Editing Effect system (section)

{{Short description|System which describes the computational effects of computer programs}}
{{More footnotes needed|date=September 2010}}
In [[computing]], an '''effect system''' is a [[formal system]] that describes the computational effects of computer programs, such as [[side effect (computer science)|side effect]]s. An effect system can be used to provide a [[compile-time]] check of the possible effects of the program.

The effect system extends the notion of type to have an "effect" component, which comprises an '''effect kind''' and a '''region'''. The effect kind describes ''what'' is being done, and the region describes ''with what'' (parameters) it is being done.

An effect system is typically an extension of a [[type system]]. The term "'''type and effect system'''" is sometimes used in this case. Often, a type of a value is denoted together with its effect as ''type ! effect'', where both the type component and the effect component mention certain regions (for example, a type of a mutable memory cell is parameterized by the label of the memory region in which the cell resides). The term "algebraic effect" follows from the type system.

Effect systems may be used to prove the external [[Pure function|purity]] of certain internally impure definitions: for example, if a function internally allocates and modifies a region of memory, but the function's type does not mention the region, then the corresponding effect may be erased from the function's effect.<ref>{{Cite book|last=Albin.|first=Turbak, Franklyn|url=http://worldcat.org/oclc/1261053520|title=Design concepts in programming languages|date=2010|publisher=PHI Learning|isbn=978-81-203-3996-5|oclc=1261053520}}</ref>