[swift-evolution] [Design Question] Metaprogramming as a non-goal

[Thorsten Seitz]

Unfortunately that’s exactly what would make a library with typed units possible (see thread about „Epic: Typesafe calculations“).

-Thorsten


> Am 24.01.2016 um 10:01 schrieb Jacob Bandes-Storch via swift-evolution <swift-evolution at swift.org>:
> 
> I'd guess the point is that the generics system isn't meant to force the compiler to perform the sorts of things C++'s (Turing-complete) template system can:
> 
>     template<size_t... F>
>     struct Fibonacci { };
> 
>     // API
>     template<size_t n>
>     struct Fibonacci<n> {
>         static constexpr size_t value = Fibonacci<n, 1, 0>::value;
>     };
> 
>     // Recursive implementation
>     template<size_t n, size_t Fn, size_t Fm>
>     struct Fibonacci<n, Fn, Fm> {
>         static constexpr size_t value = Fibonacci<n-1, Fn+Fm, Fn>::value;
>     };
> 
>     // Base case
>     template<size_t Fn, size_t Fm>
>     struct Fibonacci<0, Fn, Fm> {
>         static constexpr size_t value = Fn;
>     };
> 
>     int main() {
>         printf("F(200): %zu\n", Fibonacci<200>::value);
>         return 0;
>     }
> 
> Jacob
> 
> On Sat, Jan 23, 2016 at 11:17 PM, Michael Henson via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
> In the swift/docs/Generics.rst documentation, I see:
> 
> "
> As important as the goals of a feature are the explicit non-goals, which we don't want
> or don't need to support:
> * Compile-time "metaprogramming" in any form
> * Expression-template tricks a la Boost.Spirit, POOMA
> "
> 
> What kinds of things count as compile-time metaprogramming? I've been tinkering with some ideas related to the type system and having a more specific description here might help me pare many / most / all of them down.
> 
> Mike
> 
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