an alias for '(:>)'. Provided mostly for convenience

zipper :: a -> Top :> a

Construct a Zipper that can explore anything, and start it at the top.

Focusing

focus :: SimpleIndexedLens (Tape (h :> a)) (h :> a) a

This Lens views the current target of the Zipper.

A Tape that can be used to get to the current location is available as the index of this Lens.

focusedContext :: Zipping h a => (h :> a) -> Context a a (Zipped h a)

Extract the current focus from a Zipper as a Context

Vertical Movement

upward :: ((h :> s) :> a) -> h :> s

Move the Zipper upward, closing the current level and focusing on the parent element.

NB: Attempts to move upward from the Top of the Zipper will fail to typecheck.

downward :: SimpleLensLike (Context a a) s a -> (h :> s) -> (h :> s) :> a

Step down into a Lens. This is a constrained form of fromWithin for when you know there is precisely one target that can never fail.

 downward :: Simple Lens s a -> (h :> s) -> h :> s :> a
 downward :: Simple Iso s a  -> (h :> s) -> h :> s :> a

within :: MonadPlus m => SimpleLensLike (Bazaar a a) s a -> (h :> s) -> m ((h :> s) :> a)

Step down into the leftmost entry of a Traversal.

 within :: Simple Traversal s a -> (h :> s) -> Maybe (h :> s :> a)
 within :: Simple Lens s a      -> (h :> s) -> Maybe (h :> s :> a)
 within :: Simple Iso s a       -> (h :> s) -> Maybe (h :> s :> a)

withins :: SimpleLensLike (Bazaar a a) s a -> (h :> s) -> [(h :> s) :> a]

Step down into every entry of a Traversal simultaneously.

>>> zipper ("hello","world") & withins both >>= leftward >>= withins traverse >>= rightward <&> focus %~ toUpper <&> rezip
[("hEllo","world"),("heLlo","world"),("helLo","world"),("hellO","world")]

 withins :: Simple Traversal s a -> (h :> s) -> [h :> s :> a]
 withins :: Simple Lens s a      -> (h :> s) -> [h :> s :> a]
 withins :: Simple Iso s a       -> (h :> s) -> [h :> s :> a]

Lateral Movement

leftward :: MonadPlus m => (h :> a) -> m (h :> a)

Jerk the zipper leftward one tooth within the current Lens or Traversal.

Attempts to move past the end of the current Traversal (or trivially, the current Lens) will return Nothing.

>>> isNothing $ zipper "hello" & leftward
True

rightward :: MonadPlus m => (h :> a) -> m (h :> a)

Jerk the Zipper one tooth to the rightward within the current Lens or Traversal.

Attempts to move past the start of the current Traversal (or trivially, the current Lens) will return Nothing.

>>> isNothing $ zipper "hello" & rightward
True

>>> zipper "hello" & fromWithin traverse & rightward <&> view focus
'e'

>>> zipper "hello" & fromWithin traverse & rightward <&> focus .~ 'u' <&> rezip
"hullo"

>>> rezip $ zipper (1,2) & fromWithin both & tug rightward & focus .~ 3
(1,3)

leftmost :: (a :> b) -> a :> b

Move to the leftmost position of the current Traversal.

This is just a convenient alias for farthest leftward.

>>> zipper "hello" & fromWithin traverse & rightmost & focus .~ 'a' & rezip
"hella"

rightmost :: (a :> b) -> a :> b

Move to the rightmost position of the current Traversal.

This is just a convenient alias for farthest rightward.

>>> zipper "hello" & fromWithin traverse & rightmost & focus .~ 'y' & leftmost & focus .~ 'j' & rezip
"jelly"

Movement Combinators

tug :: (a -> Maybe a) -> a -> a

This allows you to safely 'tug leftward' or 'tug rightward' on a zipper. This will attempt the move, and stay where it was if it fails.

The more general signature allows its use in other circumstances, however.

tug f x ≡ fromMaybe a (f a)

>>> fmap rezip $ zipper "hello" & within traverse <&> tug leftward <&> focus .~ 'j'
"jello"

>>> fmap rezip $ zipper "hello" & within traverse <&> tug rightward <&> focus .~ 'u'
"hullo"

tugs :: (a -> Maybe a) -> Int -> a -> a

This allows you to safely tug leftward or tug rightward multiple times on a zipper, moving multiple steps in a given direction and stopping at the last place you couldn't move from. This lets you safely move a zipper, because it will stop at either end.

>>> fmap rezip $ zipper "stale" & within traverse <&> tugs rightward 2 <&> focus .~ 'y'
"style"

>>> rezip $ zipper "want" & fromWithin traverse & tugs rightward 2 & focus .~ 'r' & tugs leftward 100 & focus .~ 'c'
"cart"

jerks :: Monad m => (a -> m a) -> Int -> a -> m a

This allows for you to repeatedly pull a zipper in a given direction, failing if it falls off the end.

>>> isNothing $ zipper "hello" & within traverse >>= jerks rightward 10
True

>>> fmap rezip $ zipper "silly" & within traverse >>= jerks rightward 3 <&> focus .~ 'k'
"silky"

farthest :: (a -> Maybe a) -> a -> a

Move in a direction as far as you can go, then stop there.

This repeatedly applies a function until it returns Nothing, and then returns the last answer.

>>> fmap rezip $ zipper ("hello","world") & downward _1 & within traverse <&> rightmost <&> focus .~ 'a'
("hella","world")

>>> rezip $ zipper ("hello","there") & fromWithin (both.traverse) & rightmost & focus .~ 'm'
("hello","therm")

Absolute Positioning

tooth :: (h :> a) -> Int

Return the index into the current Traversal within the current level of the Zipper.

jerkTo (tooth l) l = Just'

Mnemonically, zippers have a number of teeth within each level. This is which tooth you are currently at.

teeth :: (h :> a) -> Int

Returns the number of siblings at the current level in the zipper.

teeth z >= 1

NB: If the current Traversal targets an infinite number of elements then this may not terminate.

>>> zipper ("hello","world") & teeth
1

>>> zipper ("hello","world") & fromWithin both & teeth
2

>>> zipper ("hello","world") & downward _1 & teeth
1

>>> zipper ("hello","world") & downward _1 & fromWithin traverse & teeth
5

>>> zipper ("hello","world") & fromWithin (_1.traverse) & teeth
5

>>> zipper ("hello","world") & fromWithin (both.traverse) & teeth
10

jerkTo :: MonadPlus m => Int -> (h :> a) -> m (h :> a)

Move the Zipper horizontally to the element in the nth position in the current level, absolutely indexed, starting with the farthest leftward as 0.

This returns Nothing if the target element doesn't exist.

jerkTo n ≡ jerks rightward n . farthest leftward

>>> isNothing $ zipper "not working." & jerkTo 20
True

tugTo :: Int -> (h :> a) -> h :> a

Move the Zipper horizontally to the element in the nth position of the current level, absolutely indexed, starting with the farthest leftward as 0.

If the element at that position doesn't exist, then this will clamp to the range 0 <= n < teeth.

tugTo n ≡ tugs rightward n . farthest leftward

>>> rezip $ zipper "not working." & fromWithin traverse & tugTo 100 & focus .~ '!' & tugTo 1 & focus .~ 'u'
"nut working!"

Closing the zipper

rezip :: Zipping h a => (h :> a) -> Zipped h a

Close something back up that you opened as a Zipper.

type family Zipped h a

This represents the type a Zipper will have when it is fully Zipped back up.

class Zipping h a

This enables us to pull the Zipper back up to the Top.

Instances

Zipping Top a

Zipping h s => Zipping (:> h s) a

Recording

data Tape k

A Tape is a recorded path through the Traversal chain of a Zipper.

saveTape :: (h :> a) -> Tape (h :> a)

Save the current path as as a Tape we can play back later.

restoreTape :: MonadPlus m => Tape (h :> a) -> Zipped h a -> m (h :> a)

Restore ourselves to a previously recorded position precisely.

If the position does not exist, then fail.

restoreNearTape :: MonadPlus m => Tape (h :> a) -> Zipped h a -> m (h :> a)

Restore ourselves to a location near our previously recorded position.

When moving left to right through a Traversal, if this will clamp at each level to the range 0 <= k < teeth, so the only failures will occur when one of the sequence of downward traversals find no targets.

Unsafe Movement

fromWithin :: SimpleLensLike (Bazaar a a) s a -> (h :> s) -> (h :> s) :> a

Unsafely step down into a Traversal that is assumed to be non-empty.

If this invariant is not met then this will usually result in an error!

 fromWithin :: Simple Traversal s a -> (h :> s) -> h :> s :> a
 fromWithin :: Simple Lens s a      -> (h :> s) -> h :> s :> a
 fromWithin :: Simple Iso s a       -> (h :> s) -> h :> s :> a

You can reason about this function as if the definition was:

fromWithin l ≡ fromJust . within l

but it is lazier in such a way that if this invariant is violated, some code can still succeed if it is lazy enough in the use of the focused value.

unsafelyRestoreTape :: Tape (h :> a) -> Zipped h a -> h :> a

Restore ourselves to a previously recorded position.

This *assumes* that nothing has been done in the meantime to affect the existence of anything on the entire path.

Motions leftward or rightward are clamped, but all traversals included on the Tape are assumed to be non-empty.

Violate these assumptions at your own risk!