Derived Values, Flowing

This, milord, is my family's axe. We have owned it for almost nine hundred years, see. Of course, sometimes it needed a new blade. And sometimes it has required a new handle, new designs on the metalwork, a little refreshing of the ornamentation ... but is this not the nine hundred-year-old axe of my family? And because it has changed gently over time, it is still a pretty good axe, y'know. Pretty good.

-- Terry Pratchett, The Fifth Elephant
    reflecting on identity, flow and derived values

Why Should You Care?

Perhaps:

1. You want to develop an SPA in ClojureScript, and you are looking for a framework.
2. You believe Facebook did something magnificent when it created React, and you are curious about the further implications. Is the combination of reactive programming, functional programming and immutable data going to completely change everything? And, if so, what would that look like in a language that embraces those paradigms?
3. You're taking a Functional Design and Programming course at San Diego State University and you have a re-frame/reagent assignment due. You've left the reading a bit late, right?
4. You know Redux, Elm, Cycle.js or Pux and you're interested in a ClojureScript implementation, with a data oriented design. In this space, re-frame is very old, hopefully in a Gandalf kind of way. First designed in Dec 2014, it even slightly pre-dates the official Elm Architecture, although thankfully we were influenced by early-Elm concepts like foldp and lift, as well as terrific Clojure projects like Pedestal App, Om and Hoplon. Since then, re-frame has pioneered ideas like event handler middleware, coeffect accretion, and de-duplicated signal graphs.
5. Which is a lovely segue into the most important point: re-frame is impressively buzzword compliant. It has reactivity, unidirectional data flow, pristinely pure functions, interceptors, coeffects, conveyor belts, statechart-friendliness (FSM) and claims an immaculate hammock conception. It also has a charming xkcd reference (soon) and a hilarious, insiders-joke T-shirt, ideal for conferences (in design). What could possibly go wrong?

re-frame

re-frame is a pattern for writing SPAs in ClojureScript, using Reagent.

McCoy might report "It's MVC, Jim, but not as we know it". And you would respond "McCoy, you trouble maker, why even mention an OO pattern? re-frame is a functional framework."

Being a functional framework, it is about data, and the (pure) functions which transform that data.

It is a loop

Architecturally, re-frame implements "a perpetual loop".

To build an app, you hang pure functions on certain parts of this loop, and re-frame looks after the conveyance of data around the loop, into and out of the transforming functions you provide - hence a tag line of "Derived Values, Flowing".

It does Physics

Remember this diagram from school? The water cycle, right?

Two distinct stages, involving water in different phases, being acted upon by different forces: gravity working one way, evaporation/convection the other.

To understand re-frame, imagine data flowing around that loop instead of water.

re-frame provides the conveyance of the data around the loop - the equivalent of gravity, evaporation and convection. You design what's flowing and then you hang functions off the loop at various points to compute the data's phase changes.

Sure, right now, you're thinking "lazy sod - make a proper Computer Science-y diagram". But, no. Joe Armstrong says "don't break the laws of physics" - I'm sure you've seen the videos - and if he says to do something, you do it (unless Rich Hickey disagrees, and says to do something else). So, this diagram, apart from being a plausible analogy which might help you to understand re-frame, is practically proof it does physics.

It is a 6-domino cascade

Computationally, each iteration of the loop involves a six domino cascade.

One domino triggers the next, which triggers the next, et cetera, until we are back at the beginning of the loop, whereupon the dominoes spring to attention again, ready for the next iteration of the same cascade.

The six dominoes are ...

1st Domino - Event Dispatch

An event is sent when something happens - the user clicks a button, or a websocket receives a new message.

Without the impulse of a triggering event, no six domino cascade occurs. It is only because of events that a re-frame app is propelled, loop iteration after loop iteration, from one state to the next.

re-frame is event driven.

2nd Domino - Event Handling

In response to an event, an application must decide what action to take. This is known as event handling.

Event handler functions compute side effects (known in re-frame simply as effects). More accurately, they compute a description of effects. This description is a data structure which says, declaratively, how the world should change (because of the event).

Much of the time, only the "application state" of the SPA itself need change, but sometimes the outside world must also be effected (localstore, cookies, databases, emails, logs, etc).

3rd Domino - Effect Handling

The descriptions of effects are realised (actioned). This is where mutations happens.

Now, to a functional programmer, effects are scary in a xenomorph kind of way. Nothing messes with functional purity quite like the need for side effects. On the other hand, effects are marvelous because they move the app forward. Without them, an app stays stuck in one state forever, never achieving anything.

So re-frame embraces the protagonist nature of effects - the entire, unruly zoo of them - but it does so in a largely hidden way, and in a manner which is debuggable, auditable, mockable and pluggable.

A Pivot Point

The world just changed and, very often, one particular part of it: the application state.

re-frame's app state is held in one place - think of it like you would an in-memory, central database for the app (more details later).

When domino 3 changes this app state, it triggers the next part of the cascade involving dominoes 4-5-6.

There's a formula

The 4-5-6 domino cascade implements the formula made famous by Facebook's ground-breaking React library:
v = f(s)

A view, v, is a function, f, of the app state, s.

Said another way, there are functions f that compute which DOM nodes, v, should be displayed to the user when the application is in a given app state, s.

Or, differently: over time, as s changes, f will be re-run each time to compute new v, forever keeping v up to date with the current s.

In our case, domino 3 changes s, the application state, and, in response, dominoes 4-5-6 are concerned with re-running f to compute the new v shown to the user.

Except, of course, there are nuances. For instance, there's no single f to run. There may be many functions which collectively build the overall DOM, and only part of s may change at any one time, so only part of the v (DOM) need be re-computed and updated. And some parts of v might not be showing right now.

Domino 4 - Query

Domino 4 is about extracting data from "app state", and providing it in the right format for view functions (which are Domino 5).

Domino 4 is a novel and efficient de-duplicated signal graph which runs query functions on the app state, s, efficiently computing reactive, multi-layered, "materialised views" of s.

(Relax about any unfamiliar terminology, you'll soon see how simple the code actually is)

Domino 5 - View

Domino 5 is one or more view functions (aka Reagent components) that compute the UI DOM that should be displayed to the user.

To render the right UI, they need to source application state, which is delivered reactively via the queries of Domino 4. They compute hiccup-formatted data, which is a description of the DOM required.

Domino 6 - DOM

You don't write Domino 6 - it is handled for you by Reagent/React. I mention it here for completeness and to fully close the loop.

This is the step in which the hiccup-formatted "descriptions of required DOM", returned by the view functions of Domino 5, are made real. The browser DOM nodes are mutated.

A Cascade Of Simple Functions

Each of the dominoes you write are simple, pure functions which can be be described, understood and tested independently. They take data, transform it and return new data.

The loop itself is very mechanical in operation. So, there's a regularity, simplicity and certainty to how a re-frame app goes about its business, which leads, in turn, to an ease in reasoning and debugging. This is key to why re-frame is pleasing to work with - it is just so straightforward.

Managing mutation

The two sub-cascades 1-2-3 and 4-5-6 have a similar structure.

In each, it is the second to last domino which computes "descriptions" of mutations required, and it is the last domino which does the dirty work and realises these descriptions.

In both cases, you don't need to worry yourself about this dirty work. re-frame looks after those dominoes.

As Code Fragments

Let's take this domino narrative further and introduce code fragments.

Imagine: We're working on a SPA which displays a list of items. You have just clicked the "delete" button next to the 3rd item in the list.

In response, what happens within this imaginary re-frame app? Here's a sketch of the six domino cascade:

Don't expect to completely grok the terse code presented below. We're still at 30,000 feet. Details later.

Code For Domino 1

The delete button for that 3rd item will have an on-click handler (function) which looks like this:

 #(re-frame.core/dispatch [:delete-item 2486])

dispatch emits an event.

A re-frame event is a vector and, in this case, it has 2 elements: [:delete-item 2486]. The first element, :delete-item, is the kind of event. The rest is optional, further data about the event - in this case, my made-up id, 2486, for the item to delete.

Code For Domino 2

The event handler, h, associated with :delete-item, is called to compute the effect of this event.

This handler function, h, takes two arguments: a coeffects map which holds the current state of the world (including app state), and the event. It must return a map of effects - a description of how the world should change. Here's a sketch (we are at 30,000 feet):

(defn h 
 [{:keys [db]} event]                    ;; args:  db from coeffect, event
 (let [item-id (second event)]           ;; extract id from event vector
   {:db  (dissoc-in db [:items item-id])})) ;; effect is change db

There are mechanisms in re-frame (beyond us here) whereby you can place all necessary aspects of the world into that first coeffects argument, on a per event-kind basis (different events need to know different things in order to get their job done). The current application state, db, is one aspect of the world which is invariably needed.

On program startup, h would have been associated with :delete-item events like this:

(re-frame.core/reg-event-fx  :delete-item  h)

Which says "when you see a :delete-item event, use h to handle it".

Code For Domino 3

An effect handler (function) actions the effects returned by h:

{:db  (dissoc-in db [:items item-id])}

So that's a map. The keys identify the required kinds of effect, and the values supply further details. This map only has one key, so there's only one effect.

A key of :db means to update the app state with the key's value.

This update of "app state" is a mutative step, facilitated by re-frame which has a built in effects handler for the :db effect.

Why the name :db? re-frame sees "app state" as something of an in-memory database.

Code For Domino 4

Because a new version of "app state" has been computed and installed, a query (function) over this app state is called automatically (reactively), itself computing the list of items.

Because the items are stored in app state, there's not a lot to compute in this case. This subscription acts more like an accessor.

(defn query-fn
  [db _]         ;; db is current app state
  (:items db))   ;; not much of a materialised view

On program startup, such a query-fn must be associated with an id, (for reasons obvious in the next domino) like this:

(re-frame.core/reg-sub  :query-items  query-fn)

Which says "when you see a query (subscribe) for :query-items, use query-fn to handle it".

Code For Domino 5

Because the query function re-computed a new value, a view (function) which subscribes to "items", is called automatically (reactively) to re-compute DOM.

It produces a hiccup-formatted data structure describing the DOM nodes required (no DOM nodes for the deleted item, obviously, but otherwise the same DOM as last time).

(defn items-view
  []
  (let [items  (subscribe [:query-items])]  ;; source items from app state
    [:div (map item-render @items]))   ;; assume item-render already written

Notice how items is "sourced" from "app state" via subscribe. It is called with a query id to identify what data it needs.

Code For Domino 6

The computed DOM (hiccup) is made real by Reagent/React. No code from you required. Just happens.

The DOM "this time" is the same as last time, except for the absence of DOM for the deleted item, so the mutation will be to remove some DOM nodes.

3-4-5-6 Summary

The key point to understand about our 3-4-5-6 example is:

• a change to app state ...
• triggers query functions to rerun ...
• which triggers view functions to rerun
• which causes new DOM

Boom, boom, boom go the dominoes. It is a reactive data flow.

Aaaaand we're done

At this point, the re-frame app returns to a quiescent state, waiting for the next event.

So, your job is ...

When building a re-frame app, you will:

• design your app's information model (data and schema layer)
• write and register event handler functions (control and transition layer) (domino 2)
• (once in a blue moon) write and register effect and coeffect handler functions (domino 3) which do the mutative dirty work of which we dare not speak.
• write and register query functions which implement nodes in a signal graph (query layer) (domino 4)
• write Reagent view functions (view layer) (domino 5)

It Leverages Data

You might already know that ClojureScript is a modern lisp, and that lisps are homoiconic. If not, you do now.

The homoiconic bit is significant. It means you program in a lisp by creating and assembling lisp data structures. Think about that. You are programming in data. The functions which later manipulate data, start as data.

Clojure programmers place particular emphasis on the primacy of data. When they aren't re-watching Rich Hickey videos, and wishing their hair was darker and more curly, they meditate on aphorisms like "Data is the ultimate in late binding".

I cannot stress enough what a big deal this is. It can seem like a syntax curiosity at first but, when the penny drops for you on this, it tends to be a profound moment. And once you understand the importance of this concept at the language level, you naturally want to leverage similar power at the library level.

So, it will come as no surprise, then, to know that re-frame has a data oriented design. Events are data. Effects are data. DOM is data. The functions which transform data are registered and looked up via data. Interceptors (data) are preferred over middleware (higher order functions). Etc.

Data - that's the way we roll.

It is mature and proven in the large

re-frame was released in early 2015, and has since been successfully used by quite a few companies and individuals to build complex apps, many running beyond 40K lines of ClojureScript.

Scale changes everything. Frameworks are just pesky overhead at small scale - measure them instead by how they help you tame the complexity of bigger apps, and in this regard re-frame has worked out well. Some have been effusive in their praise.

Having said that, re-frame remains a work in progress and it falls short in a couple of ways - for example it doesn't work as well as we'd like with devcards, because it is a framework, rather than a library. We're still puzzling over some aspects and tweaking as we go. All designs represent a point in the possible design space, with pros and cons.

And, yes, re-frame is fast, straight out of the box. And, yes, it has a good testing story (unit and behavioural). And, yes, it works in with figwheel to create a powerful hot-loading development story. And, yes, it has fun specialist tooling, and a community, and useful 3rd party libraries.

Where Do I Go Next?

At this point you already know 50% of re-frame. There's detail to fill in, for sure, but the core concepts, and even basic coding techniques, are now known to you.

Next you need to read read the other three articles in the Introduction section:

• Application State
• Code Walkthrough
• Mental Model Omnibus

This will get your knowledge to about 70%. The final 30% will come incrementally with use, and by reading the other tutorials (of which there's a few).

You can also experiment with these examples:
https://github.com/Day8/re-frame/tree/master/examples

Use a template to create your own project:
Client only: https://github.com/Day8/re-frame-template
Full Stack: http://www.luminusweb.net/

Use these resources:
https://github.com/Day8/re-frame/blob/develop/docs/External-Resources.md

T-Shirt Unlocked

Good news. If you've read this far, your insiders T-shirt will be arriving soon - it will feature turtles, xkcd and something about "data all the way down". But we're still working on the hilarious caption bit. Open a repo issue with a suggestion.