[swift-dev] Advice for implementing "literal values as generic types"

David Sweeris davesweeris at mac.com
Tue Aug 29 16:21:42 CDT 2017

> On Aug 29, 2017, at 1:49 PM, Slava Pestov <spestov at apple.com> wrote:
>> On Aug 29, 2017, at 11:03 AM, David Sweeris via swift-dev <swift-dev at swift.org> wrote:
>> Hi everyone! I'm trying to implement literal values as generic types.
> Can you briefly explain what you mean by this?
> Are you referring to let-polymorphism, like
> let fn = { $0 }
> let f1: (Int) -> Int = fn
> let f2: (Float) -> Float = fn

No, I mean so that a vector's or matrix's dimensions can be part of its type (strawman syntax and protocol name, but this is pretty much what I'll be trying to support, at least at first):
struct Vector<T: ExpressibleByIntegerLiteral, L: IntegerLiteralExpr> {
  var elements: [T]
  init() {
    elements = [T](repeating: 0, count: L)

let vect = Vector<Int, 5>()

And, once that's working, I'm going to add support simple "type functions":
func join <T, L1, L2> (_ lhs: Vector<T, L1>, _ rhs: Vector<T, L2>) -> Vector<T, L1 + L2 > {...}
I think restricting the supported "type functions" to expressions that could be evaluated by the compiler's "constant folding" code would be a reasonable place to start, until we figure out what we want to do about "pure"/"constexpr" stuff... even just "+" for numeric and string literals, and "-" for numeric literals, seems like a reasonable starting goal, and I think that'd be simple enough to implement (famous last words, right?)... It's all academic until I get the simple cases working first, though.

Anyway, I think it'll be worth writing up as a proposal, once it's working and I've figured out how to present these "literal types" to Swift's type system (I'd like to keep it as a literal of some sort, so that, say, `L` in this example could be used to set the value of any type that conforms to `ExpressibleByIntegerLiteral`).

- Dave Sweeris

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