Underlying type constraints and pointers

[Aaron Beitch]

Hello,

I would like to provide some helper functions that work on pointer types and having trouble getting it to work with ~ type constraints.

Here is the problem in code: https://go.dev/play/p/z0sKEsUaL_d

I have types Foo and Bar. Bar's underlying type is Foo. I want a function that takes in pointers to types whose underlying type is Foo, but I get a compile error when trying to call this function:

    *Bar does not implement ~*Foo (*Bar missing in ~*main.Foo)

Should this work?

My real use case is around a hash map library. It's implementation is a copy of Go's built-in map, but allows the user to control the equal and hash function, thus allowing keys of any type rather than just comparable types. github.com/aristanetworks/gomap

I would like to provide helper functions for gomap.Map similar to those provided by golang.org/x/exp/maps and for those helpers to work on pointers to types whose underlying type is gomap.Map.

Thanks,

Aaron

[Axel Wagner]

On Sat, Oct 22, 2022 at 1:40 AM 'Aaron Beitch' via golang-nuts <golan...@googlegroups.com> wrote:

I have types Foo and Bar. Bar's underlying type is Foo.

The underlying type is defined recursively and always either a predeclared type (int, string…), or a type literal.

The underlying type of Bar is the underlying type of Foo, which is struct { data []string }. You might try using `~struct{ data []string }`.

 

I want a function that takes in pointers to types whose underlying type is Foo, but I get a compile error when trying to call this function:

    *Bar does not implement ~*Foo (*Bar missing in ~*main.Foo)

Should this work?

My real use case is around a hash map library. It's implementation is a copy of Go's built-in map, but allows the user to control the equal and hash function, thus allowing keys of any type rather than just comparable types. github.com/aristanetworks/gomap

I would like to provide helper functions for gomap.Map similar to those provided by golang.org/x/exp/maps and for those helpers to work on pointers to types whose underlying type is gomap.Map.

Thanks,

Aaron

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[Axel Wagner]

(or, for convenience, you can use a type alias `type foo = struct{ data []string }`, which should allow you to write ~foo.

[Axel Wagner]

Sorry, I literally just fell out of bed, so I jumped on the first thing I saw, which probably isn't the most helpful answer.

You might be interested in this sentence from the Go 1.18 release notes:

The Go compiler does not support accessing a struct field x.f where x is of type parameter type even if all types in the type parameter's type set have a field f. We may remove this restriction in a future release.

 

So while what I said is correct - the underlying type of Bar *isnt* Foo - I think the more disappointing fact is that what you want to do just won't work for now. It might in the future, but not right now.

[Axel Wagner]

Actually (maybe I should stop, think, and then E-Mail) this works: https://go.dev/play/p/SBDglnj8mQx

I'm not sure if the selector X(*a).data will actually copy the value, but I don't *think* it does.

I also tried this first: https://go.dev/play/p/3qUhyOkvcVm

The fact that this doesn't work looks like a bug to me, to be honest.

[Aaron Beitch]

Thanks for your help here! You helped me realize the thing I really want access to is the methods of Foo and Bar, and not the struct fields (my example was misleading).

What I would really like to have is a constraint that the type must have an underlying type of Foo, but that isn't working how I want. So, what I can do instead is define an interface that includes the methods that Foo provides and use that as my constraint. https://go.dev/play/p/BwEb8Lm4pb6

Aaron