Functional Programming Learned with Scala: Part 1 - Pure Functions and Referential Transparency

The core of functional programming is separating computation and action. By distinguishing between the process of simply computing a value and actions that change external state, such as console output, the predictability of code can be increased.

A pure function returns a result depending only on its input values, like a mathematical function, and does not cause side effects that change external state. For example, if the logic for calculating a squared value is extracted into a separate function, that function becomes a pure function that derives its result using only the arguments passed to it.

The key concepts are as follows:

• Computation: Returns a result based only on input values, without depending on external state.
• Side effect: An action that changes external state, such as the file system, database, or network I/O, beyond the function's execution result.
• Purity: A state consisting only of computation, with no side effects.
• Referential transparency: The property whereby the same input always produces the same result, such that replacing the expression with its resulting value does not change the behavior of the program.