re-framejs

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 javascript, 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?

[OM]:https://github.com/swannodette/om
[Hoplon]:http://hoplon.io/
[Pedestal App]:https://github.com/pedestal/pedestal-app

re-frame

re-frame is a pattern for writing [SPAs] in javascript, using [React]. It's javascript
port of clojuresript's re-frame

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.

$3

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".

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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.

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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 ...

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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.

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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 affected
(localstore, cookies, databases, emails, logs, etc).

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The descriptions of effects are realised (actioned).

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.

$3

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 (details later).

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

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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.

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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)

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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.

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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 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.

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The delete button for that 3rd item will have an on-click handler (function) which looks
like this:
``javascript
() => reframe.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.

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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):
`javascript
function h(ctx, [_, itemId]){ // extract id from event vector
const db = ctx.get('db'); // db from coeffect
return {db: db.deleteIn(['items', itemId])};
}
`

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:
`javascript
reframe.regEventFx('delete-item', h);
`
Which says "when you see a :delete-item event, use h to handle it".

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An effect handler (function) puts into action the effects returned by h:
`javascript
{db: db.deleteIn(['items', itemId])}
`
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.

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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.
`javascript
function queryFn(db, _) { // db is current app state
return db.get('items'); // 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:
`javascript
reframe.regSub('query-items', queryFn);
`
Which says "when you see a query (subscribe) for query-items, use query-fn to handle it".

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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).

`javascript
const ItemsView = reframe.view('ItemsView', function() {
const items = this.derefSub(['query-items']); // source items from app state
return

; // assume itemRender already written
});
`

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

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The computed DOM is made real by 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

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.

$3

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)

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.

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 these resources:

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

[SPAs]:http://en.wikipedia.org/wiki/Single-page_application
[SPA]:http://en.wikipedia.org/wiki/Single-page_application
[Reagent]:http://reagent-project.github.io/