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Side effect (computer science)
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In computer science, an operation, function or expression is said to have a side effect if it modifies some state variable value(s) outside its local environment, that is to say has an observable effect besides returning a value (the main effect) to the invoker of the operation. State data updated "outside" of the operation may be maintained "inside" a stateful object or a wider stateful system within which the operation is performed. Example side effects include modifying a non-local variable, modifying a static local variable, modifying a mutable argument passed by reference, performing I/O or calling other side-effect functions.[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]
The degree to which side effects are used depends on the programming paradigm. 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.
In functional programming, side effects are rarely used. The lack of side effects makes it easier to do formal verifications of a program. Functional languages such as Standard ML, Scheme and Scala do not restrict side effects, but it is customary for programmers to avoid them.[4] The functional language Haskell expresses side effects such as I/O and other stateful computations using monadic actions.[5][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 overt 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.
Referential transparency
Temporal side effects
Idempotence
Example
See also
References
Last edited 9 days ago by Jarble
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