On Tue, Oct 3, 2023 at 9:06 PM Jon Watte <jwa...@gmail.com> wrote:
> I don't want to include int or struct{} in this case. I care specifically
> about "can be compared to nil"
> The only thing I can do with a type parameter that is only constrained as
> "nil" is compare it to nil, or assign nil to it.
> This means approximately "any reference type" -- interfaces, pointers,
> maps, slices, chans...
>
Exactly. That is the problem. It means your suggestion does not allow a
slew of problems that #61372 <https://go.dev/issue/61372> would solve. It
means your suggestion is less powerful.
> But, let's put this out there:
> How would you, today, write a function that compacted a slice of
> interface, such that any "nil" interface value would be removed from the
> slice?
>
> func Compact[T ?](sl []T) []T {
> j := 0
> for i := 0; i < len(sl); i++ {
> if el := sl[i]; el != nil {
> sl[j] = el
> j++
> }
> }
> return sl[:j]
> }
>
> There's nothing today I can put in place of the "?" to make this
> perfectly-reasonable generic function work.
> I propose I should be able to put "nil" in place of the "?" to make this
> work, and it would work for any reference type (that can be compared or
> assigned to nil.)
>
With #61372, you would put `any` and write `el != zero`. And it would work
for all types, including the ones you mention.
>
> Unless I'm missing something?
>
> Sincerely,
>
> Jon Watte
>
>
> --
> "I find that the harder I work, the more luck I seem to have." -- Thomas
> Jefferson
>
>
> On Mon, Oct 2, 2023 at 9:26 PM Axel Wagner <axel.wagner...@googlemail.com>
> wrote:
>
>> It doesn't solve the problem. That function signature you wrote could not
>> be instantiated by `int`, for example. You can't write `comparable | nil`,
>> as `comparable` is not allowed in a union. And if we allowed it, there
>> would be no way to write the body of the function. It doesn't help with
>> types like `struct{ F func() }`. And it doesn't make it easier to write the
>> zero value for returns etc. And it doesn't address the same kind of issue
>> for generated code.
>>
>> On Tue, Oct 3, 2023 at 3:51 AM Jon Watte <jwa...@gmail.com> wrote:
>>
>>> What's the concern with "a constant interface that is inhabited exactly
>>> by the types that can compare to nil?"
>>>
>>> This is clearly something the language has as a concept -- it knows
>>> whether "foo == nil" is an error or not.
>>>
>>> "nil" could be the ergonomic name for this constraint:
>>>
>>> func Compact[T nil](s []T) ...
>>>
>>> Sincerely,
>>>
>>> Jon
>>>
>>>
>>> On Mon, Oct 2, 2023, 16:53 Ian Lance Taylor <i...@golang.org> wrote:
>>>
>>>> There is a lot more on this topic at https://go.dev/issue/61372.
>>>> We're not sure how best to handle it.
>>>>
>>>> Ian
>>>>
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