[swift-evolution] Should we rename "class" when referring to protocol conformance?

Andrew Trick atrick at apple.com
Mon May 9 03:01:20 CDT 2016

> On May 7, 2016, at 11:51 PM, Dave Abrahams <dabrahams at apple.com> wrote:
>> Does Array<T> have value semantics then only if T also has value
>> semantics?
> This is a great question; I had to rewrite my response four times.
> In my world, an Array<T> always has value semantics if you respect the
> boundaries of element values as defined by ==.  That means that if T is
> a mutable reference type, you're not looking through references, because
> == is equivalent to ===.
> Therefore, for almost any interesting SomeConstraint that doesn't refine
> ValueSemantics, then
>  Array<T: SomeConstraint>
> only has value semantics if T has value semantics, since SomeConstraint
> presumably uses aspects of T other than reference identity.  

I just had a chance to digest Dave's answer. It explains a lot.

PureValue was defined in terms of the type's physical representation:
- A struct with no reference properties
- Recursively, a reference to immutable or uniquely referenced memory.

It's defined such that we can say Array<T> is a PureValue iff T is a PureValue.

There is currently no procedure for determining PureValue because we have no way to declare that references are immutable or uniquely referenced. It would be a promise by the developer.

Now attempting to look at it from Dave's direction, value semantics apply to the variable's type, not the object's physical representation:

let v2 = v1
assert(v1 == v2)

If everything is a value, then this always works. Great!

If the variable's type does not allow mutating shared state, then operations on the variable are operating on a value.

protocol ValueP {
  func compute() -> Result // nonmutating

func g(v1 : ValueP) {
  let v2 = v1
  assert(v1 == v2)

Nice. ‘compute' cannot change the value. Those value semantics do not tell me anything about shared state or function purity. For that, I need some additional constraint on 'compute'. Knowing that it does not mutate the 'self' value is insufficient.

One way of doing that, for example, is to declare that 'compute' transitively cannot access globals *and* ValueP must be a PureValue. Now I can safely write this:

protocol ValueP : PureValue {
  @strawman_noglobal func compute() -> Result

/// Return (v1.compute, v2.compute)
func g(v1 : ValueP, v2 : ValueP) -> (Result, Result) {
  let r1 = v1.compute()
  if v1 == v2 {
    return (r1, r1)
  return (r1, v2.compute())

So, Dave is right that we need to decide soon whether we can make stronger assumptions about value semantics. But that is a separate question from how to express function purity. I don't think there is any urgency in introducing things like the PureValue protocol or @strawman_noglobals attribute, now that we have clearly established shared-state-mutability-by-default. When we want to seriously have that discussion, we should consider other alternatives. I would prefer to wait until indirect structs and improved CoW support have had more discussion.

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