如何转换函数类型？ [关闭]

I want to use upCase as a variable in expend, But it said "Cannot use 'upCase(s1)'(type string) as type func(string) string. how can I to convert upCase type? Or what do I need do to make the wrong disappear?

package main

import (
    "fmt"
    "strings"
)

func main() {
    s := "test func"
    s1 := ""
    test := expand(s, upCase(s1))
    fmt.Print(test)
}
func expand(s string, f func(string) string) string {
    s1 := "test"
    var s2 string
    if strings.Contains(s, s1) {
        // todo 字符串中s1的值更改为f(s1)的值
        s1 = f(s1)
        strings.Replace(s1, s, s2, -1)
    } else {
        return ""
    }
    return s2
}

func upCase(s string) string {
    s = strings.ToUpper(s)
    return s
}

You are calling the upCase function in your call to expand (so it returns a string that you pass to your expand function). You need to pass in the function:

test := expand(s, upCase)

Your current code test := expand(s, upCase(s1)) is the same as doing:

v := upCase(s1)
test := expand(s, v)

I.e, you are just calling upCase(s1) that returns a string, and you pass that string to expand , which expects a function, not a string as its 2. argument.