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the two-faced application library-framework


Janus is a library-framework designed to simplify web application flow through the application of FRP and reactive programming principles. It was conceived in order to facilitate applications that could be freely rendered server- and client-side from a single codebase -- the dedication to purely functional userland code and idempotent rendering/templating operations arose as a natural outcome of this goal. This is not a complete application framework -- it contains many of the relevant building blocks, but needs to be supplemented with, amongst other things, a DOM manipulation library like jQuery, and a web application server like Express or Flatiron.

Janus is different from other FRP frameworks in two predominant ways: it is meant to look familiar and friendly to application programmers with a background writing traditional Javascript web applications, and it eschews any desire to model streams of events and signals over time, instead concentrating on providing easy, stateless mappings from the current state of the system to the UI. It does so through pragmatic purity -- in cases where imperative code can be made perfectly clear and side effects are inconsequential, Janus does not attempt to obfuscate simple operations with cognitively complex purely functional abstractions.

Of note should be the Janus Standard Library, which contains useful default implementations of core Janus components, and the Janus Samples repository, which contains a growing library of illustrative Janus projects.

Janus is current undergoing significant refitting. Now that we're up to 0.2, the library is beginning to stabilize and many parts should be ready for general use. Authors are cautioned to avoid more-advanced Model features such as shadowing and change-tracking, and these are due for overhaul in 0.3, as well as the Collection folds, as those will need considerable thinking in 0.4 to account for performance issues.

Janus is comprised of some core abstractions that are independently useful, but then leveraged to form increasingly opinionated but powerful layers for constructing web applications:

  • The core library contains basic building blocks:
    • Varying is the key abstraction and philosophical heart of Janus. Each Varying instance tracks a single current value, and manages the propagation of changes to that value to various downstream transformers and listeners. For both practical and mathematical reasons, it is Monad-compliant -- get, map, flatten, and flatMap are all provided, and the final result may be directly observed via react and reactNow.
    • The case system is an implementation of a typical functional case classing and matching system. Its purpose is to provide a formal construct through with acceptable value classes, their inner values, and matchers thereof may be communicated and implemented. In many cases, Janus framework components implement a default set of case classes that may be overridden or augmented with custom sets, allowing for great flexibility when the framework does not behave as needed out of the box.
    • from and its builder system sit on top of both Varying and case to provide a point-free way to declaratively define various necessary values and their combination into a final result, without specifically referencing object instances. This is used, for instance, in the templating engine to allow Model properties to be bound onto DOM objects declaratively and statelessly.
  • The templating system provides structure and management implementation for databinding Varying values onto a DOM tree:
    • The mutators are concrete operations that map Varying-wrapped values onto DOM state in various ways: class names, textual contents, style properties, and wholesale rendering of subviews are accomplished through mutators. Each mutator declaration represents precisely one binding.
    • Templates group mutators together into view components. Traditionally, each template has a one-to-one relationship with the DOM fragment it manages. Effort is made to make templates easily composable, such that oft-reused sets of bindings may be easily recomposed, without complicated inheritance trees to manage.
  • The view system has at its core the DomView, which wraps and manages the lifecycle of templates and their associated DOM fragments, as well as their binding to a Model object. The more-generic View sheds any DOM-based assumptions, allowing for alternative view artifact types.
  • Models are bundles of Varyings into useful object abstractions resembling traditional model objects. The primary difference is that while an object's properties may be trivially set imperatively with #set(key, value), use of #get(key) is highly discouraged -- instead, #watch(key) is the standard practice, which returns a Varying. Models also contain many useful mechanisms for declaring behaviour on particular properties, such as serialization strategies or validation conditions. Model is due for a major overhaul in version 0.3.
  • Collections are similar to models in that they resemble a fairly standard collections library, but with a dedication to facilities that enable the use of Varying and functional approaches rather than imperative operations that are time-sensitive. For instance, given collection a, we can derive collection b = -> x + 1) as expected, but updates to collection a will be result in recalculation and update of collection b. Collection is due for a major overhaul in version 0.4.
  • The remaining components in Janus fill in various gaps that manage application lifecycle at a broader level than the above systems, and tie the resulting application to its host framework.
    • To be written after reconsideration of 0.5.

Philosophically, Janus hews closer to an MVVM approach than an MVC one -- any behaviour that doesn't comfortably fit into model or template declaration is likely accomplishable by inserting an intermediate ViewModel between the data Model and its template. Most controller-like behaviour are in practice very short, understandable snippets of imperative programming within Views.

There remain three major blocs of work to be accomplished before a 1.x release can be considered:

  • 0.3 will be the great unbundling of Model:
    • The meanest, smallest kernel of the model system will become the Struct, which is purely a collection of Varying objects addressed by property keys.
    • All follow-on behaviour that Model currently supports become either increasingly powerful subclasses, or extension behaviour that may be plugged in to Model: shadow copying, attribute behaviour, property binding, validation, serialization, and change tracking.
    • Enumerability (hashes whose k/v pairs may be mapped) likely goes here, possibly to support serialization and change tracking, which will probably be unbound from inside Model.
    • Serialization/change tracking mechanisms for collections will also be considered in this rev; no tests will be written for either in the meantime. Change tracking is likely to be unbound from shadowing.
    • The Request and Store abstractions, which were updated in 0.2, will be audited for further finalization.
    • 0.3 will require minor code changes -- all the final features are superset and all concepts remain identical, but things will be cleaned up internally and possibly minorly moved about. As noted above, authors are cautioned to avoid shadow and change tracking features in the meantime.
  • 0.4 will be a refactoring of Collection:
    • For the most part, the external collection API is entirely satisfactory, in that it resembles a standard collection API. But it merits a revisit.
    • Everything is eagerly-evaluated, which simplifies a lot of operations, but probably shouldn't be the only option.
    • The various fold-related operations are nearly unusable at the moment.
    • Alternative approaches to our current system, possibly including a greater focus on lazy evaluation and/or transducers, will be evaluated.
    • 0.4 should be almost entirely backward compatible.
  • 0.5 serves as a release candidate for all of the above changes, as well as an umbrella milestone for improvements, changes, or removals to the application package.
  • 1.0 will follow, stabilizing the API for the first time.


Completely overhauled and rewrote the Varying abstraction, as well as much of the templating, view, model, and collections systems that were too tightly-bound to Varying to escape rewrite. Introduced case and from as vital core abstractions. Also dramatically increased test coverage, streamlined and removed a bunch of fluff components, and other miscellenia.

  • Varying became a true monad: it no longer automatically flattens its contents. It also no longer uses an event-based system for change propagation, as this resulted in intractable race condition problems as well as performance issues. Many improvements and changes aren't listed here.
  • The case system is new, and an attempt to formalize and abstract the internal behaviour-handling models of Janus such that they can be easily augmented or replaced in userland where needed. It is a response to the problems with instanceof-based casing.
  • The from system is a reconsideration of how databinding can be declared and executed. Its point-free programming model enables it to be freely leveraged to solve any number of problems unrelated to databinding.
  • Mutator and Template are ground-up reconsiderations of how to bundle template-like behaviour.
  • View and Model are impacted insofar as their interfaces to the above are concerned.
  • Alongside this release is the new janus-stdlib, which contains a slew of very useful default View implementations for the objects in the library.

0.2 is strictly not backwards compatible, as it represents a major formalization effort that can impact the behaviour of code that was loose but would function in 0.1. Please see the note in the introduction about the state of the library given 0.2.

Initial release. All the basics are here, but given that the philosophy codified alongside the development, the exposed API is less than precise and often in conflict with the underlying machinations.

Janus is licensed under the WTFPL.