• A principle of imperative computer programming. CQS was devised by Bertrand Meyer as part of his pioneering work on the Eiffel programming language. The principle states that every 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.” More formally, methods should return a value only if they are referentially transparent and hence possess no side effects. ← Wikipedia

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