bits by luke

Why Clojurescript?

I would like to share with you an abbreviated version of my journey that led to Clojure and Clojurescript. Hopefully, by the end, I will have convinced you to fire up a REPL and at least play around for a few hours.


I didn’t get it.

Around 2012 this thing called Clojurescript popped onto my radar via js weekly. What? Someone is trying to make a lisp-into-javascript compiler? That’s almost laughable; what a convoluted way to handle client-side code. Functional programming is wonderful in a lot of contexts and domains, but a client’s browser is not one of them.

It won’t work

The whole idea of lisp-in-a-browser reminded me of a conference talk I once attended. During the presentation some really bright folks gave a compelling demonstration of the power and wonders of functional reactive programming. They established a series of pipes, filters, nodes, trees, sinks, and drains that worked wonders on input streams and processed flows beautifully. In the final minutes of the presentation, they switched over to demo mode and tried to use this method of programming to handle events in a browser. It did not go well.

Between the three presenters, each a FRP expert, it took them over 20 minutes to get a <button> and <input> on the page that reliably accepted user input. Ouch. The layers of wrappers and adapters and signal converters were too deep. The presenters stumbled their way through exceptions and undefined or null errors. There were too many abstractions that had to be shoe-horned onto the existing environment for anything to work in the slightest, and that cognitive overhead proved too much even for the experts.

No thanks. I’ll stick to my flavor of the month javascript framework. They are much “simpler”, have a ton of useful features, and are familiar to my way of thinking. Heck, if I wanted a transpiled language, why not pick up something like Coffeescript? It saves a few keystrokes, smoothes over some rough javascript edges, and introduces familiar concepts like classes and inheritance. Importantly, it interfaces very easily with [flavor of the month framework], since it’s ultimately just javascript in ruby’s clothing.

Coincidentally, I actually did end up working with Coffeescript for a while at my regular day job. It performed exactly as expected and, looking back now, produced code that was more or less the same quality as if I wasn’t using a transpiler at all.

Time is a funny thing

Fast forward a few years in my life as a developer. I met an invaluable mentor who showed me true craftsmanship, and what it meant to be a professional in this domain. I started thinking about process and not just product. The ratios of input to output, effort to quality, and bugs to features weighed more heavily on my decisions. Architectural concerns, business value propositions, and reliable, consistent growth of systems (as well as businesses) moved into the forefront of my mind. I became engrossed in learning as much as possible about software engineering in the abstract: what worked historically, what works now, and what’s going to work in the future.

Among these experiments, blogs, tutorials, lessons, conferences, videos, and discussions, a few gems shined very brightly from beneath the piles of dirt. Perhaps the most personally influential gems were the now-famous Simple Made Easy, as well as The Value of Values. While they were presented in the abstract, applicable to any language or system, it was a logical next step for me to check out what this Rich Hickey guy was working with. Surely these fantastical ideas are purely academic and cannot be applied reliably in “the real world” of computer systems, right? I couldn’t have been more wrong.

What follows is a summary of some of the super powers afforded by Clojurescript that I have come to appreciate over the last three years of regular use.

Player 2 has entered the game

Clojurescript is truly functional programming. Nestled in with all the other benefits of the functional paradigm is context-free programming. This makes it easy for engineers to work on small parts of the system in complete isolation.

What does this mean in practice? You don’t have to fit the whole system into your head before making changes. What you see is what you get– a collection of functions that act only on inputs and return only outputs. There is no global mutable state, no side effecting, no “I hope this doesn’t break anything else” mentality. Ramp-up time is negligible for unfamiliar engineers because they can digest bite-sized pieces, and the boundaries between pieces are always well defined.

Clojurescript is expressive by design. It’s been said that, in a lisp, you don’t write software; you write the language in which you are going to write your software. The same holds true for Clojurescript.

Consider a pretty trivial example meant to demonstrate the power of expressivity: a user fills out a form. That user may or may not have unsaved changes on the form. We’d like to write a function that asks a user to confirm before exiting, but only if they have unsaved changes on the form.

Compare these two examples in clojurescript and javascript. How long it take you to understand the intent of the functions and the implementer’s approach and design? How much syntax did you have to parse in your working memory to figure everything out? How much base level of knowledge of the language did you need to grok before things made sense? How many different implementations did you work through before landing on something that worked well?

;; an example of the form
{:first-name "..."
 :last-name "..."
 :address {:street "..."}
 :languages #{"en" "de" "fr"}}

(defn confirm-leave? [original-form changed-form]
    (= original-form changed-form))
function confirmLeave(originalForm, changedForm) {
    // remember that conventionally is* variables are boolean
    // remember that let allows you to mutate this assignment
    let isChanged = false

    // use the language's forlet construct to iterate an Object. the forlet
    // iterates on keys, though, so dont forget to use key-value lookups.
    for (let key in changedForm) {
        // remember to ignore inherited/prototyped props 
        // remember to use the Object's base hasOwnProperty function
        // ... you could maybe also use originalForm.hasOwnProperty, but only if
        // it hasn't been mutated 
        if (Object.hasOwnProperty(originalForm, key)) {
            // TODO: for now we're assuming all values are scalar so we can check equality 
            // ... but equality checking non-scalars is a trickier problem
            // dont forget that strict equality requires the triple form ===,
            // otherwise you end up with auto-casting
            if (changedForm[key] !== originalForm[key]) {
                isChanged = true

                // we could technically exit the loop here because we found at
                // least 1 non-equal value, but we don't have a way to break the
                // forlet loop

    // dont forget to return the computed value
    return isChanged

There are about 7 “things” to hold in your head when writing the javascript.

If the author isn’t intimately familiar with the peculiarities of the language, a simple equality check between two forms becomes an error-prone mess of complexity.


Simplicity is at the heart of everything, and the community upholds this tenant. The fundamental unit of information in Clojurescript is data. There’s native support for the most common data types: maps {:house "stark"} , sets #{:arya :ned :sansa}, lists '("winter" "is" "coming"), vectors ["winter" "finally" "arrived"] , and keywords :didnt-see-that-coming.

Libraries, systems, and APIs are structured around Data > Functions > Macros, which means everything composes by simply using data to communicate. Generally, third-party libraries “do one thing” well and expose a simple, data-based API. You can build a dependency tree that matches your domain properly, instead of stuffing libs together that only play nice with particular partners or “sort of” do what you need. Bonus– you also have available the entire existing java ecosystem, or javascript ecosystem, depending on the host platform.

Clojurescript is extremely well designed. It is a hosted, dynamic, general-purpose, functional language that features immutable, persistent data structures and a robust infrastructure for multithreading. The language is small (clojure.core is about 5k loc, or just a weekend of reading), offers consistent abstractions, and always maintains backwards compatibility.

Let me elaborate on a particularly important part of the previous paragraph: immutable, persistent data structures. What does this mean? You cannot mutate data, only copy it into a new structure with some new changes. All references to the previous structure remain in tact, un-mutated, forever. Don’t worry, though– this doesn’t result in an excessive use of memory or CPU. Persistent data structures are well studied, and have become extremely efficient over the years. Native javascript operations, conversely, don’t use persistent structures and they pay for it in performance.

;; refs are never mutated
(let [a [1 2 3]
      b (conj a 4)]
  (println a) ;; [1 2 3]
  (println b) ;; [1 2 3 4]
  ;; equality is value-based, not identity based
  (= a b) ;; false
  (= a (drop-last b)) ;; true
  ;; but you can still compare identity
  (identical? a (drop-last b))) ;; false
// vars are mutable
var a = [1, 2, 3]
var b = a.push(4)
console.log(a) // [1, 2, 3, 4]
console.log(b) // 4, huh?

// equality has to be checked on a per-field basis
b = [1, 2, 3, 4]
console.log(a == b) // false
console.log(a === b) // false
;[1,2,3] == [1,2,3] // false

A recent trend in javascript is to move towards persistent structures through tools like immutablejs. Immutable javascript is a great step in the right direction, except it introduces a couple really big problems: the libraries expose a pervasive API that requires holistic buy-in for an application, and immutable structures don’t interop with anything that’s not also an immutable structure (goodbye lodash, jquery, or even alternative immutable implementations.).

But wait, you might say, we can use ES6/ES2017 syntax to achieve immutability! Yes, you can make immutable objects via the new … spread operators, or the likes of Object.assign({}, {...old, new: true}) and proper use of map/filter/reduce. Unfortunately, it has introduced another prickly edge to javascript; remember to use immutable operators everywhere. This is not a trivial burden, particularly when you’re talking about updating indexed arrays or objects inside arrays. Also, the benefits of structural sharing are lost, so expect heavy performance implications.

(let [old {:a 1 :b 2}
      new-o (assoc old :c 3)]
  (println old)) ;; {:a 1 :b 2}

(let [arr [1 2 3]
      new-a (conj arr 4)]
  (println arr)) ;; [1 2 3]

(let [obj-arr [{:a 1} {:b 2} {:c 3}]
      new-o-a (assoc obj-arr 1 {:b -2})]
  (println obj-arr) ;; {:a 1} {:b 2} {:c 3}]
  (println new-o-a)) ;; {:a 1} {:b -2} {:c 3}]
var old = {a: 1, b: 2}
var newO = Object.assign({}, {...old, c: 3})
console.log(old) // { a: 1, b: 2 }

var arr = [1, 2, 3]
var newA = [...arr, 4]
console.log(arr) // [1, 2, 3]

// what about updates/insertions? careful not to miss any ellipses or slices or off-by-ones
var objArr = [{a: 1}, {b: 2}, {c: 3}]
var newOA = [...objArr.slice(0, idx), {...objArr[idx], b: -2}, ...objArr.slice(idx+1)]
console.log(objArr) // [{a: 1}, {b: 2}, {c: 3}]
console.log(newOA) // [{a: 1}, {b: -2}, {c: 3}]

Clojurescript erases almost all the woes and pitfalls of javascript: inconsistent type comparisons, truthy/falsy inconsistency, null/undefined/NaN problems, unintended type conversions, shallow-vs-deep copying, passing values versus passing refs, inconsistent equality checking, and huge operator precedence tables, to name a few. But just in cast you’re a glutton for punishment, host interop is seamless via (js/myFunction args) or (.myFunction js/myModule args). Calling Clojurescript from javascript is a matter of ^:export my-function, which exposes to javascript my_namespace.my_function().

;; types are what they say they are, instances don't lie
(let [arr []]
  (type arr) ;; clojure.lang.PersistentVector
  (instance? clojure.lang.PersistentVector arr)) ;; true

;; look at that beautiful consistency
(= 1 "1") ;; false
(= 1 [1]) ;; false
(= 0 false) ;; false
(= "" false) ;; false
(= nil false) ;; false
(= "0" true) ;; false
(= "false" true) ;; false
(= [] true) ;; false
(= {} true) ;; false
(= (fn [] nil) true) ;; false
(= [] ",,") ;; false

;; you don't have to remember tables
(+ (- (/ (* 1 1) 2) 4) 2)

;; or more commonly
(-> (* 1 1)
    (/ 2)
    (- 4)
    (+ 2)) ;; -1.5

// types and instances lie all the time
var arr = new Array
arr.constructor === Array // true, but constructor can be modified
arr instanceof Array // oops, barfs === '[object Array]' // true because, I mean, why not?
Array.isArray(arr) // true, finally!
Array.isArray(Array.prototype) // true, huh?
typeof arr == 'object' // true, but it's a string?
arr = undefined
typeof arr // 'undefined', another string?

// look at how inconsistent it is
1 == '1' // true
1 == [1] // true
1 === '1' // false
1 === [1] // false
0 == false // true
'' == "" == false // true
null == false // true
undefined == false // true
NaN == false // true
'0' == true // true
'false' == true // true
[] == true  // true
{} == true // true
function() {} == true // true
Array(3) == ",," // true

// do you remember your order precedence table?
1 * 1 / 2 - 4 + 2 // ?

Clojurescript tooling is amazing. REPL driven development deserves an entirely separate post. Just know that it affords immediate code execution under the cursor (no test harness required), immediate source and documentation lookup, runtime-remote injection and debugging, and an immediate feedback loop at all stages of development. Leinengen offers scaffolding, building, packaging, and deploying under one roof, which means consistency and little-to-no tooling churn. Figwheel has paved the path to the future of javascript development. Mount or component or integrant offer runtime system setup/teardown/halt/resume capabilities. nREPL allows direct-injection into the browser’s runtime. Debugging tools like re-frame-10x make you feel like a super-hero with x-ray vision, perfect knowledge, and time-manipulation. Also, when necessary, you can reach into any profiling, debugging, or monitoring tools in the host ecosystem– some of which have been battle-hardened for decades.

Clojurescript has an opt-in, expressive, powerful, and integrated system for specification, instrumentation, and testing via clojure.spec. Docs alone are not enough for a whole variety of reasons that we won’t get into here. Instead of trying to solve the human-related documentation problem, Clojurescript offers a simple and extensible way to describe your application’s data. With spec you gain data parsing and conformance, function instrumentation, and generative testing, all with as-needed flexibility.

Clojurescript utilizes the Google Closure Compiler. The poorly-named GCC performs extremely aggressive javascript optimizations via renaming, munging, dead code elimination, and inlining. It’s a full-blown optimizing compiler that understands the javascript runtime, whereas other optimizing tools only parse and shuffle the AST. At the risk of throwing out extremely general and debatable numbers, a good explanation of the power of GCC follows: historically, if delivering javascript and jquery to a client was 1x, Clojurescript floated at around 2x. But for the last few years rich web applications have required much more beyond javascript and jquery. Thanks to the power of Clojurescript, which has not required a proliferation of libraries, and the GCC, it has remained at 2x. Meanwhile, a comparable javascript solution consisting of [flavor of the month framework] + react rendering engine + [extra libs] has reached 4x.

// given this code to optimize
function unusedFunction(note) {
function displayNoteTitle(note) {
var flowerNote = {};
flowerNote['title'] = "Flowers";

// rollup turns it into:
function displayNoteTitle(note) {
var flowerNote = {};
flowerNote['title'] = "Flowers";

// and Google Closure Compiler turns it into:

Clojurescript also integrates seamlessly with the Google Closure Library. The GCL is not very popular for some reason, but it’s been around for years and has been used internally at Google in most of their product lines going back a decade: search, gmail, maps, docs, calendar, plus, photos. In short, GCL is the missing std-lib for javascript; it fills the void by providing standard implementations for dates, i18n, math, styles, strings, dom manipulation, and much more. Portions of Clojurescript are built on top of the GCL, and any functionality not provided by the language directly is only one (:import goog.SomeClosureLibrary) away.

Lastly, the Clojurescript developer experience in terms of ergonomics, speed to delivery, extensibility, and bug-to-feature ratio is phenomenal. This is largely in part due to powerful libraries that a functional, immutable language has enabled. Reagent is what React hopes to be one day. Re-frame is a simple, powerful, extensible, uni-directional, single-source-of-truth, event-based system that both predates and outshines the capabilities of Flux/Redux-style architectures. Libraries in general tend to be single-purpose, dependency-free (or dependency-light), and composable. For those rare instances when the Clojurescript ecosystem does not already have a solution, it’s trivial to reach into the node ecosystem.

Learn from my mistakes

I started my journey as a developer by severely overlooking the edges. Non-standard, under-adopted, or even just strange-looking approaches went ignored. I was content staying in the lines and following the trends– however wacky and fickle they might have been. I balked at some “crazy” concepts that, in reality, were diamonds masked by a learning curve and some rough edges. I made Alan Perlis sad.

While I have since transformed into a furiously curious engineer that welcomes the unconventional, it bothers me to know that I probably missed out on many opportunities to learn great ideas. I know that, usually, greatness hides on the edges somewhere.

Clojure certainly started on the edges, and while it has gained an enormous amount of traction, it is also more than happy to persist along the edges if necessary. Hopefully I have convinced you to at least take a peek behind the unfamiliar, because maybe there’s some good ideas in hiding.

Where to go from here

Here is a list of material I wish someone handed me when I first started to learn Clojurescript. I would say good luck, but you won’t need it.