Side effect (computer science)

In computer science, an operation, function or expression is said to have a side effect if it has any observable effect other than its primary effect of reading the value of its arguments and returning a value to the invoker of the operation. Example side effects include modifying or reading a non-local variable, a static local variable or a mutable argument passed by reference; raising errors or exceptions; performing I/O; or calling other functions with side-effects.^[1] In the presence of side effects, a program's behaviour may depend 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.^[2]^[3] 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.

Functional programming aims to minimize or eliminate side effects. The lack of side effects makes it easier to do formal verification of a program. The functional language Haskell eliminates side effects such as I/O and other stateful computations by replacing them with monadic actions.^[4]^[5] Functional languages such as Standard ML, Scheme and Scala do not restrict side effects, but it is customary for programmers to avoid them.^[6]

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 that implicitly modifies condition codes (a hidden side effect) while it explicitly modifies a register (the intended effect). One potential drawback of an instruction set with 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 some 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.

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[Spuler-Sajeev_1994-1] Cite error: The named reference Spuler-Sajeev_1994 was invoked but never defined (see the help page).

[Hughes_1990-2] Cite error: The named reference Hughes_1990 was invoked but never defined (see the help page).

[Collberg_2005-3] Cite error: The named reference Collberg_2005 was invoked but never defined (see the help page).

[Haskell_1998-4] Cite error: The named reference Haskell_1998 was invoked but never defined (see the help page).

[Jones-Wadler_1993-5] Cite error: The named reference Jones-Wadler_1993 was invoked but never defined (see the help page).

[Felleisen_2014-6] Cite error: The named reference Felleisen_2014 was invoked but never defined (see the help page).

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