如何实现通用切片追加器？

Question

I'm relatively new to go. I'm trying to write a generic "appender" function. This is a simplification, but its an attempt to create a clean interface for processing some lists. Specifically, I have questions about the two errors that this generates:

package main

type GenericFunc func() *interface{}
func Append(ints interface{}, f GenericFunc) {
    ints = append(ints, f())
}

func ReturnInt() *int {
    i := 1
    return &i
}

func main() {
    var ints []*int
    Append(ints, ReturnInt)
}

Playground

prog.go:5:18: first argument to append must be slice; have interface {} prog.go:15:11: cannot use ReturnInt (type func() *int) as type GenericFunc in argument to Append

1. Why can't ReturnInt be of type GenericFunc? If this doesn't work, I'm not understanding how interface{} can be used with functions at all.. can it?
2. How can you accept a "generic" slice and append to it using reflection? This would involve checking that GenericFunc returns the same type that the slice is, but after that appending should be possible.

Answer 1

The types func() *interface{} (type type of GenericFunc) and (type func() *int) (the type of ReturnInt) are different types. One returns a *interface{}. The other returns a *int. The types are not assignable to each other.

Use this function to generically append the result of a function to a slice:

func Append(sp interface{}, f interface{}) {
    s := reflect.ValueOf(sp).Elem()
    s.Set(reflect.Append(s, reflect.ValueOf(f).Call(nil)[0]))
}

Call it like this:

var ints []*int
Append(&ints, ReturnInt)

The function will panic if the argument is not a pointer to a slice or the function does not return a value assignable to a slice element.

playground example