@thi.ng/atom

@thi.ng/atom

About

Clojure inspired mutable wrappers for (usually) immutable values, with support for watches, cursors (direct access to nested values), undo/redo history.

Installation

yarn add @thi.ng/atom

Usage examples

A complete minimal webapp example is in the /examples/todo-list directory.

Live demo here

Atom

import * as atom from "@thi.ng/atom";

const a = new atom.Atom(23);

// obtain value via deref()
a.deref();
// 23

// add watch to observe value changes
a.addWatch("foo", (id, prev, curr) => console.log(`${id}: ${prev} -> ${curr}`));
// true

a.swap((val)=> val + 1);
// foo: 23 -> 24

a.reset(42);
// foo: 24 -> 42

Cursor

Cursors provide direct & immutable access to a nested value within a structured atom. The path to the desired value must be provided when the cursor is created. The path is then compiled into a getter and setter to allow cursors to be used like atoms and update the parent state in an immutable manner (i.e. producing an optimized copy with structural sharing of the original (as much as possible)) - see further details below.

It's important to remember that cursors also cause their parent state (atom or another cursor) to reflect their updated local state. I.e. any change to a cursor's value propagates up the hierarchy of parent states.

a = new atom.Atom({a: {b: {c: 1}}})
// cursor to `b` value
b=new atom.Cursor(a, "a.b")
// cursor to `c` value, relative to `b`
c=new atom.Cursor(b, "c")

c.reset(2);

b.deref();
// { c: 2 }

a.deref();
// { a: { b: { c: 2 } } }

For that reason, it's recommended to design the overall data layout rather wide than deep (my personal limit is 3-4 levels) to minimize the length of the propagation chain and maximize structural sharing.

// main state
main = new atom.Atom({ a: { b: { c: 23 }, d: { e: 42 } }, f: 66 });

// cursor to `c` value
cursor = new atom.Cursor(main, "a.b.c");
// or
cursor = new atom.Cursor(main, ["a","b","c"]);

// alternatively provide path implicitly via lookup & update functions
// both fns will be called with cursor's parent state
// this allows the cursor implementation to work with any data structure
// as long as the updater DOES NOT mutate in place
cursor = new atom.Cursor(
    main,
    (s) => s.a.b.c,
    (s, x) => ({...s, a: {...s.a, b: {...s.a.b, c: x}}})
);

// add watch just as with Atom
cursor.addWatch("foo", console.log);

cursor.deref()
// 23

cursor.swap(x => x + 1);
// foo 23 24

main.deref()
// { a: { b: { c: 24 }, d: { e: 42 } }, f: 66 }

Undo history

// the History can be used with & behaves like an Atom or Cursor
// but creates snapshots of current state before applying new state
// by default history has length of 100 steps, but is configurable
db = new atom.History(new atom.Atom({a: 1}))
db.deref()
// {a: 1}

db.reset({a: 2, b: 3})
db.reset({b: 4})

db.undo()
// {a: 2, b: 3}

db.undo()
// {a: 1}

db.undo()
// undefined (no more undo possible)
db.canUndo()
// false

db.redo()
// {a: 2, b: 3}

db.redo()
// {b: 4}

db.redo()
// undefined (no more redo possible)

db.canRedo()
// false

Getters & setters

The getter() and setter() functions transform a path like a.b.c into a function operating directly at the value the path points to in nested object. For getters, this essentially compiles to val = obj.a.b.c.

The resulting setter function too accepts a single object to operate on and when called, immutably replaces the value at the given path, i.e. produces a selective deep copy of obj up until given path. If any intermediate key is not present in the given object, it creates a plain empty object for that missing key and descends further along the path.

s = setter("a.b.c");
// or
s = setter(["a","b","c"]);

s({a: {b: {c: 23}}}, 24)
// {a: {b: {c: 24}}}

s({x: 23}, 24)
// { x: 23, a: { b: { c: 24 } } }

s(null, 24)
// { a: { b: { c: 24 } } }

In addition to these higher-order functions, the module also provides immediate-use wrappers: getIn(), setIn(), updateIn() and deleteIn(). These functions are using getter / setter internally, so have same behaviors.

state = {a: {b: {c: 23}}};

getIn(state, "a.b.c")
// 23

setIn(state, "a.b.c", 24)
// {a: {b: {c: 24}}}

// apply given function to path value
updateIn(state, "a.b.c", x => x + 1)
// {a: {b: {c: 24}}}

// immutably remove path key
deleteIn(state, "a.b.c.")
// {a: {b: {}}}

Only keys in the path will be modified, all other keys present in the given object retain their original values to provide efficient structural sharing / re-use. This is the same behavior as in Clojure's immutable maps or those provided by ImmutableJS (albeit those implementation are completely different - they're using trees, we're using the ES6 spread op and recursive functional composition to produce the setter/updater).

s = setter("a.b.c");

// original
a = { x: { y: { z: 1 } }, u: { v: 2 } };
// updated version
b = s(a, 3);
// { x: { y: { z: 1 } }, u: { v: 2 }, a: { b: { c: 3 } } }

// verify anything under keys `x` & `u` is still identical
a.x === b.x // true
a.x.y === b.x.y // true
a.u === b.u; // true

Authors

• Karsten Schmidt

License

© 2018 Karsten Schmidt // Apache Software License 2.0