On Dec 6, 2015, at 11:48 PM, Manav Gabhawala via swift-evolution < > swift-evolution@swift.org> wrote:
I created a pull request with the proposal: Implicit Initializer proposal
<https://github.com/apple/swift-evolution/pull/37> (
https://github.com/apple/swift-evolution/pull/37\). Feel free to respond
with suggestions/ideas with improvements to the proposal and things that
may be missing.
I have two comments. The first is on the suitability of this proposal for
Swift as a language; the second is about technical feasibility of the
proposal.
Regarding suitability for Swift: for the most part, Swift’s type system
has a set of subtyping relationships that allow implicit conversions
(during compilation) that can be safely reversed via conditional
downcasting (the “as?” operator). For example, say I have:
protocol P { }
struct X : P { }
I can turn an X into a P:
var p: P = X()
and then reverse the operation with a cast:
if let x = p as? X { … }
It’s a rather beautiful and powerful symmetry to the language. If we add
implicit conversions:
struct X {
implicit init(_ y: Y) { … }
}
struct Y { }
var x: X = Y()
Should we allow
let y = x as? Y { … }
to work? If no, we’ve lost some useful symmetry to the type system.
If yes, we first need to figure out how the user can implement it… perhaps
some opposite operation that can fail:
extension Y {
init?(castingFrom: X) { … }
}
but how can we implement this in an efficient manner in the runtime? We
would effectively have to represent the DAG of implicit conversion
relationships in runtime data structures and evaluate those with every
“as?” cast, which implies a heavy runtime burden for using this feature.
On technical feasibility: introducing user-defined conversions is a major
complication to the type system, particularly in a language with general
type inference. We did go down this route (there are vestiges of it still
remaining in the expression type checker), and it added a huge amount of
complexity to the type checker that compromised it’s ability to provide
good type inference. We would need a massive improvement to the expression
type checker for this to become technically feasible, and I’m not sure we
even can get there. Swift has a complicated type system already—general
type inference, parametric polymorphism, ad hoc overloading, general
subtyping—and the type checker is one of the weaker areas of the
implementation in part because of that complexity.
I don’t think we can bring a proposal up for review when there is a
significant chance that it cannot be faithfully implemented. Obviously,
asking you to “go implement a revolutionary new type checker” isn’t a great
response, either, which leaves us in the unfortunate position of looking
for some kind of assurance that, should we decide we want to go in this
direction, we actually *can* go in this direction.
- Doug
Regards,
Manav Gabhawala
On December 6, 2015 at 5:33:37 PM, Matthew Johnson via swift-evolution ( > swift-evolution@swift.org) wrote:
I would be happy with any solution that provides failable conversion so I
would support that as well.
Any chance we could see something like this in Swift 2.2 or Swift 3? I was
hoping the failable initializers might be low hanging fruit but it seems
like the scoped feature would be more than that.
Sent from my iPad
> On Dec 6, 2015, at 4:04 PM, Chris Lattner <clattner@apple.com> wrote:
>
> Could definitely be interesting. I’d personally like to turn it into a
scoped feature along the lines of this post though:
>
>
[swift-evolution] Failable arithmetic
>
> -Chris
>
>> On Dec 6, 2015, at 5:29 AM, Matthew Johnson <matthew@anandabits.com> > wrote:
>>
>> Related to this, but maybe easier to do in the short term: what do you
think of adding failable overloads to the numeric conversion initializers
Chris?
>>
>> They would throw or return nil if the runtime value could not be
preserved rather than trap, truncate, etc. Floating point types might allow
a tolerance for small value changes due to precision, although I'm not sure
if that would be good or not. These initializers would be very useful when
processing data from an external source such as JSON that has an expected
type but cannot be trusted.
>>
>>
>>
>> Sent from my iPad
>>
>>>> On Dec 6, 2015, at 1:02 AM, Chris Lattner via swift-evolution < > swift-evolution@swift.org> wrote:
>>>>
>>>> On Dec 5, 2015, at 4:27 AM, Jonathan Hull <jhull@gbis.com> wrote:
>>>> I understand why you can’t auto-convert from a Double to a Float or
Int32 to Int8. It is good that we have to add the cast explicitly and think
though the implications.
>>>>
>>>> …but I don’t think through the implications because we currently have
a boy who cried wolf situation where we have to explicitly cast everything
(even the safe stuff).
>>>>
>>>>
>>>> I think all of the numeric types should be able to auto-convert if
the conversion is safe (without loss of precision or overflow).
>>>>
>>>> For example:
>>>> • If an Int is casting to a larger size (Int16 -> Int32)
>>>> • Float -> Double
>>>> • Float -> CGFloat
>>>> • Int -> Float, Double, or CGFloat (but not the other way)
>>>>
>>>> I don’t see why these aren’t allowed. The forced casts make my code
much less readable. Are the casts above dangerous in a way I am not aware
of?
>>>
>>> I agree that the current Swift numerics model is suboptimal, I
personally would like to see small integers implicitly promote to large
integers (when they are known lossless), have Float promote to Double, and
have both Float and Double promote to CGFloat (yes, I know that the Double
-> CGFloat promotion would be lossy on 32-bit apple platforms). I
personally don’t think that integer -> floating point promotions are a good
idea even if value preserving, since their domains are so different.
>>>
>>> The problem with doing this today is that there are a lot of
dependencies we need to get resolved first.
>>>
>>> 1. The type checker is really slow, partially because of too-many and
too-crazy implicit conversions. We also get very surprising behavior when
they kick in. Specifically, IMO, we need to reevaluate the T! <-> T and T
to T? conversions. We have thoughts on this, but should be discussed in a
separate thread if you’re interested.
>>>
>>> 2. These promotions should be expressible in the library, not hard
coded into the compiler. This means that we would need a language feature
to (e.g.) be able to define subtype relationships between structs. Such a
feature would be generally useful and could allow us to push some of our
existing compiler magic out to the stdlib.
>>>
>>> 3. We want the existing work to revise the numerics protocols to be
better understood and hopefully implemented.
>>>
>>> There are also a ton of unrelated specific problems that should be
addressed in various ways: e.g. macros like M_PI get imported as Double
instead of a typeless literal, forcing tons of casts in code that wants to
use it (e.g.) with Floats. These issues are separable, and blocked on
things like generic properties not being in place.
>>>
>>> It would be great for interested contributors to start pushing on any
of the above issues to help unblock progress on improving the numerics
model.
>>>
>>> -Chris
>>>
>>> _______________________________________________
>>> swift-evolution mailing list
>>> swift-evolution@swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>
_______________________________________________
swift-evolution mailing list
swift-evolution@swift.org
https://lists.swift.org/mailman/listinfo/swift-evolution
_______________________________________________
swift-evolution mailing list
swift-evolution@swift.org
https://lists.swift.org/mailman/listinfo/swift-evolution