Ok, thanks! I take it that we should not expect any dramatic advances
of Swift's type system any time soon.
Reason for asking is that we are trying to write an API for
N-dimensional graphics/audio/signal/data processing.
Metal, vDSP, simd, etc. would perhaps be used, but only behind the
scenes, eventually, as necessary, since we want something more
uniform and math-like, thus allowing for a more rapid experimental
style of coding, where you can quickly try something out for a
different number of dimensions, etc.
This has turned out to be impossibly hard to write in current Swift,
unless you are willing to either
1. Forget about performance and type safety, ie use a standard Array
(instead of a static vector with type-level Count as well as Element)
for N-dimensional positions, matrices, vectors, indices, etc.
2. Forget about code reuse / abstractions.
Option 1 is not an alternative. We want to let the compiler (and our
code) know/optimize as much as possible, otherwise it will be
unusably slow even for ("rapid") prototyping.
So we'll probably go with option 2 and spell out / generate code for
each and every permutation of
(dim, data-structure, function/algorithm), and sadly this will also
be necessary for every piece of code that uses the API, since it is
impossible to write eg
A generic StaticVector type with type parameters for its Count and
Element.
A generic N-dimensional array type with type parameters for its
(NDim)Index: StaticVector (where Index.Element == Int)
and
Element
Or we'll have to use (Obj) C++ : /
/Jens
On Mon, Sep 19, 2016 at 3:22 AM, Robert Widmann
<[email protected] <mailto:[email protected]>> wrote:
On Sep 17, 2016, at 6:37 PM, Jens Persson via swift-evolution
<[email protected] <mailto:[email protected]>>
wrote:
Has there been any discussions about the possibility of having
generic associatedtypes?
I (naively) think that it would open up a lot of possibilities.
Because if, for example, we could do this:
protocol CountType {
associatedtype Storage<E>
...
}
Then we could do this:
struct Count1 : CountType {
typealias Storage<E> = (E)
...
}
struct Count2 : CountType {
typealias Storage<E> = (E, E)
...
}
struct Count3 : CountType {
typealias Storage<E> = (E, E, E)
...
}
...
protocol StaticArrayType {
associatedtype Count: CountType
associatedtype Element
...
}
struct StaticArray<C: CountType, Element> : StaticArrayType {
typealias Count = C
var storage: C.Storage<Element>
...
}
Would adding support for generic associatedtypes be possible?
Are there any plans for it?
Possible, yes, plans, no.
Generic associated types go part and parcel with higher-kinded
quantification and higher-kinded types, the implementation
challenges of which have been discussed thoroughly on this list
and elsewhere. Is there a particular flavor you had in mind?
One major problem is that presumably you’d want to constrain such
a generic associatedtype and then we’d have to have some kind of
type-level-yet-runtime-relevant apply of a generic witness table
to another potentially generic witness. It’s not clear what that
kind of thing would look like, or how far it would have to be
taken to get the kind of support you would expect from a basic
implementation higher associatedtypes. Implementations in
languages like Haskell tend to also be horrendously inefficient -
I believe Edward Kmett calls is the “Mother May I” effect of
forcing a witness table to indirect through multiple layers of
the witness because inlining necessarily fails for the majority
of these things in the MTL.
tl;dr Basic examples like the ones you cite hide the kinds of
tremendously evil fun things you can do once you have these kinds
of features.
(
I tried searching for it but I found only this:
https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160411/015089.html
<https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160411/015089.html>
)
Thanks,
/Jens
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