Add support for generics

[Paddy]

With the release of Go 1.18, interface types can now have type constraints. It would be neat to be able to specify these type constraints when generating the implementation.

[Josh Bleecher Snyder]

Maybe I'm missing something, but impl generates methods, and as of Go 1.18, there are no generic methods.

[Paddy]

Sorry, this probably would've been more helpful with an example, huh?

type Parser[Result any] interface {
    Parse(in []byte) (Result, error)
}

That's valid in Go 1.18, but I can't use impl to generate an implementation of, say github.com/my/package.Parser[string]. You get interface Parser[string] not found: type Parser[string] not found in github.com/my/package.

I would expect it to generate (assuming it was run with s stringParser):

func (s stringParser) Parse(in []byte) (string, error) {
    panic("not implemented")
}

[Josh Bleecher Snyder]

Ah! I see. So you'd also have to tell impl the intended type of Result.

I see the usefulness, although I'm a bit worried about the command line API. There can be an unlimited number of type parameters. Using order isn't a great API, but naming them gets awkward. At some point it'll be easier to write the implementation stubs by hand. :P

I'm not spending much time on impl at this point. If there's a path forward that is simple and clear, I'm happy to review it. If not, I will probably wait until popular demand grows larger. :)

[Paddy]

So I think my preferred way of invoking it would be to just attach the type parameter to the interface type. I don't use impl on the command line, I use it through vim-go, so what it would look like there is:

:GoImpl s stringParser github.com/my/package.Parser[string]

I don't know what the command line equivalent is, sadly. :(

But the above could read that it wants a string from the type parameter, and I think that's a semi-standard way to refer to type parameters? It matches how I'd declare a variable of that type with those parameters, at least: https://go.dev/play/p/XldKb_P2Uvp It was also my first instinct for how to go about doing that.

I haven't looked at impl's source, but would you be amenable to a PR for that functionality if I ever got around to implementing it? (Can't promise I will, but in case I get the time. 😄)

[Josh Bleecher Snyder]

That works for me. I would review such a PR, with the caveat that my turnaround time is typically O(weeks) or even O(months). I'm not proud of that...just being honest.

[Paddy]

The transparency is appreciated and no judgment. Appreciate the willingness to look at it, if I (or someone else) can find the time and energy to put something together :)

[Paddy]

I've opened #49 with my implementation of this feature. I've added some test cases, but was unable to get a test case for a qualified interface working in an automated fashion; the full path worked, and the unqualifed interface worked, but the qualified one couldn't find the testdata package. I have verified it manually, though that's not ideal and it has only been tested with an interface that accepts two type parameters (that's all I had handy).