Maps and Slices don't have a lot of operations in common. In general, the
only ones I can think of are
- `make(T, someInt)`, though it means *very* different things for both, so
a generic function doing this is likely not super sensible
- `for _, v := range x`, though even this differs, as the order is not
guaranteed to be deterministic for maps, but it is for slices
- `x == nil`, `len(x) == 0`
If you'd restrict the key-type of the map to be of integer type, there's
one (?) more
- `x[i]` for the key-type. But only on the RHS, as it's not addressable for
maps.
I might overlook something, but in any case, the set of operations a
generic function could do with an argument constrained like this is
extremely limited. I guess the most useful way to express the constraint
would be
type SliceOrMap(type K constraints.Integer, V interface{}) interface {
type map[K]V, []V
}
But really, given that this is such a useless constraint, I feel it's fine
that it's hard to express nicely.
On Sat, Jun 20, 2020 at 9:08 PM David Finkel <david.fin...@gmail.com> wrote:
>
>
> On Sat, Jun 20, 2020 at 11:03 AM T L <tapir....@gmail.com> wrote:
>
>>
>>
>> On Saturday, June 20, 2020 at 10:21:56 AM UTC-4, Axel Wagner wrote:
>>>
>>> I would assume it's
>>>
>>> type MapConstraint(type K comparable, V interface{}) interface {
>>> type map[K]V
>>> }
>>>
>>> func F(type M MapConstraint(K, V), K comparable, V interface{}) (m M) {
>>> }
>>>
>>> Note that you are under no obligation to make use of a type-parameter if
>>> you don't need it.
>>>
>>
>> I don't very understand this. Can a slice value be used as the argument
>> of the F function?
>>
> For that, I think you'd need the interface to be:
> type MapSliceConstraint(type K comparable, V interface{}) interface {
> type map[K]V, []V
> }
>
> I'm not quite sure how to eliminate the useless K type-param for slices,
> though.
>
> Note: the relevant draft design section is:
> https://go.googlesource.com/proposal/+/refs/heads/master/design/go2draft-type-parameters.md#type-parameters-in-type-lists
>
> Here's an almost working example:
> https://go2goplay.golang.org/p/qcdfl0tuHlb
>
> It looks like there's a bug in the type-parameter constraint checking
> because in the above example code, I get:
>
> type checking failed for main
>
> prog.go2:11:39: K does not satisfy comparable
>
> This, despite the type parameter definition literally requiring that K be
> comparable:
> func genLen(type T MapSliceConstraint(K, V), K *comparable*, V
> interface{})(collection T) int {
> return len(collection)
> }
> (example without a main() to reduce clutter:
> https://go2goplay.golang.org/p/1gqiYuDELuI)
>
>>
>>
>>>
>>> On Sat, Jun 20, 2020 at 4:14 PM T L <tapi...@gmail.com> wrote:
>>>
>>>> I mean I don't care about the element and key types of the parameter
>>>> type.
>>>>
>>>> For a simple example, I want to define a generic function which prints
>>>> the length of a container (do do some other things):
>>>>
>>>> func Print(type T Container) (v T) {
>>>> // ... do some things
>>>>
>>>> fmt.Println(len(v))
>>>>
>>>> // ... do some things
>>>> }
>>>>
>>>> On Saturday, June 20, 2020 at 9:16:39 AM UTC-4, T L wrote:
>>>>>
>>>>> .
>>>>>
>>>> --
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>>>> .
>>>>
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>>
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