Editing Command–query separation (section)

{{Short description|IT architecture separating actions and reads}}
{{refimprove|date=February 2008}}
'''Command-query separation''' ('''CQS''') is a principle of [[Imperative programming|imperative]] [[computer programming]]. It was devised by [[Bertrand Meyer]] as part of his pioneering work on the [[Eiffel (programming language)|Eiffel programming language]].

It states that every [[Method (computer science)|method]] should either be a ''command'' that performs an action, or a ''query'' that returns data to the caller, but not both. In other words, ''asking a question should not change the answer''.<ref>{{cite web |last=Meyer|first=Bertrand|title=Eiffel: a language for software engineering|url=http://laser.inf.ethz.ch/2012/slides/Meyer/eiffel_laser_2012.pdf|page=22|access-date=16 December 2014}}</ref> More formally, methods should return a value only if they are [[Referential transparency|referentially transparent]] and hence possess no [[Side effect (computer science)|side effect]]s.