Side effect (computer science)

In

Side effects play an important role in the design and analysis of programming languages. The degree to which side effects are used depends on the programming paradigm. For example, imperative programming is commonly used to produce side effects, to update a system's state. By contrast, declarative programming is commonly used to report on the state of system, without side effects.

Absence of side effects is a necessary, but not sufficient, condition for referential transparency. Referential transparency means that an expression (such as a function call) can be replaced with its value. This requires that the expression is pure, that is to say the expression must be deterministic (always give the same value for the same input) and side-effect free.

Temporal side effects

Side effects caused by the time taken for an operation to execute are usually ignored when discussing side effects and referential transparency. There are some cases, such as with hardware timing or testing, where operations are inserted specifically for their temporal side effects e.g. sleep(5000) or for (int i = 0; i < 10000; ++i) {}. These instructions do not change state other than taking an amount of time to complete.

Idempotence

A

x = 0

def setx(n):
    global x
    x = n

setx(3)
assert x == 3
setx(3)
assert x == 3

A pure function is idempotent if it is idempotent in the mathematical sense. For instance, consider the following Python program:

def abs(n):
    return -n if n < 0 else n

assert abs(abs(-3)) == abs(-3)

abs is idempotent because the second application of abs to the return value of the first application to -3 returns the same value as the first application to -3.

Example

One common demonstration of side effect behavior is that of the

a = (b = 3);  // b = 3 evaluates to 3, which then gets assigned to a

Because the operator right associates, this is equivalent to

a = b = 3;

This presents a potential hangup for novice programmers who may confuse

while (b == 3) {}  // tests if b evaluates to 3

with

while (b = 3) {}  // b = 3 evaluates to 3, which then casts to true so the loop is infinite

See also

• Action at a distance (computer programming)
• Don't-care term
• Sequence point
• Side-channel attack
• Undefined behaviour
• Unspecified behaviour

References