> 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)
You need to convert to "any": https://go.dev/play/p/1pMhs22S-8P P.S. It would be better to use https://pkg.go.dev/strings#Builder for building a string On Tuesday, 8 November 2022 at 13:09:51 UTC Mark wrote: > 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. >>> >>> -- >>> You received this message because you are subscribed to the Google >>> Groups "golang-nuts" group. >>> To unsubscribe from this group and stop receiving emails from it, send >>> an email to golang-nuts...@googlegroups.com. >>> To view this discussion on the web visit >>> https://groups.google.com/d/msgid/golang-nuts/e746f065-24fa-474a-90ec-2d8367bf3e3bn%40googlegroups.com >>> >>> <https://groups.google.com/d/msgid/golang-nuts/e746f065-24fa-474a-90ec-2d8367bf3e3bn%40googlegroups.com?utm_medium=email&utm_source=footer> >>> . >>> >> -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/golang-nuts/3169ebd2-d854-4e7d-addd-9b11918feb45n%40googlegroups.com.
