Computer science or computing science is the study of the theoretical foundations of information and computation and of practical techniques for their implementation and application in computer systems...
In computer science, a subroutine is a portion of code within a larger program that performs a specific task and is relatively independent of the remaining code....
An expression in a programming language is a combination of explicit values, constants, variables, operators, and functions that are interpreted according to the particular rules of precedence and of association for a particular programming language, which computes and then produces another value...
is said to have a side effect if, in addition to returning a value, it also modifies some state
State (computer science)
In computer science and automata theory, a state is a unique configuration of information in a program or machine. It is a concept that occasionally extends into some forms of systems programming such as lexers and parsers....
or has an observable interaction with calling functions or the outside world. For example, a function might modify a global or static variable
Static variable
In computer programming, a static variable is a variable that has been allocated statically — whose lifetime extends across the entire run of the program...
, 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 a program with side effects requires knowledge about the context and its possible histories; and is therefore hard to read, understand and debug.
Side effects are the most common way to enable a program to interact 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
Imperative programming
In computer science, imperative programming is a programming paradigm that describes computation in terms of statements that change a program state...
In computer science, functional programming is a programming paradigm that treats computation as the evaluation of mathematical functions and avoids state and mutable data. It emphasizes the application of functions, in contrast to the imperative programming style, which emphasizes changes in state...
, side effects are rarely used. Functional languages such as Standard ML
Standard ML
Standard ML is a general-purpose, modular, functional programming language with compile-time type checking and type inference. It is popular among compiler writers and programming language researchers, as well as in the development of theorem provers.SML is a modern descendant of the ML...
and Scheme do not restrict side effects, but it is customary for programmers to avoid them. The functional language Haskell
Haskell (programming language)
Haskell is a standardized, general-purpose purely functional programming language, with non-strict semantics and strong static typing. It is named after logician Haskell Curry. In Haskell, "a function is a first-class citizen" of the programming language. As a functional programming language, the...
A type system associates a type with each computed value. By examining the flow of these values, a type system attempts to ensure or prove that no type errors can occur...
; it uses the concept of monads to do stateful and IO computations.
An assembly language is a low-level programming language for computers, microprocessors, microcontrollers, and other programmable devices. It implements a symbolic representation of the machine codes and other constants needed to program a given CPU architecture...
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 all 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 instruction pipeline
Instruction pipeline
An instruction pipeline is a technique used in the design of computers and other digital electronic devices to increase their instruction throughput ....
(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 necessary but not sufficient 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
Pure function
In computer programming, a function may be described as pure if both these statements about the function hold:# The function always evaluates the same result value given the same argument value...
(always returns the same results on the same input).
Temporal side effects
Side effects due to the time taken for an operation to execute are usually ignored when discussing side effects and referential transparency. In most programs it is desirable to replace a long operation with an equivalent shorter one e.g. replacing (60 / 3 * 2) with 40. 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(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
Idempotence
Idempotence is the property of certain operations in mathematics and computer science, that they can be applied multiple times without changing the result beyond the initial application...
In mathematics, function composition is the application of one function to the results of another. For instance, the functions and can be composed by computing the output of g when it has an argument of f instead of x...
f ∘ f).
Example
One common demonstration of side effect behavior is that of the assignment operator in C++
C++
C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...
. For example, the assignment operator returns the value getting assigned 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 C++ assignment operator right associates, this is equivalent 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" is then assigned into "i". This is a potential hangup for novice programmers who are apt to confuse
while (i true) {}; //Tests if i evaluates to true
with
while (i = true) {}; //The result of the assignment function is "true", so the loop conditional always evaluates to true
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