Ian, Thanks for your reply.
Yes, it's just probably because it's still early days but I was a bit confused. Happy to note this new draft allows to define functions (and methods) that can only take pointers to values, though. -s -------- Original Message -------- On Jun 20, 2020, 02:12, Ian Lance Taylor wrote: > On Wed, Jun 17, 2020 at 12:09 AM Sebastien Binet <d...@sbinet.org> wrote: >> >> I was playing a bit w/ pointer methods[1] to see how this could be applied >> to a pet peeve of mine: ensuring at compile-time that, e.g., >> json.Decoder.Decode takes a pointer to a value: >> >> https://go2goplay.golang.org/p/J2Lr5QrkKTj >> >> this works: >> >> func Decode(type *T)(ptr *T) error { >> return json.NewDecoder(new(bytes.Buffer)).Decode(ptr) >> } >> >> func main() { >> var v int >> Decode(int)(&v) >> } >> >> as well as this: >> >> type Decoder(type *T) struct{} >> >> func (dec *Decoder(T)) Decode(ptr *T) error { >> // ... >> return nil >> } >> >> func main() { >> var v int >> dec := Decoder(int){} >> dec.Decode(&v) >> } >> >> but I was initially caught off guard trying to write the method on Decoder >> like so: >> >> // ERROR: receiver type parameter *T must be an identifier >> func (dec *Decoder(*T)) Decode(ptr *T) error { >> return nil >> } >> >> (notice the extra '*' in Decoder(*T)) >> >> perhaps something to mention in the document? >> >> -s >> >> [1]: >> https://go.googlesource.com/proposal/+/refs/heads/master/design/go2draft-type-parameters.md#pointer-methods > > Using a pointer method, as in "type *T", does not mean that the type > argument must be a pointer. If you write "(type *T C)", it means > that, given a type argument A, the type *A must implement the > constraint C. There is actually no reason to ever write "(type *T)" > with no constraint. That means that, given a type argument A, the > type *A must implement the empty constraint, which it does, because > all types implement the empty constraint. > > Pointer methods do seem to be confusing. I think they have a clear > use, but it may be that they are too confusing for that use. > > Ian -- 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/weyIcUv1VNhNaVAI3XcqoKy-G9xdijAuz5CERk9xIlcBoQKxsnCRHwQVdu9p5MNz-UwKe4oGh3LAqmL-NqVaHzjptskE9950n5nGtOrdcxY%3D%40sbinet.org.
