Editing Expression (computer science) (section)

===Caveats===

The formal notion of a side effect is a change to the abstract state of the running program.

Another class of side effects are changes to the concrete state of the computational system, such as loading data into [[cache memory|cache memories]].  Languages that are often described as "side effect–free" will generally still have concrete side effects that can be exploited, for example, in [[side-channel attack]]s.

Furthermore, the elapsed time evaluating an expression (even one with no other apparent side effects), is sometimes essential to the correct operation of a system, as behaviour in time is easily visible from outside the evaluation environment by other parts of the system with which it interacts, and might even be regarded as the ''primary'' effect such as when performing [[Benchmark (computing)|benchmark testing]].

It depends on the particular programming language specification whether an expression with no abstract side effects can [[Program optimization|legally be eliminated]] from the execution path by the processing environment in which the expression is evaluated.