Doing arithmetic with concrete numbers is fine, but when you move from arithmetic to algebra, you use variables so that you can manipulate abstract symbols. Similarly, in ClojureScript, you want to be able to describe transformations of data in abstract terms. Before we talk about ClojureScript, let’s look a bit more closely at algebraic variables.

In algebra, when you say something like

x = 5

that means that you are using the symbol x to represent 5. Wherever you have a 5, you can substitute x; wherever you have an x, you can substitute 5. Similarly, when you say:

y = x + 3

That means the symbol y represents the quantity x + 3, and you can substitute one for the other.

Another feature of algebraic symbols is that, once you set them, you can’t change your mind. You can’t be halfway through a calculation and all of a sudden claim that:

x = 6

because you said that x stands for 5, and five certainly does not equal six!

Symbols in ClojureScript

Symbols in ClojureScript work much as they do in algebra. You use the def function to bind a symbol to a value. The def function takes two arguments: the symbol and the value. Once a symbol is bound, you can use it in expressions, including definitions of other symbols. The value can be an expression as well. In the following code, the println function (the name stands for “print line”) is used to display the value bound to days.

In many other programming languages, symbols are called variables, and they are not bound to values as in ClojureScript. Instead, a variable is a name for a location in memory; when you say x = 5, that puts 5 into the memory location labeled x. This operation is called assigning to a variable rather than binding to a symbol, and it doesn’t work like algebra at all. In these other languages, it’s perfectly valid to say x = 6, which puts a new value into the same memory location. In this book, I will use the words “symbol” and “bind” to help you keep ClojureScript’s algebra-like model in mind rather than the model used by other languages.
You are probably used to a statement like x = 5 being called an assignment statement that means “put 5 into a memory location that has been labeled x,” and x is called a variable rather than a symbol. One of the larger hurdles in learning a language like ClojureScript is understanding that its symbols (variables) work more like variables in algebra than like labeled memory locations. That’s why I am using the words “symbol” and “bind” in this book rather than “variable” and “assign,” to help you get used to this different mental model.


Unlike true algebraic symbols, it is possible to re-bind a new value to a symbol in ClojureScript, but you don’t need to do it often. Normally you will consider a binding to be immutable—once bound, you never change a symbol’s value.

What’s in a (Symbol) Name?

Symbols must start with a non-numeric character, followed by letters, digits, or any of *, +, !, -, _, ‘, and ?.

Making your symbol names meaningful is an important part of programming. If, instead of using years I had used y, the code wouldn’t be as clear; y could also refer to a graphical y-coordinate.

Sometimes you might have a symbol that is best described by two words, such as an interest rate. Just calling it rate is not good; a financial program could have interest rates, penalty rates, or any number of types of rates. You can’t call it interest rate because you can’t put spaces in a symbol name. There are three ways that people solve the problem of writing multi-word names:

  • interestRate This is called camel case, where every word after the first begins with a capital letter (it’s called camel case because the capital letters are distantly reminiscent of a camel’s humps).
  • interest_rate In this method, words are separated by underscores.
  • interest-rate Words are separated by hyphens. This is what you will see most often in ClojureScript.

Again, in terms of meaningfulness, a name like ir would be far too short, and interest-rate-calculated-on-a-per-month-basis would be ridiculously long.

ClojureScript doesn’t just mean A through Z when it talks about a “letter”; you can use alphabetic characters in any language supported by Unicode. Here’s the first example with the names in Russian:

You Try It

Give this a try: define a symbol named hours and another named minutes. Bind them to any values you like. Then define a third symbol named total-minutes that is bound to 60 times hours, plus minutes, and use the println function to display the result.

Next Section - Writing Your Own Functions