Pfthb

I vaguely remember reading somewhere a while ago that either a method/function should return a value without modifying the state or should process some data changing a state but not returning data. It is beyond simple getters and setters.

I cannot figure where I read that, the rationale and if this was a sound principle. It may be simpler to test for sure but is there any other reason? Is there is there a related design principle or pattern? Any clue or link appreciated.

Thanks
Olivier

That principle is called Command–query separation.

From wikipedia:

Command–query separation (CQS) is a principle of imperative computer programming. It was devised by Bertrand Meyer as part of his pioneering work on the Eiffel programming language.

It 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.

Personally, I think this principle is very helpful. It is based on a common observation that: when we see a function that returns some data, we tend to think that it does not change any state of data. In other words, that function does not have any side-effect. In constrast, functions with side-effects are considered "risky" and therefore should have a clear indication (returning void type). You see, C# has the concept of Properties where you change state of a property by using the assignment symbol which is a stronger indication than setter methods.

To conclude, the CQS principle helps our code to be easier to read and reason since managing how state changes is crucial in programming.

You can find more useful links in the wikipedia article, and this and this.