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

How to Obtain a Function's Signature as a String in Go

Your query concerns the retrieval of the signature, a string representation of a function's type, given a function variable.

Understanding reflect.Type.String()

The reflect.Type.String() method returns the name of the type, not the full signature. When the function value is a function literal (unnamed type), it prints the signature.

Constructing the Signature Manually

To retrieve the signature of a named type, information from the reflect.Type is needed. Here's a function that constructs the signature:

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()
}

Testing the Function

Sample output of the signature function:

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(""))

Expected 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: Printing parameter and result type names is not possible due to the lack of accessible information.

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