Thanks for your help and very interesting ideas. In the end I used this:
type Set[T comparable] map[T]struct{}
func New[T comparable](elements ...T) Set[T] {
set := make(Set[T], len(elements))
for _, element := range elements {
set[element] = struct{}{}
}
return set
}
func (me Set[T]) String() string {
elements := make([]T, 0, len(me))
for element := range me {
elements = append(elements, element)
}
sort.Slice(elements, func(i, j int) bool {
return less(elements[i], elements[j])
})
s := "{"
sep := ""
for _, element := range elements {
s += sep + asStr(element)
sep = " "
}
return s + "}"
}
func asStr(x any) string {
if s, ok := x.(string); ok {
return fmt.Sprintf("%q", s)
}
return fmt.Sprintf("%v", x)
}
func less(a, b any) bool {
switch x := a.(type) {
case int:
return x < b.(int)
case float64:
return x < b.(float64)
case string:
return x < b.(string)
default:
return fmt.Sprintf("%v", a) < fmt.Sprintf("%v", b)
}
}
Interestingly, I couldn't put the asStr() code in the String() function
since doing so produced this error:
invalid operation: cannot use type assertion on type parameter value
element (variable of type T constrained by comparable)
Anyway, I'm happy that it all works now. (I know I ought to include every
int & float32, but this is enough for now).
On Tuesday, November 8, 2022 at 11:29:34 AM UTC rog wrote:
> If you're sure that T is an int or a string, then why not constrain it as
> such? https://go.dev/play/p/1kT6EacMHco
>
> You could go further and constrain it to allow any type with ordering
> defined: https://go.dev/play/p/il5koj1RPkh
>
> If you want to allow any kind of comparable key in your set, one could
> observe that essentially you're trying to solve the same problem that the
> fmt package is solving when it converts maps to string. It uses the
> internal fmtsort <https://pkg.go.dev/internal/fmtsort> package, which, as
> luck would have it, has been factored out into an externally available
> package <https://pkg.go.dev/github.com/rogpeppe/go-internal/fmtsort>. So
> you could do this: https://go.dev/play/p/oKTGSm_o22a
>
> To answer the specific question you asked, there is an issue that tracks
> the ability to do a switch directly on a type parameter:
> https://github.com/golang/go/issues/45380
>
> But you can also work around the lack of that feature by doing something
> like this: https://go.dev/play/p/3C2a61Ojbxs
>
> Hope this helps,
>
> rog.
>
>
> On Tue, 8 Nov 2022 at 08:53, 'Mark' via golang-nuts <
> golan...@googlegroups.com> wrote:
>
>> Given a function:
>>
>> func F[T comparable](a T) {
>> }
>>
>> is it possible to check T's type inside F?
>>
>> My use case is that I have a function with signature G[T comparable](x
>> []T) and inside G I want to sort the elements in slice x where T could be
>> int or string.
>>
>> This arises in a tiny generic set module I've created:
>> https://github.com/mark-summerfield/gset
>> In the String() method I want to return a string with the elements sorted
>> (for human readability and for testing convenience); but at the moment I
>> can only do this by converting all elements to strings and sorting them
>> which isn't good for ints.
>>
>> --
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>> .
>>
>
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