On Thursday, June 18, 2020 at 7:20:38 AM UTC-4 tapi...@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.
>
I attempted to write a similar function using assignability rather than
conversion.¹
Neither constraint is possible in the current draft.
(See
https://golang.org/design/go2draft-type-parameters#no-way-to-express-convertability.)
However, I believe that it would be possible (and coherent with the design)
to add built-in interface types for “assignable to T” and “convertible to
T” (but not “assignable from T” or “convertible from T”).
(https://github.com/bcmills/go2go/blob/master/assignable.md contains more
detail.)
¹ My use-cases are the same as those described in:
https://golang.org/doc/faq#convert_slice_of_interface
https://golang.org/doc/faq#convert_slice_with_same_underlying_type
https://golang.org/issue/38753
https://golang.org/issue/38385, esp.
https://github.com/golang/go/issues/38385#issuecomment-612668321
https://golang.org/issue/21651
https://golang.org/issue/3756
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.
>
>
>>
>>> 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".
> 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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