On Thursday, June 18, 2020 at 8:01:22 AM UTC-4, Axel Wagner wrote:
>
>
>
> On Thu, Jun 18, 2020, 13:21 T L <tapi...@gmail.com <javascript:>> wrote:
>
>>
>>
>> On Thursday, June 18, 2020 at 5:53:28 AM UTC-4, Axel Wagner wrote:
>>>
>>>
>>>
>>> On Thu, Jun 18, 2020, 11:28 T L <tapi...@gmail.com> wrote:
>>>
>>>> How to declare a generic functions which converting a slice with type
>>>> Ta to a slice with type Tb. Like
>>>> func ConvertSlice(type Ta, Tb constraint)(ins []Ta) []Tb {...}
>>>>
>>>>
>>>>
>> For example, I need a ConvertSlice function which will do
>> * Convert []int to []int64, and the inverse
>> * Convert string to []byte, and the inverse
>> * Convert between several struct types with the same field set, except
>> their field tags are different.
>> * more generally, Convert []Ta to []Tb, and the inverse, where Ta can be
>> converted to Tb.
>>
>
> None of these are interface types. I think I don't understand your
> question. Was there a typo and you meant "integer"? In that case, the
> answer is here:
>
> https://go.googlesource.com/proposal/+/refs/heads/master/design/go2draft-type-parameters.md#type-lists-in-constraints
> You mention four very different use cases. I don't understand how to unify
> them into one question.
>
>
>>
>>
>>> How to constraint a type parameter must be of an interface type?
>>>>
>>>
>>> I don't think you can. Why would you need to? Note that there will
>>> always be constraints that you can't express.
>>>
>>
>> One need is the same as the above Convert case, there should be more.
>>
>> func ConvertSlice(type T, I constraint)(ins []T) []I {...}
>>
>> Where I must/may be some special interface types.
>>
>
> You still haven't said *why* you need it to be an interface type. What
> does knowing it's an interface allow you to do? Why would it break if
> someone passed a concrete type?
>
>
>
>>
>>>
>>>> Is it possible to define generic unnamed types?
>>>>
>>>
>>> No. It also would be useless. You can't refer to a generic,
>>> uninstantiated type. So a generic type either needs a name to be referred
>>> by to instantiate it, or be instantiated right immediately in its
>>> declaration (and thus wouldn't need to be generic).
>>>
>>> If it is impossible, then how to define a generic map type which
>>>> elements must be an unnamed integer slice?
>>>>
>>>
>>> type numeric interface {
>>> type int, int8, int16, int32, ...
>>> }
>>>
>>> type MyMap(type K comparable, T numeric) map[K][]T
>>>
>>
>> Sorry, here I meant "... must be of a slice with an unnamed element type".
>>
>
> I don't think that's possible. And the same question as above applies: Why
> is that an important constraint to be able to express?
>
> I'm sorry, but I just don't understand what you are trying to ask.
>
> Sometimes, the map element slice type itself might be required to be shown
>> as a type parameter.
>>
> One of the case is still related to slice conversions.
>>
>>
>
It is a little hard to describe it clearly. I will continue this question
when I sort it out.
>
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>>
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