[swift-evolution] Preserving non-mutability of methods of an existential or generic object

[Slava Pestov]

Hi Hooman,

Since the protocol P is not class-bounded, the requirement can be witnessed by a protocol extension method which re-assigns ‘self’:

protocol Initable {
  init()
}

extension P where Self : Initable {
  mutating func f(_ x: Int) -> Int {
    self = Self()
    return x
  }
}

class C : P, Initable {
  required init() {}
}

Now imagine you could do this,

let x: P & AnyObject

x.f(12)

This would be invalid because ‘x’ is a let binding but the requirement ‘f’ is witnessed by the protocol extension method, which performs a mutating access of ‘self’.

Slava

> On Dec 21, 2017, at 6:01 PM, Hooman Mehr via swift-evolution <swift-evolution at swift.org> wrote:
> 
> The title is confusing, let me clarify by example:
> 
> We have this protocol with a mutating method:
> 
> protocol P { mutating func f(_ x: Int) -> Int }
> 
> And a conforming class (which has to conform with a non-mutating method):
> 
> class C: P { func f(_ x: Int) -> Int { return x } }
> 
> An instance of this class can be used with a let constant:
> 
> let c = C()
> c.f(1) // OK
>  
> If we make it an existential object conforming to P, the immutability of the method will be erased:
> 
> let c: AnyObject & P = C()
> c.f(1) // Cannot use mutating member on immutable value: 'c' is a 'let' constant
> 
> A generic context has the same issue:
> 
> func f<T: AnyObject & P>(_ arg: T)-> Int { return arg.f(1) } // Cannot use mutating member on immutable value: ‘arg' is a 'let' constant
> 
> My question: 
> 
> Is it too much work to preserve method non-mutability in in these cases?
> 
> The workaround I am using is this:
> 
> protocol Q: class, P { func f(_ x: Int) -> Int } // 'Refine' it to be non-mutating.
> extension C: Q {}
> 
> // Now these work:
> let c: Q = C()
> c.f(1) // OK
> func f<T: Q>(_ arg: T)-> Int { return arg.f(1) } // OK
> 
> This workaround creates a lot of duplication and is hard to maintain. It is not something that I do often, but when I do, it is pretty annoying. 
> 
> Supplemental questions:
> 
> Have you guys ever ran into this?
> Is there already a bug tracking this? 
> 
> 
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