[swift-evolution] Proposal: Add generator functions to the language

[Alex Gordon]

IMO the yields should be literally *in* the return type. The present state
of things is

fn countToTen() -> CountToTenGenerator

Generator functions are normal functions. What is special about them is
that instead of returning a named type such as "CountToTenGenerator", the
compiler replaces the return type with an anonymous type conforming to
GeneratorType. So conceptually it's the return type that is special, not
the function.

Try either one of these for size

fn countToTen() -> yields Int {
fn countToTen() -> yields<Int> {

- Alex

On Sat, Dec 12, 2015 at 3:17 AM, Joe Groff via swift-evolution <
swift-evolution at swift.org> wrote:

>
> On Dec 11, 2015, at 6:26 PM, Jordan Rose via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> Eh, I was trying to avoid grabbing another keyword, but I guess it's
> context-sensitive anyway.
>
>
> I've thought about this some. It might not have to be a keyword, if this
> were a generalized language feature. Anything that interrupts control flow
> and optionally resumes it later, such as 'throws', 'yields', and
> potentially also 'async', could be implemented as instances of algebraic
> effects. As a rough sketch of an idea, you could declare an effect and its
> operations:
>
> effect throws { @noreturn operation throw (ErrorType) -> () }
> effect yields<T> { operation yield (T) -> () }
> effect awaits { operation await<T> (Promise<T>) -> T }
>
> and 'catch' could be generalized to let you handle any effect operations
> that might be performed in the body of a block:
>
> class Generator<T> {
> var generator: () yields<T> -> ()
> func next() -> T? {
> do {
>    generator()
> return nil
> } catch yield (let x) {
> generator = currentContinuation
> return x
> }
> }
> }
>
> See Eff (http://www.eff-lang.org) for an example of a language with this
> already implemented.
>
> -Joe
>
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