On Wed, Feb 2, 2022 at 5:29 AM Ed Pelc <edrock...@gmail.com> wrote: > I was able to get this to work but I'm curious if it could be simplified > at all as it seems complicated to require two type parameters for this. I > figured this out after trying to follow the `pointer method` > <https://go.googlesource.com/proposal/+/8b5daebee3198c7b6265b40c06b077be5e4c5b4b/design/43651-type-parameters.md> > section > from the generics proposal. It seems to be written for a slightly different > use case though. > > Working, but complicated(requires two type params): > https://gotipplay.golang.org/p/iz2GKYtaL-A >
This seems to be what you are supposed to do, yes. Note that the caller doesn't need to really concern themselves with the second type-parameter. It's virtual, in a sense. Basically I do not understand why you can't do a simpler single type > parameter version using comparable directly within DataRecord. I know the > spec says comparable cannot be used as a variable, or value(and I presume > this means a struct field too) but it works with the two type parameter > redirection above. I'd like to know why the two parameter version works for > some reason. > > Single type parameter version: https://gotipplay.golang.org/p/UO8e6zQwi0k > The compiler tells you why this doesn't work: You are comparing to `nil` and not all values can be compared to `nil`. For that, you need the type argument to be a pointer specifically. Telling the compiler that it is a pointer, is what the second type parameter is for. You can do this, if you want to: https://gotipplay.golang.org/p/aDGzOF7EWIF But, of course, this means that your type might be instantiated with a non-pointer and it's no longer possible to distinguish between "the zero value" and "not given". > My research/help others find this. I could not find much for people trying > to do something similar even though it seems a common use case(we have many > different data types that need basic crud http handlers): > > From reading the 1.18 spec it says you can't embed `comparable` types as a > variable or value. I assume this means you also can't use as a struct > field. I'm not sure why you can't do this or if it's just a limitation for > now. Are you expected to add an `IsNil() bool` method as a replacement for > this? > > From the spec re comparable type parameter(Record field in example) as a > struct field: > https://tip.golang.org/ref/spec#Type_constraints > > "They cannot be the types of values or variables, or components of other, > non-interface types." > > If you try to use a pointer field(line 12) to make the record comparable > then you get this error. > > "req.Record.SetID undefined (type *Record is pointer to type parameter, > not type parameter)" > > -- > 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/4b4ce168-64fd-4534-9925-989b1371a32bn%40googlegroups.com > <https://groups.google.com/d/msgid/golang-nuts/4b4ce168-64fd-4534-9925-989b1371a32bn%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/CAEkBMfE%2BFvOqmPOE69LYo5B913uXesy%3DoG3Q9dUtz%3D%2BxBOmk%3DA%40mail.gmail.com.
