On Mon, 24 Aug 2020 at 12:57, 'Richard Oudkerk' via golang-nuts <
[email protected]> wrote:
>
> Applying the same rule to type lists and type switches you should be able
> to write
>
> type Comparer[T any] interface {
> Compare(T) int
> }
>
> type CompareConstraints interface {
> type int, int8, …, float64, string, Comparer
> }
>
> func Min[T CompareConstraints](x, y T) bool {
> switch T {
> case int:
> …
> …
> case Comparer:
> if x.Compare(y) < 0 {
> return x
> }
> return y
> }
> }
>
> But the current proposal does not allow interfaces in type lists.
>
FWIW, even if the proposal did allow interfaces in type lists, you wouldn't
be able to invoke the Compare method on x, because both:
a) the allowed operations on T are the intersection of all the operations
in the type list, and only one of those types has a Compare method
and
b) according to the first post in this thread, methods are specifically
excluded from the operations allowed by a type list.
> On Monday, 24 August 2020 at 07:09:15 UTC+1 rog wrote:
>
>> On Mon, 24 Aug 2020 at 06:35, Denis Cheremisov <[email protected]>
>> wrote:
>>
>>> I probably didn't read what you have wrote in the first message carefuly
>>> enough. Does it mean something like that will work
>>>
>>> type SomeTypes interface {
>>> type int, float32, float64
>>> }
>>>
>>> func Min[T SomeTypes](x, y T) T {
>>> switch T {
>>> case int:
>>> if x < y {
>>> return x
>>> }
>>> return y
>>> case float32:
>>> return math.Min(float64(x), float64(y))
>>> case float64:
>>> return math.Min(x, y)
>>> }
>>> }
>>>
>>
>> This was discussed above.
>>
>> I don't believe you can do that. I didn't see any suggestion that knowing
>> the type implies the ability
>> to convert from the generic type to the known underlying type.
>>
>>>
>>> Would something like below work as well?
>>>
>>> type Compare[T any] interface {
>>> Compare(x, y T) int
>>> }
>>>
>>> type CompareConstraints[T any] {
>>> type int, int8, …, float64, string, Compare[T]
>>> }
>>>
>>> func Min[T CompareConstraints]Min(x, y T) bool {
>>> switch T {
>>> case int:
>>> …
>>> …
>>> case Compare[T]:
>>> return x.Compare(y) < 0
>>> }
>>> }
>>>
>>
>> No. As proposed, the type switch doesn't change anything about the type T.
>>
>> Also, AIUI that type list wouldn't be very useful in practice, because
>> using that interface type in the type list would only allow types whose
>> underlying type is exactly Compare[T], which doesn't include any
>> non-interface types or interface types that have more methods than just
>> Compare.
>>
>> cheers,
>> rog.
>>
>> --
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