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A `List` zipper with holes in it.

Home Page: http://package.elm-lang.org/packages/zwilias/elm-holey-zipper/latest

License: BSD 3-Clause "New" or "Revised" License

Elm 100.00%

elm-holey-zipper's Introduction

Holey Zipper

Like a List zipper, but with more holes in it.

The basic idea is a zipper that can represent an empty list, can focus before and after every item, and doesn't make life hard.

import List.Holey.Zipper as Zipper

Zipper.empty           -- Zipper Hole a
    |> Zipper.plug 5   -- Zipper Full Int
    |> Zipper.append
        [ 1, 2, 3 ]    -- Zipper Full Int
    |> Zipper.nextHole -- Zipper Hole Int
    |> Zipper.toList   -- List Int
--> [ 5, 1, 2, 3 ]

So, there's that.

Made with love and released under BSD-3.

elm-holey-zipper's People

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lue-bird

elm-holey-zipper's Issues

Different data structure?

@zwilias Thanks for the package, and for the friendly suggestions to plug it our holes. Sounds fun! ;)

The idea of being able to have a non-empty list fits my case. However in the current implementation doing something like this causes an error:

revisionIdPerAuthorityZipper =
    case flags.revisionIdPerAuthority of
        [] ->
            Zipper.empty

        x :: xs ->
            Zipper.zipper x xs

The thing is that it doesn't like it having Zipper Hole.. in one branch and Zipper Full ... in another.

What if we've changed the type to be:

type Zipper a
    = Zipper (List a) (ZipperType a) (List a)


type ZipperType a
    = Hole
    | Full a

Then we could also have Zipper.fromList that would take care of the logic.

type-safe `current`

List.Holey.Zipper is implemented using unsafe recursive code, but it really doesn't have to be:

type ZipperOnHole possiblyOrNever a
    = Zipper (List a) (Emptiable possiblyOrNever a) (List a)

type Emptiable possiblyOrNever a
    = Empty possiblyOrNever
    | Filled a

type Possibly
    = Possible

current : ZipperOnHole Never a -> a
current (Zipper _ selected _) =
    case selected of
        Filled value ->
            value
            
        Empty hole ->
            hole |> never -- Never <3

empty : ZipperOnHole Possibly a
empty =
    Zipper [] (Empty Possibly) []

zipper : a -> ZipperOnHole never_ a
zipper head tail =
    Zipper [] (Filled head) tail

Notice the never_ type variable, which solves the problem that branches that have Hole and Full don't combine into one type #2:

case list of
    [] -> Zipper.empty
    x :: xs -> Zipper.zipper x xs

--> : ZipperOnHole Possibly ...

I published Possibly and Empty, Stack, FocusList in new libs allowable-state, elm-emptiness-typed and scroll as it seemed more adequate to put them in a separate package.

zwilias / elm-holey-zipper Goto Github PK

elm-holey-zipper's Introduction

Holey Zipper

elm-holey-zipper's People

Contributors

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Forkers

elm-holey-zipper's Issues

Different data structure?

type-safe `current`

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