On 16 Mar 2017, at 13:27, David Hart wrote:
> On 16 Mar 2017, at 20:55, Itai Ferber via swift-evolution
<[email protected]> wrote:
I’m going to reply to this thread as a whole — apologies if
there’s someone’s comment that I’ve missed.
This is something that has come up in internal review, and we’ve
certainly given it thought. As Zach has already mentioned, the
primary concern with overloading based on return type is ambiguity.
There are many cases in which Swift’s type system currently does
not handle ambiguity in the way that you would expect, and it can be
very surprising. For instance,
func foo() -> Int { return 42 }
func foo() -> Double { return .pi }
func consumesInt(_ x : Int) { print(x) }
let x = foo() // Ambiguous use of foo()
consumesInt(x) // Even though x is going to be used as an Int
let y: Int = x // Same here
let x = foo() as Int works now, but it actually didn’t always —
until a somewhat recent version of Swift AFAICT, the only way to
resolve the ambiguity was through let x: Int = foo(). This has since
been fixed, but it was very confusing to try to figure out the
unambiguous way to call it.
Keep in mind that this isn’t an unreasonable thing to want to do:
struct Foo {
var x: Int
init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy:
CodingKeys.self)
// Want to process an element before it’s assigned.
let x = container.decode(forKey: .x) // Ambiguous call
// Or whatever.
if x < 0 {
self.x = x + 100
else {
self.x = x * 200
}
}
}
You can write let x: Int = container.decode(…) or let x =
container.decode(…) as Int, but this isn’t always intuitive.
That’s where I disagree. Let me try to prove my point:
You bring up the example of having to store the decoded value in a
variable before setting it to a typed property. But its also not
unreasonable to want to do the same thing when encoding the value,
possibly storing it into a different type. If we follow that argument,
its also not very intuitive to have to do
container.encode(x as Double, forKey: .x).
Wouldn’t that be an argument to have an API like this:
func encode<T>(_ value: Data?, forKey key: Key, as type: T.Type)
throws
I don’t agree that these are equivalent cases.
Here, for an `as` cast to be valid, the type of `x` must be an
existential (I’m guessing `Any`).
The original `container.encode(x, forKey: .x)` call is not ambiguous
because `x` has no type, but rather because the type of `x` does not
match any of the overloads. You would get the same error as if you wrote
```swift
struct NonCodableFoo {}
let x = NonCodableFoo()
container.encode(x, forKey: .x)
```
You have to convert the type to something that fits one of the
overloads.
On encode, there cannot be any true ambiguity because it’s not
possible to satisfy more than one of these concrete overloads. You
cannot have a thing with a type which would satisfy both, say, `Int` and
`Double`.
I would argue that type inference is a core feature in Swift and that
we should embrace it. I believe that in most cases the return value of
encode will be stored into a typed property and type inference will do
the right thing. In the few cases where the type has to be enforced,
the patterns you mention above are not weird syntax; they are used and
useful all over Swift:
Sure, but I think these cases are not equivalent.
let cgFloat: CGFloat = 42
42 has a default value of `Int`, but since `CGFloat` is
`ExpressibleByIntLiteral`, this becomes the equivalent of writing `let
cfFloat = CGFloat(42)`, which would not be ambiguous without the
`CGFloat()`; you would just get an `Int`.
With `let x = container.decode(forKey: .x)`, has _has no type_ unless
otherwise specified.
let pi = 3.14159265359 as Float
Same here, but with `Double` instead of `Int`, and `Float` instead of
`CGFloat`…
let person = factory.get<Person>() // potential feature in Generics
Manifesto
This isn’t type inference. This is type specification, which is
exactly what we are trying to do. At the moment, explicit type
specification has a different syntax: passing a metatype as an argument.
If this feature were available, this is what we would use.
The way I think about it is that the type argument is already there as
a generic parameter. Adding an extra argument that needs to be
explicitly given on every single call feels like unneeded verbosity to
me.
For consideration: why does `let person = factory.get<Person>()` seem
reasonable, but `let person = factory.get(Person.self)` does not?
Consider also that the metatype would also be necessary for
decode<Value : Codable>(_ type: Value.Type, forKey: Key) -> Value
because the return value of that certainly could be ambiguous in many
cases.
Finally, the metatype arg allows you to express the following
succinctly: let v: SuperClass = container.decode(SubClass.self,
forKey: .v).
In the general case (decode<Value : Codable>) we would need the
metatype to avoid ambiguity. It’s not strictly necessary for
primitive types, but helps in the case of ambiguity, and solves the
conceptual overhead of "Why do I specify the type sometimes but not
others? Why are some of these types special? Should I always provide
the type? Why wouldn’t I?"
Matthew offered func decode<T>(_ key: Key, as type: T.Type = T.self)
throws -> T which looks appealing, but:
Doesn’t help resolve the ambiguity either
Allows for 3 ways of expressing the same thing (let x: Int =
decode(key), let x = decode(key) as Int, and let x = decode(key, as:
Int.self))
The cognitive overhead of figuring out all of the ambiguity goes away
when we’re consistent everywhere.
FWIW, too, I am not convinced that Foundation should add API just
because 3rd parties will add it.
Agreed. Foundation should not add API just because 3rd parties do it.
But 3rd parties should not be dismissed entirely nonetheless. They are
a good breeding ground for ideas to spawn and shape Swift in
interesting ways.
True. I don’t want to seem dismissive of third parties here. The
limitations that we operate within are more conservative than those
third parties tend to work within; we would prefer to offer consistency
over concision.
The ambiguity in the general case cannot be solved by wrappers, and I
would prefer to provide one simple, consistent solution; if 3rd
parties would like to add wrappers for their own sake, then I
certainly encourage that.
On 16 Mar 2017, at 11:46, Matthew Johnson via swift-evolution wrote:
On Mar 16, 2017, at 1:34 PM, Zach Waldowski via swift-evolution
<[email protected] <mailto:[email protected]>>
wrote:
On Thu, Mar 16, 2017, at 02:23 PM, Matthew Johnson via
swift-evolution wrote:
I don’t have an example but I don’t see a problem either.
There are two options for specifying the return type manually. We
can use the signature you used above and use `as` to specify the
expected type:
let i = decode(.myKey) as Int
The awkwardness of this syntax is exactly what I'm referring to.
Would a beginner know to use "as Int" or ": Int"? Why would they?
The "prettiness" of the simple case doesn't make up for how
difficult it is to understand and fix its failure cases.
Any official Swift or Foundation API shouldn't, or shouldn't need
to, make use of "tricky" syntax.
I don’t think this is especially tricky. Nevertheless, we can
avoid requiring this syntax by moving the type argument to the end
and providing a default. But I think return type inference is worth
supporting. It has become widely adopted by the community already in
this use case.
If we don’t support this in Foundation we will continue to see
3rd party libraries that do this.
The proposal's been out for less than 24 hours, is it really
productive to already be taking our ball and go home over such a
minor thing?
I don’t think that’s what I’m doing at all. This is a
fantastic proposal. I’m still working through it and writing up my
more detailed thoughts.
That said, as with many (most?) first drafts, there is room for
improvement. I think it’s worth pointing out the syntax that many
of us would like to use for decoding and at least considering
including it in the proposal. If the answer is that it’s trivial
for those who want to use subscripts to write the wrappers for return
type inference and / or subscripts themselves that’s ok. But
it’s a fair topic for discussion and should at least be addressed
as an alternative that was rejected for a specific reason.
Zach Waldowski
[email protected] <mailto:[email protected]>
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