effectful

Effectful is an adjective used in programming to describe computations, functions, or modules that produce or depend on observable side effects. Side effects include input/output, mutation of state, exceptions, interaction with the outside world, non-determinism, and communications. By contrast, an effect-free or pure function yields the same result for the same inputs and has no observable interactions with external state.

In functional programming, effectful computations are often isolated from pure computations, using constructs that make effects explicit. In Haskell, functions with type IO a are considered effectful; the IO action describes a sequence of effects rather than performing them immediately. In imperative languages, effects are more directly visible, as functions may write to a global variable or perform a network request.

Managing effects: effect systems and algebraic effects provide ways to model and compose effects, while monads and effect handlers offer patterns for sequencing and interpreting effects. These approaches aim to separate the declaration of what a computation computes from how its effects are realized, enabling safer composition and reasoning about code.

Practical implications include improved modularity and testability when effects are explicit. Testing often requires mocks or stubs to simulate environment or I/O, and reasoning about performance and concurrency can become more complex as the number of interacting effects grows.

The term is widely used descriptively rather than as a formal, universal classification, and may refer to any computation that yields observable interactions with the outside world. It contrasts with pure, referentially transparent code and underpins discussions of purity, effect management, and language design.