FAQ - The Go Programming Languageの Can I convert a []T to an []interface{}?にあるように[]intを[]interface{}にキャストはできないので、あまり実用的ではないです。あくまでサンプルということで。

slice.go

package sample

type Slice []interface{}
    
func (s Slice) Each(f func(interface{}, int)) {
    for i, v := range(s) {
        f(v, i)
    }
}   
    
func (s Slice) Map(f func(interface{}, int) interface{}) Slice {
    r := make([]interface{}, len(s))
    for i, v := range(s) {
        r[i] = f(v, i)
    }
    return r
}   

func (s Slice) Select(f func(interface{}, int) bool) Slice {
    r := make([]interface{}, 0)
    for i, v := range(s) {
        if f(v, i) {
            r = append(r, v)
        }
    }
    return r 
}

slice_test.go

package sample

import (
    "fmt"
    "testing"
)   
        
func TestEach(test *testing.T) {
    sum := 0
    values := Slice{1, 2, 3}
    values.Each(func(v interface{}, _ int) {
        sum += v.(int)
    })
    expected := 6
    if sum != expected {
        test.Fatalf("expected:%d, actual:%d", expected, sum)
    }
}

func TestMap(test *testing.T) {
    expected := Slice{2, 3, 4}
    values := Slice{1, 2, 3}
    values = values.Map(func(v interface{}, _ int) interface{} {
        return v.(int) + 1
    })
    for i := 0; i < len(values); i++ {
        if values[i] != expected[i] {
            test.Fatalf("i:%d, expected:%d, actual:%d", i, expected[i], values[i])
        }
    }
}

func TestSelect(test *testing.T) {
    expected := Slice{1, 3}
    values := Slice{1, 2, 3}
    values = values.Select(func(v interface{}, _ int) bool {
        return v.(int) % 2 == 1
    })
    for i := 0; i < len(values); i++ {
        if values[i] != expected[i] {
            test.Fatalf("i:%d, expected:%d, actual:%d", i, expected[i], values[i])
        }
    }
}