Understanding Purity in Nested Function Systems

Nested functions—a function within a function—are a common sight in modern programming. The question arises: when is such a nested system considered "pure"?

Criteria for Purity in Nested Functions

A nested function system is pure if, and only if, calling the inner function does not produce any side effects. In simpler terms, invoking the inner function should not alter any state or produce different outcomes beyond its own scope.

Common Side Effects

Side effects are any operations that extend beyond the local environment of the function itself. Here are some common and not-so-common side effects to watch out for:

• HTTP Requests: Making a call to an external API or database.
• Global Variable Manipulation: Changing the state of variables not confined within the function.
• Output Operations: Writing to a screen, console, or file.
• DOM Alterations: Any changes to the structure or content of a webpage.
• Random Number Generation: Functions like Math.random() which produce non-deterministic results.
• Time-based Operations: Anything dependent on the current time or date.
• State Changes in Storage: Altering cookies, local storage, or other client-side storage mechanisms.
• Event Triggering: Emitting or broadcasting events to other parts of the application.

By avoiding these side effects in your nested functions, you ensure that they maintain their purity. This makes the code easier to understand, reason about, and test.