On Thu, 14 Apr 2022, 09:50 'Sebastien Binet' via golang-nuts, <
golang-nuts@googlegroups.com> wrote:
> hi there,
>
> I am playing a bit with generics.
> I am trying to implement a parametrized "Read" function that unmarshals
> bytes into some type value that implements an interface:
>
> type T1 struct {
> v string
> }
>
> func (t *T1) UnmarshalBinary(p []byte) error {
> t.v = string(p)
> return nil
> }
>
> type Unmarshaler interface {
> UnmarshalBinary(p []byte) error
> }
>
> func f1[T Unmarshaler](p []byte) (T, error) {
> var t T
> err := t.UnmarshalBinary(p)
> return t, err
> }
>
> func main() {
> p := []byte("hello")
> t1, err := f1[T1](p)
> if err != nil {
> log.Panic(err)
> }
>
> log.Printf("t1: %+v", t1)
> }
>
> my naive attempt failed like so:
>
> ./prog.go:26:16: T1 does not implement Unmarshaler (UnmarshalBinary method
> has pointer receiver)
>
> it's only when I rewrite my f1 function like so that it "works":
>
> func f2[T any](p []byte) (t T, err error) {
> switch t := any(&t).(type) {
> case Unmarshaler:
> err = t.UnmarshalBinary(p)
> default:
> panic("boo")
> }
> return t, err
> }
>
> but it doesn't take advantage of the nice type constraints Go's generics
> provide.
>
> of course, replacing:
> t1, err := f1[T1](p)
>
> with:
> t1, err := f1[*T1](p)
> compiles. but fails at runtime.
>
> what am I missing?
> how do I convey the constraint "unmarshaling usually imply passing a
> pointer to some value" ?
>
This is a classic use case for structural type constraints:
https://go.dev/play/p/-stEjeeqQD0
>
> -s
>
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>
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