接口和类型的问题

Question

Can someone please explain as to why duck typing does not work when a slice is passed to the variadic function.
Cases 1 and 2 as indicated below seems to work, but case-3 below initializes a slice and thereafter passes the dereferenced slice to the function that accepts the interface.

The error message is : cannot use gophers (type []Gopher) as type []Animal in argument to runForest

package main

    import (
        "fmt"
    )

    type Animal interface {
        Run() string
    }

    type Gopher struct {
    }

    func (g Gopher) Run() string {
        return "Waddle.. Waddle"
    }

    func runForest(animals ...Animal) {
        for _, animal := range animals {
            fmt.Println(animal.Run())
        }
    }

    func main() {

        //works
        runForest(Gopher{})

        //works
        runForest(Gopher{},Gopher{})

        // does not work
        gophers := []Gopher{{},{}}
        runForest(gophers...)
    }

Answer 1

As Volker mentions in the comments, slices can't be implicitly converted the way their elements can. A Gopher can quack like an Animal, but a []Gopher can't quack like an []Animal.

The minimal change to get this working is:

func main() {
    gophers := []Gopher{{}, {}}
    out := make([]Animal, len(gophers))
    for i, val := range gophers {
        out[i] = Animal(val)
    }
    runForest(out...)
}