![]() , which simply indicates that the computation has terminated correctly.Ī side-effect is when an operation has an effect on a variable/object that is outside the intended usage. These are often modeled as functions that return a special value None, or unit or () or. sleep() in Python, are only useful for their (side) effects. They are often called pure functions because all they do is compute a function in the mathematical sense. Some procedures only compute a result value and have no other effects.Some procedures are useful for both their return value and their side-effect.Some programming languages use the term procedure to avoid confusion with functions in the mathematical sense. I suppose that, since the term function was originally used in Mathematics, computing a value is considered to be the primary effect of a function whereas any other effects are considered side-effects. The term side-effect may sound negative but normally the effect of calling a function is the very purpose of the function itself. So effects can be related to state but also to other aspects like firing a missile or pausing execution for a few seconds. prints something to the screen, changes a value on a database, launches missiles, sleeps for 10 seconds, sends an SMS. given input values, it returns a result, or It computes a function in the mathematical sense, i.e.What is called a function (or procedure) in many programming language is a construct (piece of code) that can be executed because: You may well replace f(x) with y everywhere in an expression and nothing will change. So, given a function f and a value x, f(x) will always be the same result y. In mathematics a function is just a mapping from a tuple of values to a value. I would like to elaborate on some aspects that IMO have not been stressed enough. I think that the existing answers are quite good. One does not imply the other in general computer science, but functional programming languages will typically tend to enforce both constraints. As commenter benjol points out, several people may be conflating the definition of a side effect with the definition of a pure function, which is a function that is (a) idempotent and (b) has no side-effects. Side effect = changing something somewhere. An idempotent function can have side effects, and a non-idempotent function may have no side effects (such as getting the current system date and time). Enabling or disabling a button in the User Interface.Ĭontrary to what some people seem to be saying:Ī side effect does not have to be hidden or unexpected (it can be, but that has nothing to do with the definition as it applies to computer science) Ī side effect has nothing to do with idempotency.A side effect refers simply to the modification of some kind of state - for instance:
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