[swift-evolution] SE-0023 API Design Guidelines: Mutating vs Functional

[Haravikk]

> On 25 Jan 2016, at 06:12, T.J. Usiyan via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 'Functional' is pretty vague in this context. Mutating cuts to differentiating trait. 
> 
> "A function produces something" can fall down if I have a function with side effects. 
> 
> ``` swift
> private var state = 0 
> func generateCount() -> String {
>     state += 1
>     return "I've been called \(state) times" 
> }
> ```
> 
> We could narrow it by saying that a pure function has no side effects but this just draws us further from the relevant point, in my opinion.
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Also, how do we differentiate between functions that have @warn_unused_result? In your example the string that’s returned isn’t actually required to be used, so someone could call the function purely with the intention of increment state; not that it would be a very useful thing to do, but some functions have return values that aren’t necessarily required, like removeLast() on Arrays (I may not actually want the last array element, or I already know what it is and just want it removed).