Golang generics with interface and implementation at same time

Question

I am trying to write the following function:

func Fill[X any](slice []*X){
   for i := range slice {
      slice[i] = new(X)
   }
}

xs := make([]*int, 10) // fill with nils
Fill(xs) // now fill with new(int)

That works fine but… if I want to use a slice of interfaces and provide a concrete type?

func Fill[X, Y any](slice []X){
   for i := range slice {
      slice[i] = new(Y) // not work!
   }
}

xs := make([]sync.Locker, 10) // fill with nils
Fill[sync.Locker,sync.Mutex](xs) // ouch

I try some combinations without success, is there a way or go1.18 does not support such relations?

blackgreen · Accepted Answer · 2022-03-22 07:09:37Z

When you constrain both X and Y to any, you lose all interface-implementor relationship. The only thing that is known at compile time is that X and Y are different types, and you can't assign one to the another within the function body.

A way to make it compile is to use an explicit assertion:

func Fill[X, Y any](slice []X) {
    for i := range slice {
        slice[i] = any(*new(Y)).(X)
    }
}

But this panics if Y doesn't really implement X, as in your case, since it is *sync.Mutex (pointer type) that implements sync.Locker.

Moreover, when Y is instantiated with a pointer type, you lose information about the base type, and therefore the zero value, including *new(Y) would be nil, so you don't really have a baseline improvement over make (just typed nils vs. nil interfaces).

What you would like to do is to constrain Y to X, like Fill[X any, Y X](slice []X) but this is not possible because 1) a type parameter can't be used as a constraint; and/or 2) a constraint can't embed a type parameter directly. It also initializes nils as the above.

A better solution is to use a constructor function instead of a second type parameter:

func main() {
    xs := make([]sync.Locker, 10)
    Fill(xs, func() sync.Locker { return &sync.Mutex{} })
}

func Fill[X any](slice []X, f func() X) {
    for i := range slice {
        slice[i] = f()
    }
}

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