Infix vs. prefix notation

In Clojure, +, -, * and / appear before two numbers. This is called prefix notation. What you're used to seeing is called infix notation, as the arithmetic operator is in-between the two operands.

Languages such as JavaScript use infix notation, while Clojure only uses prefix notation.

Prefix notation is useful for many reasons. Look at this example of an infix expression and the prefix equivalent:

Infix:  1 + 2 * 3 / 4 + 5 - 6 * 7 / 8 + 9

Prefix: (+ (- (+ (+ 1 (/ (* 2 3) 4)) 5) (/ (* 6 7) 8)) 9)

The benefits of prefix notation

Imagine both the above expressions are unclear. However in the prefix version you do not have to ever think about the precedence of operators.

Because each expression has the operator before all the operands and the entire expression is wrapped in parentheses, all precendence is explicit.

Infix:  1 + 2 / 3
Prefix: (+ 1 (/ 2 3))

Less repetitive

Another reason prefix notation can be nice is that it can make long expressions less repetitive.

With prefix notation, if we plan to use the same operator on many operands, we do not have to repeat the operator between them.

Infix:  1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9
Prefix: (+ 1 2 3 4 5 6 7 8 9)

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