How to Retrieve a Go Function\'s Signature as a String?

Retrieving a Function's Signature as a String in Go

Problem:

In a Go module designed to load plugins, it's essential to ensure that a specific function with a predetermined name and signature exists within the main package. If this function is absent or does not match the expected signature, a descriptive error message is necessary. Given a function type variable, how can the underlying signature be obtained?

Solution:

Merely relying on reflect.Type.String() only provides the type's name, such as main.ModuleInitFunc. To retrieve the complete signature, we must construct it manually using the information provided by reflect.Type.

Implementation:

func signature(f interface{}) string {
    t := reflect.TypeOf(f)
    if t.Kind() != reflect.Func {
        return "<not a function>"
    }

    buf := strings.Builder{}
    buf.WriteString("func (")
    for i := 0; i < t.NumIn(); i++ {
        if i > 0 {
            buf.WriteString(", ")
        }
        buf.WriteString(t.In(i).String())
    }
    buf.WriteString(")")
    if numOut := t.NumOut(); numOut > 0 {
        if numOut > 1 {
            buf.WriteString(" (")
        } else {
            buf.WriteString(" ")
        }
        for i := 0; i < t.NumOut(); i++ {
            if i > 0 {
                buf.WriteString(", ")
            }
            buf.WriteString(t.Out(i).String())
        }
        if numOut > 1 {
            buf.WriteString(")")
        }
    }

    return buf.String()
}

Usage:

By passing a function variable to the signature() function, we can obtain the corresponding signature as a string. For example:

var myFunc ModuleInitFunc

fmt.Println(signature(func(i int) error { return nil }))
fmt.Println(signature(myFunc))
fmt.Println(signature(time.Now))
fmt.Println(signature(os.Open))
fmt.Println(signature(log.New))
fmt.Println(signature(""))

Output:

func (int) error
func (int) error
func () time.Time
func (string) (*os.File, error)
func (io.Writer, string, int) *log.Logger
<not a function>

Note:

It's crucial to recognize that it's impossible to extract the names of the parameters and result types from the signature since Go does not store or provide access to that information.

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