Functional Reactive Bindings

In their simplest form, bindings provide the illusion that two objects have the same property. Changing the property on one object causes the same change in the other. This is useful for coordinating state between views and models, among other entangled objects. For example, if you enter text into a text field, the same text might be added to the corresponding database record.

bind(object, "a.b", {"<->": "c.d"});

Functional Reactive Bindings go farther. They can gracefully bind long property paths and the contents of collections. They can also incrementally update the results of chains of queries including maps, flattened arrays, sums, and averages. They can also add and remove elements from sets based on the changes to a flag. FRB makes it easy to incrementally ensure consistent state.

bind(company, "payroll", {"<-": "departments.map{employees.sum{salary}}.sum()"});
bind(document, "body.classList.has('dark')", {"<-": "darkMode", source: viewModel});

FRB is built from a combination of powerful functional and generic building blocks, making it reliable, easy to extend, and easy to maintain.

Getting Started

frb is a CommonJS package, with JavaScript modules suitable for use with Node.js on the server side or Mr on the client side.

❯ npm install frb

Tutorial

In this example, we bind model.content to document.body.innerHTML.

var bind = require("frb/bind");
var model = {content: "Hello, World!"};
var cancelBinding = bind(document, "body.innerHTML", {
    "<-": "content",
    "source": model
});

When a source property is bound to a target property, the target gets reassigned to the source any time the source changes.

model.content = "Farewell.";
expect(document.body.innerHTML).toBe("Farewell.");

Bindings can be recursively detached from the objects they observe with the returned cancel function.

cancelBinding();
model.content = "Hello again!"; // doesn't take
expect(document.body.innerHTML).toBe("Farewell.");

Two-way Bindings

Bindings can go one way or in both directions. Declare one-way bindings with the <- property, and two-way bindings with the <-> property.

In this example, the "foo" and "bar" properties of an object will be inexorably intertwined.

var object = {};
var cancel = bind(object, "foo", {"<->": "bar"});

// <-
object.bar = 10;
expect(object.foo).toBe(10);

// ->
object.foo = 20;
expect(object.bar).toBe(20);

Right-to-left

Note that even with a two-way binding, the right-to-left binding precedes the left-to-right. In this example, "foo" and "bar" are bound together, but both have initial values.

var object = {foo: 10, bar: 20};
var cancel = bind(object, "foo", {"<->": "bar"});
expect(object.foo).toBe(20);
expect(object.bar).toBe(20);

The right-to-left assignment of bar to foo happens first, so the initial value of foo gets lost.

Property chains

Bindings can follow deeply nested chains, on both the left and the right side.

In this example, we have two object graphs, foo, and bar, with the same structure and initial values. This binds bar.a.b to foo.a.b and also the other way around.

var foo = {a: {b: 10}};
var bar = {a: {b: 10}};
var cancel = bind(foo, "a.b", {
    "<->": "a.b",
    source: bar
});
// <-
bar.a.b = 20;
expect(foo.a.b).toBe(20);
// ->
foo.a.b = 30;
expect(bar.a.b).toBe(30);

In this case, the source of the binding is a different object than the target, so the binding descriptor specifies the alternate source.

Structure changes

Changes to the structure of either side of the binding are no matter. All of the orphaned event listeners will automatically be canceled, and the binders and observers will reattach to the new object graph.

Continuing from the previous example, we store and replace the a object from one side of the binding. The old b property is now orphaned, and the old b property adopted in its place.

var a = foo.a;
expect(a.b).toBe(30); // from before

foo.a = {}; // orphan a and replace
foo.a.b = 40;
// ->
expect(bar.a.b).toBe(40); // updated

bar.a.b = 50;
// <-
expect(foo.a.b).toBe(50); // new one updated
expect(a.b).toBe(30); // from before it was orphaned

Sum

Some advanced queries are possible with one-way bindings from collections.

var object = {array: [1, 2, 3]};
bind(object, "sum", {"<-": "array.sum()"});
expect(object.sum).toEqual(6);

Average

var object = {array: [1, 2, 3]};
bind(object, "average", {"<-": "array.average()"});
expect(object.average).toEqual(6);

Map

You can also create mappings from one array to a new array and an expression to evaluate on each item. The mapped array is bound once, and all changes to the source array are incrementally updated in the target array. Unaffected items in the array are not affected.

var object = {objects: [
    {number: 10},
    {number: 20},
    {number: 30}
]};
bind(object, "numbers", {"<-": "objects.map{number}"});
expect(object.numbers).toEqual([10, 20, 30]);
object.objects.push({numbers: 40});
expect(object.numbers).toEqual([10, 20, 30]);

Any function, like sum or average, can be applied to the result of a mapping. The straight-forward path would be objects.map{number}.sum(), but you can use a block with any function as a short hand, objects.sum{number}.

Flatten

You can flatten nested arrays. In this example, we have an array of arrays and bind it to a flat array.

var arrays = [[1, 2, 3], [4, 5, 6]];
var object = {};
bind(object, "flat", {
    "<-": "flatten()",
    source: arrays
});
expect(object.flat).toEqual([1, 2, 3, 4, 5, 6]);

Note that changes to the inner and outer arrays are both projected into the flattened array.

arrays.push([7, 8, 9]);
array[0].unshift(0);
expect(object.flat).toEqual([0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);

Also, as with all other bindings that produce arrays, the flattened array is never replaced, just incrementally updated.

var flat = object.flat;
arrays.splice(0, arrays.length);
expect(object.flat).toBe(flat); // === same object

Reversed

You can bind the reversal of an array.

var object = {forward: [1, 2, 3]};
bind(object, "backward", {
    "<->": "reversed()"
});
expect(object.backward).toEqual([3, 2, 1]);
object.forward.push(4);
expect(object.forward).toEqual([1, 2, 3, 4]);
expect(object.backward).toEqual([4, 3, 2, 1]);

Note that you can do two-way bindings, <-> with reversed arrays. Changes to either side are updated to the opposite side.

object.backward.pop();
expect(object.backward).toEqual([4, 3, 2]);
expect(object.forward).toEqual([2, 3, 4]);

Has

You can bind a property to always reflect whether a collection contains a particular value.

var object = {
    haystack: [1, 2, 3],
    needle: 3
};
bind(object, "hasNeedle", {"<-": "haystack.has(needle)"});
expect(object.hasNeedle).toBe(true);
object.haystack.pop(); // 3 comes off
expect(object.hasNeedle).toBe(false);

The binding also reacts to changes to the value you seek.

// continued from above
object.needle = 2;
expect(object.hasNeedle).toBe(true);

has bindings are not incremental, but with the right data-structure, updates are cheap. The Collections package contains Lists, Sets, and OrderedSets that all can send content change notifications and thus can be bound.

var Set = require("collections/set");
object.haystack = new Set([1, 2, 3]);
expect(object.hasNeedle).toBe(true);

has bindings can also be left-to-right and bi-directional.

bind(object, "hasNeedle", {"<->": "haystack.has(needle)"});
object.hasNeedle = false;
expect(object.haystack.has(2)).toBe(false);

The collection on the left-hand-side must implement has or contains, add, and delete or remove. FRB shims Array to have has, add, and delete, just like all the collections in Collections. It happens that the classList properties of DOM elements, when they are supported, implement add, remove, and contains.

var model = {darkMode: false};
bind(document.body, "classList.has('dark')", {
    "<-": "darkMode",
    source: model
});

The DOM classList does not however implement addContentChangeListener or removeContentChangeListener, so it cannot be used on the right-hand-side of a binding, and such bindings cannot be bidirectional. With some DOM Mutation Observers, you might be able to help FRB overcome this limitation in the future.

Equals

You can bind to whether expressions are equal.

var fruit = {apples: 1, oranges: 2};
bind(fruit, "equal", {"<-": "apples == oranges"});
expect(fruit.equal).toBe(false);
fruit.orange = 1;
expect(fruit.equal).toBe(true);

Equality can be bound both directions. In this example, we do a two-way binding between whether a radio button is checked and a corresponding value in our model.

bind(model, "fruit = 'orange'", {
    "<->": "checked",
    source: orangeElement
});
bind(model, "fruit = 'apple'", {
    "<->": "checked",
    source: appleElement
});

orangeElement.checked = true;
expect(model.fruit).toEqual("orange");

appleElement.checked = true;
expect(model.fruit).toEqual("apple");

Because equality and assignment are interchanged in this language, you can use either = or ==.

Operators

FRB can also recognize many operators. These are in order of precedence unary - negation, + numeric coercion, and ! logical negation and then binary ** power, // root, %% logarithm, *, /, % modulo, %% remainder, +, -, <, >, <=, >=, = or ==, !=, && and ||.

var object = {height: 10};
bind(object, "heightPx", {"<-": "height + 'px'"});

Algebra

FRB can automatically invert algebraic operators as long as they operate strictly on numbers and the left-most expressions on both the source and target are bindable properties.

In this examlple, the primary binding is notToBe <- !toBe, and the inverse binding is automatically computed toBe <- !notToBe.

var caesar = {toBe: false};
bind(caesar, "notToBe", {"<->": "!toBe"});
expect(caesar.toBe).toEqual(false);
expect(caesar.notToBe).toEqual(true);

caesar.notToBe = false;
expect(caesar.toBe).toEqual(true);

FRB does algebra by rotating the expressions on one side of a binding to the other until only one independent property remains (the left most expression) on the target side of the equation.

convert: y <- !x
revert: x <- !y

convert: y <- x + a
revert: x <- y - a

The left-most independent variable on the right hand side becomes the dependent variable on the inverted binding. At present, this only works for numbers and when the left-most expression is a bindable property because it cannot assign a new value to the literal 10. For example, FRB cannot yet implicitly revert y <-> #10 + x.

Literals

You may have noticed literals in the previous examples. String literals take the form of any characters between single quotes. Any character can be escaped with a back slash.

var object = {};
bind(object, "greeting", {"<-": "'Hello, World!'"});
expect(object.greeting).toBe("Hello, World!");

To distingish array indicies from number literals, the number literal must have a prefix #.

var array = [1, 2, 3];
var object = {};
bind(object, 'zero', {"<-": "#0", source: array});
bind(object, 'one', {"<-": "0", source: array});

Tuples

Bindings can produce fixed-length arrays. These are most useful in conjunction with mappings. Tuples are comma-delimited and parantheses-enclosed.

var object = {array: [[1, 2, 3], [4, 5]]};
bind(object, "summary", {"<-": "array.map{(length, sum())}"});
expect(object.summary).toEqual([
    [3, 6],
    [2, 9]
]);

Records

Bindings can also produce fixed-shape objects. The notation is comma-delimited, colon-separated items, enclosed by curly-braces.

var object = {array: [[1, 2, 3], [4, 5]]};
bind(object, "summary", {
    "<-": "array.map{{length: length, sum: sum()}}"
});
expect(object.summary).toEqual([
    {length: 3, sum: 6},
    {length: 2, sum: 9}
]);

The left hand side of an item in a record is any combination of letters or numbers. The right side is any expression.

Parameters

Bindings can also involve parameters. The source of parameters is by default the same as the source. The source, in turn, defaults to the same as the target object. It can be specified on the binding descriptor. Parameters are declared by any expression following a dollar sign.

var object = {a: 10, b: 20, c: 30};
bind(object, "foo", {
    "<-": "($a, $b, $c)"},
    parameters: object
});

Bindings also react to changes to the parameters.

object.a = 0;
object.b = 1;
object.c = 2;
expect(object.foo).toEqual([0, 1, 2]);

The degenerate case of the property language is an empty string. This is a valid property path that observes the value itself. So, as an emergent pattern, a $ expression by itself corresponds to the whole parameters object.

var object = {};
bind(object, "ten", {"<-": "$", parameters: 10});
expect(object.ten).toEqual(10);

Observers

FRB’s bindings use observers and binders internally. You can create an observer from a property path with the observe function exported by the frb/observe module.

var object = {foo: {bar: 10}};
var cancel = observe(object, "foo.bar", function (value) {
    // 10
    // 20
});
object.foo.bar = 10;
object.foo.bar = 20;

For more complex cases, you can specify a descriptor instead of the callback. For example, to observe a property’s value before it changes, you can use the beforeChange flag.

var object = {foo: {bar: 10}};
var cancel = observe(object, "foo.bar", {
    set: function (value) {
        // 10
        // 20
    },
    beforeChange: true
});
object.foo.bar = 20;
object.foo.bar = 30;

If the product of an observer is an array, that array is always updated incrementally. It will only get emitted once. If you want it to get emitted every time its content changes, you can use the contentChange flag.

var array = [[1, 2, 3], [4, 5, 6]];
observe(array, "map{sum()}", {
    set: function (sums) {
        // 1. [6, 15]
        // 2. [6, 15, 0]
        // 3. [10, 15, 0]
    },
    contentChange: true
});
array.push([0]);
array[0].push(4);

Nested Observers

To get the same effect as the previous example, you would have to nest your own content change observer.

var array = [[1, 2, 3], [4, 5, 6]];
var cancel = observe(array, "map{sum()}", function (array) {
    function contentChange() {
        // 1. expect(array).toEqual([6, 15]);
        // 2. expect(array).toEqual([6, 15, 0]);
        // 3. expect(array).toEqual([10, 15, 0]);
    }
    array.addContentChangeListener(contentChange);
    return function cancelContentChange() {
        array.removeContentChangeListener(contentChange);
    };
});
array.push([0]);
array[0].push(4);
cancel();

This illustrates one crucial aspect of the architecture. Observers return cancelation functions. You can also return a cancelation function inside a callback observer. That canceler will get called each time a new value is observed, or when the parent observer is canceled. This makes it possible to nest observers.

var object = {foo: {bar: 10}};
var cancel = observe(object, "foo", function (foo) {
    return observe(foo, "bar", function (bar) {
        expect(bar).toBe(10);
    });
});

Bindings

FRB provides utilities for declaraing and managing multiple bindings on objects. The frb (frb/bindings) module exports this interface.

var Bindings = require("frb");

The Bindings.create and Bindings.define methods have a similar interface. The create function creates a new object with properties and bindings. The define function augments an existing object. The properties object is just key value pairs to copy to the object, for convenience.

var object = Bindings.create(Object.prototype, {
    a: 10,
    b: 20
});
expect(object.a).toEqual(10);
expect(object.b).toEqual(20);

Bindings.define(object, {
    b: 30,
    c: 50
});
expect(object.b).toEqual(30);
expect(object.c).toEqual(50);

Binding Descriptors

The third argument of both create and define is an object containing property descriptors. These are the same property descriptors you see with EcmaScript 5’s Object.defineProperty. They additionally can contain bindings or dependent paths.

If a descriptor has a <- or <->, it is a binding descriptor. FRB creates a binding, adds the canceler to the descriptor, and adds the descriptor to an internal table that tracks all of the bindings defined on that object.

var object = Bindings.create(null, {
    darkMode: false,
    document: document
}, {
    "document.body.classList.has('dark')": {
        "<-": "darkMode"
    }
});

You can get all the binding descriptors with Bindings.getBindings, or a single binding descriptor with Bindings.getBinding. Bindings.cancel cancels all the bindings to an object and Bindings.cancelBinding will cancel just one.

var bindings = Bindings.getBindings(object);
var descriptor = Bindings.getBinding(object, "document.body.classList.has('dark')");
Bindings.cancelBinding(object, "document.body.classList.has('dark')");
Bindings.cancel(object);

Converters

A binding descriptor can have a convert function, a revert function, or alternately a converter object. Converters are useful for transformations that cannot be expressed in the property language, or are not reversible in the property language.

In this example, a and b are synchronized such that a is always half of b, regardless of which property gets updated.

Bindings.create(null, {
    a: 10
}, {
    b: {
        "<-": a
        convert: function (a) {
            return a * 2;
        },
        revert: function (b) {
            return a / 2;
        }
    }
});

Converter objects are useful for reusable or modular converter types and converters that track additional state.

Bindings.create(null, {
    a: 10
}, {
    b: {
        "<-": a
        converter: {
            factor: 2,
            convert: function (a) {
                return a * this.factor;
            },
            revert: function (b) {
                return a / this.factor;
            }
        }
    }
});

Computed Properties

A computed property is one that gets updated with a function call when one of its arguments changes. Like a converter, it is useful in cases where a transformation or computation cannot be expressed in the property language, but can additionally accept multiple arguments as input. A computed property can be used as the source for another binding.

In this example, we create an object as the root of multiple bindings. The object synchronizes the properties of a "form" object with the window’s search string, effectively navigating to a new page whenever the "q" or "charset" entries of the form change.

Bindings.create(null, {
    window: window,
    form: {
        q: "",
        charset: "utf-8"
    }
}, {
    queryString: {
        args: ["form.q", "form.charset"],
        compute: function (q, charset) {
            return "?" + QS.stringify({
                q: q,
                charset: charset
            });
        }
    },
    "window.location.search": {
        "<-": "queryString"
    }
});

Debugging with Traces

A binding can be configured to log when it changes and why. The trace property on a descriptor instructs the binder to log changes to the console.

Bindings.create(null, {
    a: 10
}, {
    b: {
        "<-": "a + #1",
    }
});

Reference

Functional Reactive Bindings is an implementation of synchronous, incremental object-property and collection-content bindings for JavaScript. It was ripped from the heart of the Montage web application framework and beaten into this new, slightly magical form. It must prove itself worthy before it can return.

functional: The implementation uses functional building blocks to compose observers and binders.
generic: The implementation uses generic methods on collections, like addContentChangeListener, so any object can implement the same interface and be used in a binding.
reactive: The values of properties and contents of collections react to changes in the objects and collections on which they depend.
synchronous: All bindings are made consistent in the statement that causes the change. The alternative is asynchronous, where changes are queued up and consistency is restored in a later event.
incremental: If you update an array, it produces a content change which contains the values you added, removed, and the location of the change. Most bindings can be updated using only these values. For example, a sum is updated by decreasing by the sum of the values removed, and increasing by the sum of the values added. FRB can incrementally update map, reversed, flatten, sum, and average observers. It can also incrementally update has bindings.
unwrapped: Rather than wrap objects and arrays with observable containers, FRB modifies existing arrays and objects to make them dispatch property and content changes. For objects, this involves installing getters and setters using the ES5 Object.defineProperty method. For arrays, this involves replacing all of the mutation methods, like push and pop, with variants that dispatch change notifications. The methods are either replaced by swapping the __proto__ or adding the methods to the instance with Object.defineProperties. These techniques should work starting in Internet Explorer 9, Firefox 4, Safari 5, Chrome 7, and Opera 12.

Architecture

property and content change events for objects and arrays using getters and setters for observable objects and either prototype swapping or method override for observable arrays. Other collection types can implement the same interface to be compatible with all subsequent layers. Caveats: you have to use a set method on Arrays to dispatch property and content change events. Does not work in older Internet Explorers since they support neither prototype assignment or ES5 property setters.
observer functions for watching an entire object graph for incremental changes, and gracefully rearranging and canceling those observers as the graph changes. Observers can be constructed directly or with a very small query language that compiles to a tree of functions so no parsing occurs while the graph is being watched.
one- and two-way bindings for incrementally updating properties with the results of observed queries.
declarative interface for creating an object graph with bindings, properties, and computed properties with dependencies.

Bindings

The highest level interface for FRB resembles the ES5 Object constructor and can be used to declare objects and define and cancel bindings on them with extended property descriptors.

var Bindings = require("frb");

// create an object
var object = Bindings.create(null, { // prototype
    // simple properties
    foo: 0,
    graph: [
        {numbers: [1,2,3]},
        {numbers: [4,5,6]}
    ]
}, {
    // extended property descriptors
    bar: {"<->": "foo", enumerable: false},
    numbers: {"<-": "graph.map{numbers}.flatten()"},
    sum: {"<-": "numbers.sum()"},
    reversed: {"<-": "numbers.reversed()"}
});

expect(object.bar).toEqual(object.foo);
object.bar = 10;
expect(object.bar).toEqual(object.foo);
expect.foo = 20;
expect(object.bar).toEqual(object.foo);

// note that the identity of the bound numbers array never
// changes, because all of the changes to that array are
// incrementally updated
var numbers = object.numbers;

// first computation
expect(object.sum).toEqual(21);

// adds an element to graph,
// which pushes [7, 8, 9] to "graph.map{numbers}",
// which splices [7, 8, 9] to the end of
//  "graph.map{numbers}.flatten()",
// which increments "sum()" by [7, 8, 9].sum()
object.graph.push({numbers: [7, 8, 9]});
expect(object.sum).toEqual(45);

// splices [1] to the beginning of [1, 2, 3],
// which splices [1] to the beginning of "...flatten()"
// which increments "sum()" by [1].sum()
object.graph[0].numbers.unshift(1);
expect(object.sum).toEqual(46);

// cancels the entire observer hierarchy, then attaches
//  listeners to the new one.  updates the sum.
object.graph = [{numbers: [1,2,3]}];
expect(object.sum).toEqual(6);

expect(object.reversed).toEqual([3, 2, 1]);

expect(object.numbers).toBe(numbers) // still the same object

Bindings.cancelBindings(object); // cancels all bindings on this object and
// their transitive observers and event listeners as deep as
// they go

Bindings.create(prototype, properties, descriptors)
Bindings.define(object, properties, descriptors)
Bindings.defineBinding(object, name, descriptor)
Bindings.getBindings(object)
Bindings.getBinding(object, name)
Bindings.cancelBindings(object)
Bindings.cancelBinding(object, name)

A binding descriptor contains:

target: the
targetPath: the target
targetSyntax: the syntax tree for the target path
source: the source object, which defaults to target
sourcePath: the source path, from either <- or <->
sourceSyntax: the syntax tree for the source path
twoWay: whether the binding goes in both directions, if <-> was the source path.
parameters: the parameters, which default to source.
cancel: a function to cancel the binding

Bind

The bind module provides direct access to the bind function.

var bind = require("frb/bind");

var source = [{numbers: [1,2,3]}, {numbers: [4,5,6]}];
var target = {};
var cancel = bind(target, "summary", {
    "<-": "map{(numbers.sum(), numbers.average())}",
    source: source
});

expect(target.summary).toEqual([
    [6, 2],
    [15, 5]
]);

cancel();

bind is built on top of parse, compileBinder, and compileObserver.

Compute

The compute module provides direct access to the compute function, used by Bindings to make computed properties.

var compute = require("frb/compute");

var source = {operands: [10, 20]};
var target = {};
var cancel = compute(target, "sum", {
    source: source,
    args: ["operands.0", "operands.1"],
    compute: function (a, b) {
        return a + b;
    }
});

expect(target.sum).toEqual(30);

// change one operand
source.operands.set(1, 30); // needed to dispatch change notification
expect(target.sum).toEqual(40);

Observers

The observe modules provides direct access to the observe function. observe is built on top of parse and compileObserver. compileObserver creates a tree of observers using the methods in the observers module.

var observe = require("frb/observe");

var source = [1, 2, 3];
var sum;
var cancel = observe(source, "sum()", function (newSum) {
    sum = newSum;
});

expect(sum).toBe(6);

source.push(4);
expect(sum).toBe(10);

source.unshift(0); // no change
expect(sum).toBe(10);

cancel();
source.splice(0, source.length); // would change
expect(sum).toBe(10);

observe produces a cancelation hierarchy. Each time a value is removed from an array, the underlying observers are canceled. Each time a property is replaced, the underlying observer is canceled. When new values are added or replaced, the observer produces a new canceler. The cancel function returned by observe commands the entire underlying tree.

Observers also optional accept a descriptor argument in place of a callback.

set: the change handler, receives value for most observers, but also key and object for property changes.
parameters: the value for $ expressions.
beforeChange: instructs an observer to emit the previous value before a change occurs.
contentChange: instructs an observer to emit an array every time its content changes. By default, arrays are only emitted once.

var object = {};
var cancel = observe(object, "array", {
    set: function (value) {
        // may return a cancel function for a nested observer
    },
    parameters: {},
    beforeChange: false,
    contentChange: true
});

object.array = []; // emits []
object.array.push(10); // emits [10]

The Language

Bindings and observers used a small query language intended to resemble the same code that you would write in JavaScript to update a binding by brute force.

Grammar

expression = logical or expression
logical or expression = logical and expression ( || relation expression )?
logical and expression = relation expression ( && relation expression )?
relation expression = arithmetic expression ( relation operator arithmetic expression )?
relation operator = = | == | < | <= | > | >=
arithmetic expression = multiplicative expresion delimited by arithmetic operator
arithmetic operator = + | -
multiplicative expression = exponential expression delimited by multiplicative operator
multiplicative operator = * | / | % | rem
exponential expression = unary expression delimited by exponential operator
exponential operator = ** | // | %%
unary expression = unary operator ? chain expression
unary operator = + | - | !
chain expression = primary expression delimited by . ( .* )?
primary expression = literal or ( expression ) or property name or function call or block call
property name = ( non space character )+
block name = function name or map
function name = flatten or reversed or sum or average or has
function call = function name ( tuple )
block call = function name { expression }
tuple = expression delimited by ,
literal = string literal or number literal
number literal = # ( non space character )+
string literal = ' ( non quote character or \ character )* '

Semantics

An expression is observed with a source value and emits a target one or more times. All expressions emit an initial value. Array targets are always updated incrementally. Numbers and boolean are emited anew each time their value changes.

The first term is evaluated with the source value.
Each subsequent term uses the target of the previous as its source.
Literals are interpreted as their corresponding value.
A property expression observes the named key of the source object.
A "map" block observes the source array and emits a target array. The target array is emitted once and all subsequent updates are reflected as content changes that can be independently observed with addContentChangeListener. Each element of the target array corresponds to the observed value of the block expression using the respective element in the source array as the source value.
Any function call with a "block" implies calling the function on the result of a "map" block.
A "flatten" function call observes a source array and produces a target array. The source array must only contain inner arrays. The target array is emitted once and all subsequent updates can be independently observed with addContentChangeListener. The target array will always contain the concatenation of all of the source arrays. Changes to the inner and outer source arrays are reflected with incremental splices to the target array in their corresponding positions.
A "reversed" function call observes the source array and produces a target array that contains the elements of the source array in reverse order. The target is incrementally updated.
A "sum" function call observes the numeric sum of the source array. Each alteration to the source array causes a new sum to be emitted, but the sum is computed incrementally by observing the smaller sums of the spliced values, added and removed.
An "average" function call observes the average of the input values, much like "sum".
A "has" function call observes the source collection for whether it contains an observed value.
A "tuple" expression observes a source value and emits a single target array with elements corresponding to the respective expression in the tuple. Each inner expression is evaluated with the same source value as the outer expression.
.* at the end of a chain has no effect on an observed value.

On the left hand side of a binding, the last term has alternate semantics. Binders receive a target as well as a source.

A "property" observes an object and a property name from the target, and a value from the source. When any of these change, the binder upates the value for the property name of the object.
A "equals" expression observes a boolean value from the source. If that boolean becomes true, the equality expression is made true by assigning the right expression to the left property of the equality, turning the "equals" into an "assign" conceptually. No action is taken if the boolean becomes false.
A "reversed" expression observes an indexed collection and maintains a mirror array of that collection.
A "has" function call observes a boolean value from the source, and an collection and a sought value from the target. When the value is true and the value is absent in the collection, the binder uses the add method of the collection (provided by a shim for arrays) to make it true that the collection contains the sought value. When the value is false and the value does appear in the collection one or more times, the binder uses the delete or remove method of the collection to remove all occurrences of the sought value.

If the target expression ends with .*, the content of the target is bound instead of the property. This is useful for binding the content of a non-array collection to the content of another indexed collection. The collection can be any collection that implements the "observable content" interface including dispatchContentChange(plus, minus, index), addContentChangeListener, and removeContentChangeListener.

Interface

var parse = require("frb/parse");
var compileObserver = require("frb/compile-observer");
var compileBinder = require("frb/compile-binder");

parse(text) returns a syntax tree.
compileObserver(syntax) returns an observer function of the form observe(callback, source, parameters) which in turn returns a cancel() function. compileObserver visits the syntax tree and creates functions for each node, using the observers module.
compileBinder(syntax) returns a binder function of the form bind(observeValue, source, target, parameters) which in turn returns a cancel() function. compileBinder visits the root node of the syntax tree and delegates to compileObserver for its terms. The root node must be a property at this time, but could conceivably be any function with a clear inverse operation like map and reversed.

Syntax Tree

The syntax tree is JSON serializable and has a "type" property. Nodes have the following types:

value corresponds to observing the source value
parameters corresponds to observing the parameters object
literal has a value property and observes that value

All other node types have an "args" property that is an array of syntax nodes.

property: corresponds to observing a property named by the right argument of the left argument.
map: the left is the input, the right is an expression to observe on each element of the input.
tuple: has any number of arguments, each an expression to observe in terms of the source value.

For all operators, the "args" property are operands. The node types for unary operators are:

+: number, arithmetic coercion
-: neg, arithmetic negation
!: not, logical negation

For all binary operators, the node types are:

**: pow, exponential power
//: root, binary
%%: log, logarithm with base
*: mul, multiplication
/: div, division
%: mod, modulo (toward negative infinity, always positive)
rem: rem, remainder (toward zero, negative if negative)
+: add, addition
-: sub, subtraction
<: lt, less than
```<=````: le, less than or equal
>: gt, greater than
>=: ge, greater than or equal
= and ==: equals, equality comparison and assignment
!= produces unary negation and equality comparison or assignment so does not have a corresponding node type. The simplification makes it easier to rotate the syntax tree algebraically.
&&, and, logical and
||, or, logical or

For all function calls, the right hand side is a tuple of arguments, presently ignored.

reversed
flatten
sum
average

Observers and Binders

The observers module contains functions for making all of the different types of observers, and utilities for creating new ones. All of these functions are or return an observer function of the form observe(emit, value, parameters) which in turn returns cancel().

observeValue
observeParameters
makeLiteralObserver(value)
makeRelationObserver(callback, thisp) is unavailable through the property binding language, translates a value through a JavaScript function.
makeComputerObserver(observeArgs, compute, thisp) applies arguments to the computation function to get a new value.
makeConverterObserver(observeValue, convert, thisp) calls the converter function to transform a value to a converted value.
makePropertyObserver(observeObject, observeKey)
makeMapObserver(observeArray, observeRelation)
makeTupleObserver(...observers)
makeObserversObserver(observers)
makeReversedObserver(observeArrayh)
makeWindowObserver is not presently available through the language and is subject to change. It is for watching a length from an array starting at an observable index.
makeFlattenObserver(observeArray)
makeSumObserver(observeArray)
makeAverageObserver(observeArray)

These are utilities for making observer functions.

makeNonReplacing(observe) accepts an array observer (the emitted values must be arrays) and returns an array observer that will only emit the target once and then incrementally update that target. All array observers use this decorator to handle the case where the source value gets replaced.
makeArrayObserverMaker(setup) generates an observer that uses an array as its source and then incrementally updates a target value, like sum and average. The setup(source, emit) function must return an object of the form {contentChange, cancel} and arrange for emit to be called with new values when contentChange(plus, minus, index) receives incremental updates.
makeUniq(callback) wraps an emitter callback such that it only forwards new values. So, if a value is repeated, subsequent calls are ignored.
autoCancelPrevious(callback) accepts an observer callback and returns an observer callback. Observer callbacks may return cancelation functions, so this decorator arranges for the previous canceler to be called before producing a new one, and arranges for the last canceler to be called when the whole tree is done.
once(callback) accepts a canceler function and ensures that the cancelation routine is only called once.

The binders module contains similar functions for binding an observed value to a bound value. All binders are of the form bind(observeValue, source, target, parameters) and return a cancel() function.

makePropertyBinder(observeObject, observeKey)
makeHasBinder(observeCollection, observeValue)

Change Events

Object Property Changes

To use object observers, require("frb/properties"). Observers depend on EcmaScript 5's Object.defineProperty and Object.defineProperties or a suitable shim. Observable collections benefit from the ability to swap __proto__ in all engines except Internet Explorer, in which case they fall back to using Object.defineProperties to trap change functions.

Listen for individual property changes on an object. The listener may be a function or a delegate.

addOwnPropertyChangeListener(object, key, listener, beforeChange)
removeOwnPropertyChangeListener(object, key, listener, beforeChange)
addBeforeOwnPropertyChangeListener(object, key, listener)
removeBeforeOwnPropertyChangeListener(object, key, listener)

The arguments to the listener are (value, key, object), much like a forEach callback. The this within the listener is the listener object itself. The dispatch method must be one of these names, favoring the most specific provided.

handle + key (TwoHump) + (Change or WillChange before change), for example, handleFooWillChange for foo.
handleOwnPropertyChange or handleOwnPropertyWillChange before change
handleEvent
function

Custom Property Change Observers

The property change listener functions delegate to a makePropertyObservable(key) and makePropertyUnobservable(key) if they exist. So, these can be used to augment host objects, like parts of the DOM, to accommodate property change listeners. The dom module monkey-patches HTML element prototoypes to make some properties observable, like the "checked" property of radio and checkbox input elements using the addEventListener("change") interface.

Array Content Changes

To use array content observers, require("frb/array"). This will install the necessary methods on the Array prototype. The performance of arrays in general will not be affected—only observed arrays will require more time to execute changes.

Listen for ranged content changes on arrays. The location of the change is where the given arrays of content are removed and added. For unordered collections like sets, the location would not be defined. Content changes are not yet implemented for other collections.

array.addContentChangeListener(listener, beforeChange)
array.removeContentChangeListener(listener, beforeChange)
array.addBeforeContentChangeListener(listener)
array.removeBeforeContentChangeListener(listener)

The arguments to the listener are (plus, minus, at), which are arrays of the added and removed values, and optionally the location of the change for ordered collections (lists, arrays). For a list, the position is denoted by a node. The dispatch method must be one of these names, favoring the most specific provided.

handleContentChange or handleContentWillChange if before change
handleEvent
function

Listen for content changes from each position within an array, including changes to and from undefined. Content changes must be emitted by method calls on an array, so use array.set(index, value) instead of array[index] = value.

array.addEachContentChangeListener(listener, beforeChange)
array.removeEachContentChangeListener(listener, beforeChange)
array.addBeforeEachContentChangeListener(listener)
array.removeBeforeEachContentChangeListener(listener)

The listener is a listener as for property changes.

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