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

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

In the context of Go development, you may encounter scenarios where you need to programmatically retrieve a function's signature as a string. Understanding how to accomplish this is crucial for performing advanced type introspection and error handling.

Background

The reflect package in Go provides extensive reflection capabilities, including the ability to obtain a function's reflect.Type. However, reflect.Type.String() only returns the type name which may not always provide sufficient information.

Obtaining the Signature

To obtain the full function signature, we must delve into the reflect.Type to extract the parameter and result types manually. Here's a function that does this:

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()
}

Example Usage

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>

Limitations

It's important to note that this approach doesn't capture the names of parameters or result types. This is because Go doesn't store this information at runtime.

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