package main

import (
    "log"
    "reflect"
)

func main() {
    xs := []int{3, 3, 1, 3, 6, 3}
    RemoveElem(&xs, func(x interface{}) bool {
        if x.(int) == 3 {
            return true
        }
        return false
    })
    log.Println(xs)
}

func RemoveElem(xs interface{}, removefunc func(x interface{}) bool) {
    resultv := reflect.ValueOf(xs)
    slicev := resultv.Elem()
    for i := slicev.Len() - 1; i >= 0; i-- {
        if removefunc(slicev.Index(i).Interface()) {
            slicev = reflect.AppendSlice(slicev.Slice(0, i), slicev.Slice(i+1, slicev.Len()))
        }
    }
    resultv.Elem().Set(slicev)
}

func RetainElem(xs interface{}, retainfunc func(x interface{}) bool) {
    RemoveElem(xs, func() func(x interface{}) bool {
        return func(x interface{}) bool {
            return !retainfunc(x)
        }
    }())
}