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Side-effect (computer science)

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Side-effect (computer science)

In computer science, a function or expression is said to have a side effect if, in addition to returning a value, it also modifies some state or has an observable interaction with calling functions of the outside world. For example, a function might modify a global or static variable, modify one of its arguments, raise an exception, write data to a display or file, read data, or call other side-effecting functions. In the presence of side effects, a program's behavior depends on history; that is, the order of evaluation matters. Understanding and debugging a function with side effects requires knowledge about the context and its possible histories.[1][2]

Side effects are the most common way that a program interacts with the outside world (people, filesystems, other computers on networks). But the degree to which side effects are used depends on the programming paradigm. Imperative programming is known for its frequent utilization of side effects. In functional programming, side effects are rarely used. Functional languages such as Standard ML, Scheme and Scala do not restrict side effects, but it is customary for programmers to avoid them.[3] The functional language Haskell restricts side effects with a static type system; it uses the concept of monads to do stateful and IO computations.[4][5]

Assembly language programmers must be aware of hidden side effects — instructions that modify parts of the processor state which are not mentioned in the instruction's mnemonic. A classic example of a hidden side effect is an arithmetic instruction which explicitly modifies a register (an overt effect) and implicitly modifies condition codes (a hidden side effect). One defect of an instruction set with many hidden side effects is that, if many instructions have side effects on a single piece of state, like condition codes, then the logic required to update that state sequentially may become a performance bottleneck. The problem is particularly acute on processors designed with pipelining (since 1990) or with out-of-order execution. Such a processor may require additional control circuitry to detect hidden side effects and stall the pipeline if the next instruction depends on the results of those effects.

Referential transparency

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 has no side effects and is pure (always returns the same results on the same input).

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 side effect free function f is always idempotent (under sequential composition f ; f, not function composition ff).

Example

One common demonstration of side effect behavior is that of the assignment operator in C++. For example, assignment returns the right operand and has the side effect of assigning that value to a variable. This allows for syntactically clean multiple assignment:

int i, j;
i = j = 3;

Because the operator right associates, this equates to

int i, j;
i = (j = 3); //j = 3 returns 3, which then gets assigned to i

Where the result of assigning 3 into "j" then gets assigned into "i". This presents a potential hangup for novice programmers who may confuse

while (b == 10) {} //Tests if b evaluates to 10

with

// The assignment function returns 10
// which automatically casts to "true"
// so the loop conditional always evaluates to true
while (b = 10) {}

See also

References

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