What is a Pure Function?

About

Pure functions are called pure because:

• they do not change their inputs,
• they always return the same result for the same inputs.

The result is then predictable.

Pure functions take data and transform it, without modifying any shared state, meaning there are no side effects besides returning a value.

Example

function sum(a, b) {
  return a + b;
}

function withdraw(account, amount) {
  account.total -= amount;
}

Expression

A pure expression is an expression that declares data and transform it with functions.

Impurities

Impurities are language constructs that are not reproducible.

Data Structure

Pure functional data structures - Purely Functional Data Structures - Chris Okasaki - September 1996 - CMU-CS-96-177 (PDF)

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