append第二个参数不接受切片类型。
截图代码改成:

for _, v := range files {
    for _, urlInFile := range util.ReadFile(v) {
        urls = append(urls, urlInFile)
    }
}

或者自己定义一个合并Slice得方法


func MergeSlice(s1 []string, s2 []string) []string {
    slice := make([]string, len(s1)+len(s2))
    copy(slice, s1)
    copy(slice[len(s1):], s2)
    return slice
}

for _, v := range files {
    urls = MergeSlice(urls, util.ReadFile(v))
}