To help keep proposals moving forward, the Swift core team has set aside some
time specifically for design discussions of upcoming proposals. Below are some
rough notes from the yesterday's discussion.
These are informal comments, intended to guide the proposals in directions that
draw constructive feedback. You are welcome to ignore the feedback, agree with
it, or disagree with it. As always, the formal decision doesn't happen until
after the review period ends.
SE-0047
<file:///Users/alexmartini/DevPubs%20Git%20Repositories/Swift%20Language%20Review/_build/html/LR_MeetingNotes/MeetingNotes-03-15-2016.html#se-0047>
This is about switching the polarity of @warn_unused_result and renaming it to
@discardable as something on the return type.
Is there a subtype conversion? Is Void -> Int a subtype or supertype of Void ->
Void? No, it’s not; we just have a special rule for closures.
If we put this in the type system, we would want it to be a function attribute
rather than a declaration attribute. It’s not obvious why it’s a bad idea to
have this in the type system... It’s a little bit outside the bounds of what
the type system normally does; it’s not that important of a semantic property.
It looks like the proposal has an error in the second to last line before
“Mutating Variants” – it should either be a hard error or nothing. Also the
case below:
let x = f
x()
c = x()
In the more general case, if x is a closure...
How many cases are there where you want to mark the result as discardable so
you don’t have to write _ = foo() to explicitly silence the warning about an
unused result?
We almost certainly don’t want to allow overloading based on this. Does the
propagation even work? If you have a compose operator that takes T -> U and U
-> V and try to pass it a T -> discardable U you won’t get a discardable V
unless we make it a full qualifier in the type system.
Ok... if we don’t make it part of the function type, where would we want the
attribute to go? This is similar to how we place inout. It would be similar to
how we place @autoclosure but that attribute is currently not quite right.
It would be consistent to put it on the function declaration. There are other
places though where we might want attributes on the function type that are
understood to not effect the actual function type, such as some kind of
ownership rules.
func f () -> @discardable Int // we're saying this is valid
let x: () -> @discardable Int // but we wouldn't let you write this
func g() -> @convention(block) () -> () {...} // we wouldn't allow this either
In that last case, you can put attributes on types. We like how putting the
@discardable on the type because of how it reads, but it’s not on the type.
Counterpoint: It’s actually more semantically part of the function. You don’t
have a “discardable integer”, you have a function whose result is discardable.
SE-0042
<file:///Users/alexmartini/DevPubs%20Git%20Repositories/Swift%20Language%20Review/_build/html/LR_MeetingNotes/MeetingNotes-03-15-2016.html#se-0042>
Right now if you access an instance method on a type, you get this curried
thing that accepts “self” and then the members. It’s not useful for the
standard library because they want flattened functions. And for mutable
methods, because of how the in-out duration lasts, we end up with a miscompile
that scribbles over random memory.
Looking to the future, we can get operators on the type. Then operators would
have the same type in both cases.
We probably don’t need migration. If you wrote #selector(foo as Bar) before,
the Bar part will change. But that’s not very common and #selector is still
pretty new.
SE-0043
<file:///Users/alexmartini/DevPubs%20Git%20Repositories/Swift%20Language%20Review/_build/html/LR_MeetingNotes/MeetingNotes-03-15-2016.html#se-0043>
This seems like a simple generalization of our rules. People coming from the ML
family would just expect this to work. It’s a resonable expectation.
SE-0048
<file:///Users/alexmartini/DevPubs%20Git%20Repositories/Swift%20Language%20Review/_build/html/LR_MeetingNotes/MeetingNotes-03-15-2016.html#se-0048>
This is straightforward and 95% implemented. The contentious point is that it
starts simple: you can’t add constraints to the type alias.
We want to avoid type-based metaprogramming.
Unclear if this would work:
typealias StringDictionary<T> = Dictionary<String, T>
It need to infer that T must be hashable. Maybe it’s only 85% implemented.
SE-0049
<file:///Users/alexmartini/DevPubs%20Git%20Repositories/Swift%20Language%20Review/_build/html/LR_MeetingNotes/MeetingNotes-03-15-2016.html#se-0049>
In short, @noescape and @autoclosure are attributes on the parameter. We got
away with it before, but now that we’re requiring you to manually uncurry, you
can’t write things that you used to be able to write. None of this makes sense;
we should only accept these attributes in the type position.
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