Pure functions
Functions always producing same output for same input without modifying external state or accessing external variables. Pure functions are deterministic and testable.
Real World
Google's MapReduce framework relies on pure functions so that the same mapping task can run identically across thousands of servers without any machine needing to know what another is doing.
Exam Focus
Define 'no side effects' precisely — same inputs always produce same output AND no external state is changed.
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