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

Erica Sadun erica at ericasadun.com
Mon Jan 25 15:22:05 CST 2016

I incorporated your feedback. Please reload: https://github.com/erica/SwiftStyle/blob/master/Grammatical.md

-- E

> On Jan 25, 2016, at 2:52 AM, Haravikk <swift-evolution at haravikk.com> wrote:
>> 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).

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