cannot use a type parameter as RHS in type declaration

[Diego Augusto Molina]

Hi everyone! I wanted to ask about the error in the title: "cannot use a type parameter as RHS in type declaration".

My use case: I am implementing range queries on a single dimension, and use a common type, e.g. int64. There is a method that I have to implement that is Contains(v int64) bool. For space efficiency, if for any reason a range of int64 numbers contains a single value, then I make let's say two implementations, one for 32 and one for 64 bytes. If the only value in the range can be represented with an int32, then I use it. Please, bear in mind that this is a simplification of my use case, but it correctly captures my concern.

My attempt:

type oneValue[T interface{int32 | int64}] T

func (x oneValue[T]) Contains(v int64) bool { return v == int64(x) }

As that causes the aformentioned error, my current approach is:

type oneValue[T interface{int32 | int64}] [1]T

func (x oneValue[T]) Contains(v int64) bool { return v == int64(x[0]) }

Of course, this can also be expressed as:

type oneValue[T interface{int32 | int64}] struct {

    Value T

}

func (x oneValue[T]) Contains(v int64) bool { return v == int64(x.Value) }

I found that the two approaches are functionally equivalent for the most part (maybe I'm missing something). Some key points:

1. There doesn't seem to be any struct alignment, space use, or performance difference in neither of the three.

2. For generic versions, in general, a custom (un)marshaler may be needed (a dumb one  pointing the (un)marshaler is enough). For JSON encoding, for example, the first generic version creates a JSON Array with a single element; and the second a JSON Object with a single `Value` member.

Considering the three approaches: (A) non-generic; (B) generic with struct; (C) generic with an array of one element:

1. Can you explain why this error happens ? what problems it would create that was allowed? Any oneliner pointing to docs or just "look for x" is great! I may be missing something too obvious.

2. Are there other atlternatives you can think of, even dumb ones, and how would they compare to the three above?

3. Are there other differences or concerns you can think of, that were not addressed above?

Thank you!

[Ian Lance Taylor]

On Tue, Jul 2, 2024 at 5:36 PM Diego Augusto Molina
<diegoaugu...@gmail.com> wrote:
>
> Hi everyone! I wanted to ask about the error in the title: "cannot use a type parameter as RHS in type declaration".

...

> Considering the three approaches: (A) non-generic; (B) generic with struct; (C) generic with an array of one element:
> 1. Can you explain why this error happens ? what problems it would create that was allowed? Any oneliner pointing to docs or just "look for x" is great! I may be missing something too obvious.
> 2. Are there other atlternatives you can think of, even dumb ones, and how would they compare to the three above?
> 3. Are there other differences or concerns you can think of, that were not addressed above?

See the discussion at https://go.dev/issue/43621.

Ian