Names, Values, Identities, States and Time

“No man can cross the same river twice, because neither the man nor the river are the same.”


The following post is extracted & paraphrased from Rich Hickey's excellent Are We There Yet? - specifically the section of the talk that focuses on the model for discussing and thinking about the titular concepts. These concepts are in turn taken from the philosophical writings of Alfred North Whitehead (a co-author of Principia Mathematica).

I often find myself wanting to explain this core concept to people who are new to Clojure, and particularly people who I am trying to make into people who are new to Clojure. While I think I have a good handle on this concept in my head - I sometimes struggle to explain it succinctly, hopefully this post achieves that goal.


These definitions are not really globally applicable, but they represent the precise meaning I try to capture when discussing values changing over time in the context of software development and programming.


A value is some measurable quantity, magnitude or amount for which its equivalence to some other value can be determined. A value can also be some immutable composite of other values. The number 42 is a value, the string "Hello World" is a value, the list (1, 2, 3) is also a value.


An identity is some composite psychological concept we apply to a succession of values over time where they are in some way causally related. You are an identity, I am an identity, a river is an identity (see below).


A name is simply a label that we apply to an identity or a value to make it easier to refer to. The same identity can have many names. "Glen", "Glen Mailer", "Mr Mailer", "Glenjamin" and "@glenathan" are all names which could legitimately be considered to refer to the identity that is myself in the appropriate context. Likewise the "Answer to the Ultimate Question of Life, The Universe, and Everything" is a name for the value 42.


A state is simply the value of an identity at a particular time. A snapshot, if you will. Under this definition state does not change, and thus there is no such thing as "mutable state".


Purely relative, it has no dimensions - it can only tell us whether something happened before or after some other thing (or at the same time).

The River

Let us consider the title quote in the context of these definitions. To help us examine the proverbial river under this light, we shall give ourselves the same powers as when running a computer program but in the real world - which requires us to sidestep some fairly fundamental physics - hopefully this will not cause any lasting damage.

The third-longest river in Asia runs through China. Depending on context it is known as the "Yellow River", "Huang He", "རྨ་ཆུ།", "the cradle of Chinese civilization" and "China's Sorrow". All of these are names for the same river, which itself is an identity.

If we were to freeze an instant in time into a snapshot of our proverbial river crossing, this state would contain a value composed of a large number of atomic (in the irreducible sense) smaller values. For simplicity, lets assume that water molecules are immutable. In this case then the state of the river we are crossing can be said to be the current arrangement of all these immutable water molecule values.

At some point in the future when returning for our second crossing, we take another snapshot of the river as our new state. The river's value is again the arrangement of all the immutable water molecules - but this time they are all different molecules with different values.

The state of the identity which is the river named "Huang He" at this later point in time is measurably different from the value we took during the first crossing.

In Clojure

Since immutability is at it's core, we'll start here for some code examples.

The following code should work correctly when pasted into a running Clojure REPL.

In JavaScript

JavaScript doesn't have the same set of immutable primitives, but we can achieve a similar effect with a little sleight of hand.

The following code should work correctly when pasted into the browser console or a Node.js REPL line-by-line.


The ancient Greeks knew about the perils of mutable state a long time ago - we're only now rediscovering this for ourselves.

In a language like Clojure, that was designed from the ground up with this in mind, it's easy to take back control and tease apart the separate concepts I've described. Even in a language like JavaScript, designed in a week at a time when mutability was commonplace, we can achieve a similar effect with a measure of self-control. There are also libraries like mori and immutable-js which provide much fuller implementations of the data-types required to avoid mutability.

If you remain unconvinced, I recommend watching Are We There Yet?. If you're still not sure after that, you're either a far better programmer than me, or you're yet to experience the pain of trying to reason about a large codebase riddled with unchecked mutability.


As well as the above definitions, Are We There Yet? contains this gem, which is Rich visualising the idea of obvious complexity while saying "Grrrrr".

I think I now actually understand JavaScript objects

Over the last week or so, I've been writing a lot of NodeJS-based JavaScript, and in doing so I realised I didn't really have a picture of how the object model actually fits together. So I did some reading, and now I'm pretty sure I get it.

The articles I read are linked at the bottom of the post, but often code is clearer to read than paragraphs of explanation. I've thrown together some implementations of the key players in the javascript object model that mirror what the actual engine is doing under the hood.

The following examples should explain the prototype chain, how it's assembled and how it's interacted with - I've added some console output to the loops so we can see what's happening as they're called.

The code below assumes that its running under an ECMAScript 5 environment, so thats the latest versions of Safari, Chrome and Firefox - as well as NodeJS. It takes advantage of Object.create and Object.getPrototypeOf, these are the standardised versions of messing around with the __proto__ property.

And to see it in action:

Which produces:

> node demo.js 
john instance_of Employee
Trying to match Employee.prototype
Comparing to Retired.prototype
Comparing to Employee.prototype

john instance_of Array
Trying to match Array.prototype
Comparing to Retired.prototype
Comparing to Employee.prototype
Comparing to Person.prototype
Comparing to Object.prototype

new_instance(Person, 'john') instanceof Person

new_instance(Employee, 'boss', 'john') instanceof Employee

get_attribute(john, 'role')
Looking on object directly

get_attribute(john, 'greeting')
Looking on object directly
Looking on Retired.prototype
Looking on Employee.prototype
Welcome to work

call_method(john, 'greet')
Looking on object directly
Looking on Retired.prototype
Looking on Employee.prototype
Looking on Person.prototype
Hello John

If you'd like to have a play with these functions without firing up NodeJS, here's the obligatory JSFiddle:

See also:

Taking advantage of JavaScript function hoisting

Just a quick one that I wanted to write down before I forget.

Function statements (like var statements) get hoisted up to the top of their declaring scope before evaluation. This means that the location of a function (or constructor function) definition within a file is entirely down to aesthetics - as long as the scope is correct.

This can be used to make defining normal JavaScript classes look a little bit more classical, without messing around with vastly different syntax.

"Static" methods in Javascript.

I've been trying to find a way to have "static" methods in Javascript, NodeJS to be exact.

It turns out that using the util.inherits function in NodeJS, this "just works".

The "definition" of static I'm using here is a bit lax, but it's a function that does the same thing when called from the prototype or an instance, and has access to the properties attached to the class it was called against.

To try it out in action without dropping this into a browser or firing up NodeJS: