4 ms·

Only to return information about the effect's success, which would again make it easy to see that the function must have some effect, if all it returns be true

• tsimionescu 6y ago
  I think a general case is concurrent mutation of a data structure. If you are adding some item to a list in parallel with others, you will usually need to return some token (e.g. index) that will allow your caller to retrieve that item later. You MAY be able to get away with making this a pure function by returning a shallow copy, but I believe that depends on how central the sharing aspect is.
  • brundolf 6y ago
    Yeah, so the problem here is that the value creation logic is non-deterministic; not only does it depend on hidden state, it doesn't even do so predictably because you don't know the order in which other threads might interplay. I established three categories of imperative-logic-return-values above: > The return value is either going to pertain to the state before the side-effect, the state after the side-effect, or the side-effect itself. The first two could easily be broken out into pure functions. The third, again, mostly comes down to error reporting. So this definitely falls into category three, which makes the coupling legitimate. It's possible I was wrong in my statement that that category "mostly comes down to error reporting"; it was just based on my own past experience.