Here's the code for the little meta-programming tool, SwiftInSwift:
https://gist.github.com/anonymous/07d9df1a80820bb5abf5a2c671fd223f
/Jens

On Tue, Sep 20, 2016 at 6:28 PM, Jens Persson <j...@bitcycle.com> wrote:

> You can put DEF-blocks and PRINT-blocks in your code, eg:
>
> // DEF-{
> func generateSomeCode() -> [String] {
>     var linesOfCode = [String]()
>     // ... fill linesOfCode with some interesting code ...
>     return linesOfCode
> }
> // }-DEF
>
> // PRINT-{ generateSomeCode()
> // The result of the print-block-expression will
> // replace these lines when cmd+B is pressed.
> // }-PRINT
>
> When you press cmd+B, the meta-programming-tool will put together a Swift
> script of the DEF-blocks and PRINT-block-expressions, and evaluate the
> expressions of the PRINT-blocks, which can be any expression that resolve
> into a [String], ie the lines of code which will replace the content of the
> PRINT-block.
>
> /Jens
>
>
> On Tue, Sep 20, 2016 at 4:34 PM, Vladimir.S <sva...@gmail.com> wrote:
>
>> On 20.09.2016 16:43, Jens Persson via swift-evolution wrote:
>>
>>> Sure, but the reason to go for C++ in this case would only be to be able
>>> to
>>> use eg its templates and constexprs, things that doesn't translate well
>>> into Swift. And I think it's a long term goal of Swift to become a
>>> systems
>>> language.
>>>
>>> We ended up making a meta-programming-tool that we use as a Build Phase,
>>> before compilation, that lets us write code-generating Swift code, within
>>> our ordinary Swift code. (A bit like GYB but Swift-only, using just
>>> regular
>>> Swift within our regular Swift source files.)
>>>
>>> This DIY meta programming facility let's us overcome the current
>>> limitations of Swift's type system in a somewhat convenient/nice way.
>>>
>>
>> Very interesting. Could you share some examples of how your source code
>> looks like(this "code-generating Swift code") and what is produced by this
>> "meta-programming-tool" ?
>>
>>
>>> /Jens
>>>
>>>
>>> On Mon, Sep 19, 2016 at 10:07 PM, Goffredo Marocchi <pana...@gmail.com
>>> <mailto:pana...@gmail.com>> wrote:
>>>
>>>     If you have to compromise that much, it makes for a very compelling
>>>     case to go for C++ wrapped in Objective-C(++) as far as that section
>>> of
>>>     the code is concerned and call it from Swift using the already
>>> provided
>>>     bridging support.
>>>
>>>     I do not think anyone will question the purity of our bodily
>>>     fluids/minds if we do not write 100% of code in Swift :), support for
>>>     interoperability with other languages is there for a reason IMHO and
>>>     should be expanded and not begrudged.
>>>
>>>     Sent from my iPhone
>>>
>>>     On 19 Sep 2016, at 14:14, Jens Persson via swift-evolution
>>>     <swift-evolution@swift.org <mailto:swift-evolution@swift.org>>
>>> wrote:
>>>
>>>     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
>>>>     <devteam.cod...@gmail.com <mailto:devteam.cod...@gmail.com>> wrote:
>>>>
>>>>
>>>>         On Sep 17, 2016, at 6:37 PM, Jens Persson via swift-evolution
>>>>>         <swift-evolution@swift.org <mailto:swift-evolution@swift.org>>
>>>>>
>>>>>         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-Mo
>>>>> n-20160411/015089.html
>>>>>         <https://lists.swift.org/pipermail/swift-evolution/Week-of-M
>>>>> on-20160411/015089.html>
>>>>>         )
>>>>>
>>>>>         Thanks,
>>>>>         /Jens
>>>>>
>>>>>         _______________________________________________
>>>>>         swift-evolution mailing list
>>>>>         swift-evolution@swift.org <mailto:swift-evolution@swift.org>
>>>>>         https://lists.swift.org/mailman/listinfo/swift-evolution
>>>>>         <https://lists.swift.org/mailman/listinfo/swift-evolution>
>>>>>
>>>>
>>>>
>>>>     _______________________________________________
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>>>>     swift-evolution@swift.org <mailto:swift-evolution@swift.org>
>>>>     https://lists.swift.org/mailman/listinfo/swift-evolution
>>>>     <https://lists.swift.org/mailman/listinfo/swift-evolution>
>>>>
>>>
>>>
>>>
>>>
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>>>
>>>
>
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