[generics] cannot use generic type list.Element(type TElem) without instantiation

[aurelien...@gmail.com]

Hi,

Is this expected?

import "list" // for 'std' list.go2

func main() {

type intElement list.Element(int)

_ = make(map[int]intElement) // ok: compiles

// nok

_ = make(map[int]list.Element(int)) // cannot use generic type list.Element(type TElem) without instantiation

}

I don't think it is, but in case it is, how would I instantiate a make(map[int]intElement) without having to typedef a generic type before.

PS: i'm using the latest commit at the time of writing in the dev.go2go branch (6cf6bf162c)

[Ian Lance Taylor]

On Wed, Jul 1, 2020 at 9:39 AM <aurelien...@gmail.com> wrote:
>
> Is this expected?
>
> import "list" // for 'std' list.go2
>
> func main() {
> type intElement list.Element(int)
> _ = make(map[int]intElement) // ok: compiles
>
> // nok
> _ = make(map[int]list.Element(int)) // cannot use generic type list.Element(type TElem) without instantiation
> }
>
> I don't think it is, but in case it is, how would I instantiate a make(map[int]intElement) without having to typedef a generic type before.

From the parser's perspective the argument to make is an expression,
so in this case you need parentheses to keep this from looking like a
type conversion.

make(map[int](list.Element(int)))

Ian

[Andrey T.]

Ian,

continuation question -- does `type intElement list.Element(int)` introduce a new type just like `type myInt int` does? If so, in a previous example, these 2 maps are not of the same type, am I right?

Thank you very much,

  Andrey

On Wednesday, July 1, 2020 at 10:50:07 AM UTC-6, Ian Lance Taylor wrote:

[Ian Lance Taylor]

On Wed, Jul 1, 2020 at 8:22 PM Andrey T. <xnow4f...@sneakemail.com> wrote:
>
> continuation question -- does `type intElement list.Element(int)` introduce a new type just like `type myInt int` does? If so, in a previous example, these 2 maps are not of the same type, am I right?

Yes, "type intElement list.Element(int)" introduces a new type. You
can avoid introducing a new type by using a type alias: "type
intElement = list.Element(int)".

Ian

> On Wednesday, July 1, 2020 at 10:50:07 AM UTC-6, Ian Lance Taylor wrote:
>>
>> On Wed, Jul 1, 2020 at 9:39 AM <aurelie...@gmail.com> wrote:
>> >
>> > Is this expected?
>> >
>> > import "list" // for 'std' list.go2
>> >
>> > func main() {
>> > type intElement list.Element(int)
>> > _ = make(map[int]intElement) // ok: compiles
>> >
>> > // nok
>> > _ = make(map[int]list.Element(int)) // cannot use generic type list.Element(type TElem) without instantiation
>> > }
>> >
>> > I don't think it is, but in case it is, how would I instantiate a make(map[int]intElement) without having to typedef a generic type before.
>>
>> From the parser's perspective the argument to make is an expression,
>> so in this case you need parentheses to keep this from looking like a
>> type conversion.
>>
>> make(map[int](list.Element(int)))
>>
>> Ian
>

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