Generic type alias with type conversion

[hey...@gmail.com]

Not sure generic type alias is the solution, but what I'm trying to do is I have a type like this

type Map[K comparable, V any] struct { m map[K]V }

and I want it to implement json.Unmarshaller to decode json objects, when K's underlying type is string:

type omap[K ~string, V any] = Map[K, V]

func (m *omap[K, V]) UnmarshalJSON(data []byte) error {

  k := "a"

  v :=1

  m[K(k)] = v

}

Go 1.23 with the aliastypeparams flag enabled, currently complains "cannot convert k (variable of type string) to type K".

I wonder if using alias this way is wrong and if there exists another solution?

[Ian Lance Taylor]

A type alias is just an alias. When you declare a method on the alias
map, you are declaring a method on the type Map. That means that the
~string in the omap definition is ignored for the method definition;
all that matters is the comparable constraint in the Map definition.
You can't in general convert a string to a comparable type.

Ian

[hey...@gmail.com]

Thanks for the reply.

Does that mean it's currently not possible to conditionally implement a method based on type parameters? And the solution should probably be that I use CanConvert from reflect do that at runtime?

[tapi...@gmail.com]

Why do you have to use alias here? What is the problem if omap is not an alias?

[tapi...@gmail.com]

On Monday, November 4, 2024 at 1:47:30 PM UTC+8 Ian Lance Taylor wrote:

There is a little weirdness here. Maybe delcaring method on geneirc aliases should be forbidden.

[Ian Lance Taylor]

On Mon, Nov 4, 2024 at 2:40 AM hey...@gmail.com <hey...@gmail.com> wrote:
>
> Does that mean it's currently not possible to conditionally implement a method based on type parameters?

I think that is https://go.dev/issue/65394.

Ian

> On Monday, November 4, 2024 at 1:47:30 PM UTC+8 Ian Lance Taylor wrote:
>>
>> On Sun, Nov 3, 2024 at 12:41 AM hey...@gmail.com <hey...@gmail.com> wrote:
>> >
>> > Not sure generic type alias is the solution, but what I'm trying to do is I have a type like this
>> >
>> > type Map[K comparable, V any] struct { m map[K]V }
>> >
>> > and I want it to implement json.Unmarshaller to decode json objects, when K's underlying type is string:
>> >
>> > type omap[K ~string, V any] = Map[K, V]
>> > func (m *omap[K, V]) UnmarshalJSON(data []byte) error {
>> > k := "a"
>> > v :=1
>> > m[K(k)] = v
>> > }
>> >
>> > Go 1.23 with the aliastypeparams flag enabled, currently complains "cannot convert k (variable of type string) to type K".
>> >
>> > I wonder if using alias this way is wrong and if there exists another solution?
>>
>> A type alias is just an alias. When you declare a method on the alias
>> map, you are declaring a method on the type Map. That means that the
>> ~string in the omap definition is ignored for the method definition;
>> all that matters is the comparable constraint in the Map definition.
>> You can't in general convert a string to a comparable type.
>>
>> Ian
>

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