12.8 Displaying the Methods of a Type

Our final example of reflection uses reflect.Type to print the type of an arbitrary value and enumerate its methods:

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// Print prints the method set of the value x.
func Print(x interface{}) {
    v := reflect.ValueOf(x)
    t := v.Type()
    fmt.Printf("type %s
", t)

    for i := 0; i < v.NumMethod(); i++ {
        methType := v.Method(i).Type()
        fmt.Printf("func (%s) %s%s
", t, t.Method(i).Name,
            strings.TrimPrefix(methType.String(), "func"))
    }
}

Both reflect.Type and reflect.Value have a method called Method. Each t.Method(i) call returns an instance of reflect.Method, a struct type that describes the name and type of a single method. Each v.Method(i) call returns a reflect.Value representing a method value (§6.4), that is, a method bound to its receiver. Using the reflect.Value.Call method (which we don’t have space to show here), it’s possible to call Values of kind Func like this one, but this program needs only its Type.

Here are the methods belonging to two types, time.Duration and *strings.Replacer:

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methods.Print(time.Hour)
// Output:
// type time.Duration
// func (time.Duration) Hours() float64
// func (time.Duration) Minutes() float64
// func (time.Duration) Nanoseconds() int64
// func (time.Duration) Seconds() float64
// func (time.Duration) String() string

methods.Print(new(strings.Replacer))
// Output:
// type *strings.Replacer
// func (*strings.Replacer) Replace(string) string
// func (*strings.Replacer) WriteString(io.Writer, string) (int, error)