zippa

Generic zipper library for JavaScript with functional visitor utilities.

The zippa core is similar to Neith, but implemented with JS Arrays instead infinite streams, and offers chainable API in addition to a functional API. The JS Array implementation should be more performant for data structures with low fan-out.

Functional visiting is implemented in the powerful visit(visitors, initialState, zipper) function, which visits the structure in depth-first order without recursion, maintaining a visit state, allowing breaking out of the visit, skipping subtrees, and handling pre and post visits separately for each element. walk, preWalk and postWalk are implemented using visit.

Inspired by:

• clojure.zip
• Tree visitors in Clojure
• zip-visit
• Neith

Example Usage with ArrayZipper

Instantiation

import { zip, ArrayZipper } from 'zippa';

let z = ArrayZipper.from([1, 2, 3, 4]);

The functional API is simple. You can import the functions straight from zippa with import { up, down, right, left } from 'zippa', or get them under a namespace with import { zip } from 'zippa'; All functions are curried.

import pipe from 'ramda/src/pipe'; // reverse compose
import until from 'ramda/src/until';

const thirdChild = pipe(zip.down, zip.right, zip.right, zip.value);
thirdChild(z);
// 3

const increment = zip.edit(x => x + 1);
const incrementChildren = pipe(
    z.down,
    until(zip.isRightmost, pipe(increment, zip.right))
    increment,
    zip.root,
    zip.value
);

incrementChildren(ArrayZipper.from([1, 2, 3, 4]));
// [2, 3, 4, 5]

The chainable API works with the identically named methods on the zipper value. You can exchange between between both styles. Methods are not curried.

let z = ArrayZipper.from([1, 2, 3, 4]);
z = z.down();
z.value()
// 1
z = z.right();
z.value()
// 2
z = z.rightmost();
z.value()
// 4
z = z.remove();

z.root().value()
// [1, 2, 3]

Example: Making a Tree Zipper

Zippers are very useful for functional modification of trees. Making a Zipper for a k-ary tree is simple.

First we define the tree node:

class Node {
    constructor(value, children) {
        this.value = value;
        this.children = children;
    }
}

Then write the three functions required by makeZipper to make a concrete Zipper class:

function isBranch(node) {
    return node.children && !!node.children.length;
}

function getChildren(node) {
    return node.children;
}

function makeNode(oldParent, children) {
    return new Node(oldParent.value, children);
}

Import zippa, make a TreeZipper, and zip away.

import { makeZipper } from 'zippa';

const TreeZipper = makeZipper(isBranch, getChildren, makeNode);

const tree = new Node(
    1,
    [
        new Node(2),
        new Node(3)
    ]
);

const z = TreeZipper.from(tree);

z.value().value
// 1

z.down().right().value().value
// 3

Example: Implementing reduce for TreeZipper with visit

For reduce, we want to visit each node in the tree, keep a reference to an acc value, and call reducerFn(acc, node) on each node. You could do it without zippa.visit, but this is a good demonstration of visit usage.

reduce's function signature looks like this:

declare function reduce<T>(fn: (acc: T, item: any) => T, initialAcc: any, zipper: Zipper): T;

The function passed to reduce takes acc first and the current item second, while a zippa visitor function takes an event type first (zippa.PRE or zippa.POST), item second and state third. To change the state of a visit, the visitor needs to return an object describing the changes. We want to update the state, so we have to return an object with the state key, whose value is the new state.

To make a visitor function out of a reducer function, we make a higher order function that returns a function conforming to the visitor interface. We also utilize zippa.onPre:

import { onPre } from 'zippa';
const makeReduceVisitor = fn => onPre((item, state) => ({ state: fn(state, item) }));

onPre(g) returns a function that calls g with item and state only on the PRE event, so we don't need to deal with the event identifiers ourselves.

Now that we have a way to transform a reducer function to a visitor function, we'll need to pass the visitor function to zippa.visit. zippa.visit takes a list of visitor functions, an initial state, and a zipper value as arguments.

import { visit } from 'zippa';

export const reduce = (fn, initialAcc, zipper) => {
    const {
        state,
        item,
        zipper
    } = visit(
        [makeReduceVisitor(fn)],
        initialAcc,
        zipper
    );

    return state;
}

Using the reduce function, we can gather all the data in our tree:

const tree = new Node(
    1,
    [
        new Node(2),
        new Node(3)
    ]
);

const z = TreeZipper.from(tree);
const numbers = reduce((acc, node) => acc.concat(node.value), [], z);
// [1, 2, 3]

API

See API reference here.

License

MIT. See LICENSE