On Sun, Apr 4, 2021 at 2:18 PM K. Alex Mills <[email protected]> wrote:
> This seems like a welcome change and leaves the door open for the future
> inclusion of type unions. Nicely done!
>
> Regarding this approximation operator, I have a question about the
> intended meaning of:
>
> ~struct{
> F int
> A string
> }
>
> Would this match the following types?
>
> type A struct{
> F int
> A string
> }
>
Yes.
>
> type B struct{
> A string
> F int
> }
>
No. The underlying types must be identical
<https://golang.org/ref/spec#Type_identity>:
Two struct types are identical if they have the same sequence of fields,
> and if corresponding fields have the same names, and identical types, and
> identical tags.
The order must match.
A good litmus test is that if A's type is a defined type and B's type
isn't, then B is assignable <https://golang.org/ref/spec#Assignability> to
A, if they have identical underlying types.
You can test this like this playground snippet
<https://play.golang.org/p/cqeLv9nlHSn>.
type C struct{
> X int
> Y string
> }
>
No, fields must be named the same.
type D struct{
> F int
> A string
> B float
> }
>
No, numbers of fields must be the same.
My guess is that we're talking about an exact type match, so it would
> definitely match A, I'm not sure about B without checking the compiler, and
> I don't think it would match either C or D under the current proposal.
>
> I think it could potentially be useful to have this approximation operator
> match type D, although that would require modifying the proposal to specify
> some notion of "field sets", which seems like it could be a departure from
> how Go currently views structs and therefore may introduce further
> complications I am not aware of. Just curious if it's already been
> considered and discarded.
>
> Thanks!
>
> On Thu, Apr 1, 2021, 5:59 PM Ian Lance Taylor <[email protected]> wrote:
>
>> We've just posted a potential update to the generics proposal at
>> https://golang.org/issue/45346. This clarifies and simplifies the
>> type lists that appear in interfaces, and let's us drop the "type"
>> keyword used to mark such lists.
>>
>> Ian
>>
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>>
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