Generics

• Reciever methods can’t have genercis reference

Using generic to instantiate concrete classes

Can we use generics to instantiate a concrete version of an interface?

e.g. suppose we have an interface DBDoc and different types that implement it e.g. Doc1 and Doc2.

Now suppose we want to define functions like the following

func NewDoc1() DBDoc {
    return &Doc1{}
}

func NewDoc2() DBDoc {
    return &Doc2{}
}

We’d like to define that using a generic the following works

func NewDBDoc[Doc any]() *Doc {
    d := new(Doc)
    return d
}

But it doesn’t define a typeconstraint. As a result suppose we have the following

type DocCreator fun() DBDoc

func List(creator DocCreator) {
    ....
}

The following won’t compile

List(NewDBDoc[NewDoc1])

because the return type of NewDBDoc is *Doc1.

The following won’t compile

func NewDBDocConstrained[Doc DBDoc]() DBDoc{
    d := new(Doc)
    return d
}

• I think the problem is that suppose *Doc1 implements DBDoc
• Then Doc is type *Doc1; so new(Doc) creates **Doc1 which doesn’t implement DBDoc
• Put another way we’d have to do NewDBDocConstrained[*Doc1]
• If Doc is a pointer type how do we instantiate a new version of it?
• NewDBDoc works because the type constraint is any so we can do NewDBDoc[Doc1]