>>
>> I'm starting to think my original motivation might’ve had something to do
>> with normally needing storage for both some property and a T, but not
>> needing (local) storage for said property if T conformed to a protocol which
>> itself already required conforming types to have that property? Or maybe as
>> a way to have the “bottom” variable in a hierarchy of wrapper types to break
>> the “reference cycle” for some property? (Come to think of it, those could
>> be the same thing)
>>
>> That was (and is) an odd project... Anyway, it's been a while since I've
>> thought about it. I'll try to find time today to poke through some old code
>> and see if still have a copy of what I’d gotten stuck on.
>>
>> And thanks for those links :-)
It sounds like what you want is similar to C++ template specialization (also
something I’ve been asking for). Another slightly different form you can
imagine it taking is:
class Array {
associatedtype StorageType:Storage
subscript(index:[Int]) -> StorageType.ElementType { /* … */ }
}
extension Array where StorageType:FloatStorage {
// define specific variables for this specialization
// and define behavior for subscript
}
Another example might be (if ints were allowed as generic parameters):
struct Factorial<N:Int> {
var value { return N * Factorial <N-1>.value }
}
struct Factorial<N:Int> where N == 1 {
var value { return 1 }
}
let value = Factorial<10>.value
The first example can in theory be done using runtime information (though as
stated in my previous posts, I still can’t get it to work correctly in Swift).
The second clearly needs to be done at compile time and could potentially
benefit from the `constexpr` discussed in the `pure` function thread. A
slightly different formulation could be:
constexpr factorial<N:Int>() { return N*factorial<N-1>() }
constexpr factorial<N:Int>() where N == 1 { return 1 }
Right now generics in Swift feel closer to Java’s generics than c++ templates.
I think these types of constructs are extremely useful to a language and would
disagree with anyone who says they aren’t needed (e.g. look at Boost and
Eigen). However, I can also appreciate that adding these features to a language
probably should be done with lots of care and thought.
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