An Implementation Model for Rational Protocol Conformance Behavior

Evolution Pitches
25

Just to add my reaction as a humble user. For this particular example, my mental model of Swift's generics (which took a while to develop) allows me to not get surprised or confused. This example is in my opinion not showcasing any of the really-hard-to-understand parts. I include the example here again for completeness:

protocol P {
  func whatAmI()
}

extension P                      { func whatAmI() { print("P")        }  }
extension Array: P               { func whatAmI() { print("Array")    }  }
extension Array where Element: P { func whatAmI() { print("Array<P>") }  }
extension Int: P                 { func whatAmI() { print("Int")      }  }

func whatIsIt<C: P>(_ value: C) {
  value.whatAmI()
}

func test() {
  let x = [1, 2, 3]
  x.whatAmI() // Array<P>
  whatIsIt(x) // Array
}
test()

By looking at this program I can predict (and not get surprised) by the output. The generic function whatIsIt has a typeparameter C which is constrained only by C: P which means that the compiler knows nothing about C except what it knows about P. That is, it knows about P's requirement whatAmI() and can call that required method (as implemented by various particular Cs), but it does so without knowing anything about eg Element or Array, it cannot get any information from any if its call sites, it's just its signature that matters.

The only way to make whatIsIt print "Array<P>" too (which I guess might be what some would expect?) would be to rewrite the example in a way that lets the compiler know more/enough about C. As the example is currently written, it is clear that within the body of whatIsIt, the compiler has no idea about Element: P or that C can sometimes be an Array. It only knows that some C conforms to P and if that C happens to implement P's requirement rather than use a default implementation, it will call C's implementation.

That said, there are many other things I get confused by in Swift's generics, like this example which @dabrahams linked to in the OP. (Note that I do not have a clear expectation about what the intended/best/fixed behavior should be in that example, especially so when taking @Nevin's "fun fact" into account.)

But perhaps my confusion there is just caused by the same rules I claim to understand here, I haven't thought about it enough to know if that might be the case.

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