On Thu, Jun 18, 2020, 13:21 T L <tapir....@gmail.com> 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.
>
>
>>
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>>
>
>
>
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