> On Feb 21, 2017, at 8:57 PM, Michel Fortin via swift-evolution
> <swift-evolution@swift.org> wrote:
>
> (this was accidentally sent off-list, reposting here a day later)
And this was my reply.
>
>> Le 20 févr. 2017 à 12:17, Matthew Johnson <matt...@anandabits.com> a écrit :
>>
>>> e) Generic Containers:
>>> Generic containers that impose requirements on their elements will pose
>>> some additional problems, for instance: `Set.insert` needs to call
>>> `hashValue` and `==` on its elements, making the purity of `Set.insert`
>>> constrained by the purity of those functions. Without a way to express this
>>> kind of conditional purity, `Set` and `Dictionary` cannot be pure.
>>
>> Could we use a mechanism similar to `rethrows` to address this kind of
>> transitive purity? We have already discussed something along those lines
>> for handling functions passed as arguments. Maybe that mechanism could be
>> designed to handle this use case as well.
>
> Similar, yes. But more complicated too. In pseudo code, this is what you'd
> have to express for `Set.insert`:
>
> pure(where: Element.hashValue is pure, Element.== is pure)
> func insert(_ element: Element) { ... }
>
> Then the compiler can enforce that `insert` only does things that are allowed
> in a pure function, but can ignore the purity of Element.hashValue and
> Element.== because the caller will check for that that.
Yep, that’s the kind of thing I had in mind.
I can imagine being this explicit could get pretty cumbersome though. I wonder
if there is a way to streamline this. For example, in this case if we could
just talk about `Element`’s conformance to `Hashable` and we could talk about a
`pure` conformance (all requirements are met with pure implementations) then we
could just say something like this:
pure(Element.Hashable)
func insert(_ element: Element) { ... }
Note: I also removed some redundancies - we could just say that the parameters
are a list of things the purity of which `insert`’s purity depends.
For function arguments it would look something like this:
pure(transform)
func map<T>(transform: Element -> T)
>
> --
> Michel Fortin
> https://michelf.ca
>
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