reflect

func Sprint(x interface{}) string {
    type stringer interface {
        String() string
    }

    switch x := x.(type) {
    case stringer:
        return x.String()
    case string:
        return x
    case int:
        return strconv.Itoa(x)
    // ...similar cases for int16, uint32, and so on...
    default:
        // array, chan, func, map, pointer, slice, struct
        return "???"
    }
}

func TypeOf(interface{}) Type

t := reflect.TypeOf(3)  // a reflect.Type
fmt.Println(t.String()) // "int"
fmt.Println(t)          // "int"

var w io.Writer = os.Stdout
fmt.Println(reflect.TypeOf(w)) // "*os.File"

fmt.Printf("%T\n", 3) // "int"

v := reflect.ValueOf(3)
fmt.Println(v)          // "3"
fmt.Printf("%v\n", v)   // "3"
fmt.Println(v.String()) // NOTE: "<int Value>"

t := v.Type()           // a reflect.Type
fmt.Println(t.String()) // "int"

v := reflect.ValueOf(3) // a reflect.Value
x := v.Interface()      // an interface{}
i := x.(int)            // an int
fmt.Printf("%d\n", i)   // "3"

func Any(value interface{}) string {
    return formatAtom(reflect.ValueOf(value))
}

func formatAtom(v reflect.Value) string {
    switch v.Kind() {
    case reflect.Invalid:
        return "invalid"
    case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
        return strconv.FormatInt(v.Int(), 10)
    case reflect.Uint,
        reflect.Uint8,
        reflect.Uint16,
        reflect.Uint32,
        reflect.Uint64,
        reflect.Uintptr:
        return strconv.FormatUint(v.Uint(), 10)
    // ...floating-point and complex cases omitted for brevity...
    case reflect.Bool:
        return strconv.FormatBool(v.Bool())
    case reflect.String:
        return strconv.Quote(v.String())
    case reflect.Chan, reflect.Func, reflect.Ptr, reflect.Slice, reflect.Map:
        return v.Type().String() + " 0x" + strconv.FormatUint(uint64(v.Pointer()), 16)
    default: // reflect.Array, reflect.Struct, reflect.Interface
        return v.Type().String() + " value"
    }
}

var x int64 = 1
var d time.Duration = 1 * time.Nanosecond
fmt.Println(format.Any(x))                  // "1"
fmt.Println(format.Any(d))                  // "1"
fmt.Println(format.Any([]int64{x}))         // "[]int64 0x8202b87b0"
fmt.Println(format.Any([]time.Duration{d})) // "[]time.Duration 0x8202b87e0"

type Movie struct {
    Title, Subtitle string
    Year            int
    Color           bool
    Actor           map[string]string
    Oscars          []string
    Sequel          *string
}

strangelove := Movie{
    Title:    "Dr. Strangelove",
    Subtitle: "How I Learned to Stop Worrying and Love the Bomb",
    Year:     1964,
    Color:    false,
    Actor: map[string]string{
        "Dr. Strangelove":            "Peter Sellers",
        "Grp. Capt. Lionel Mandrake": "Peter Sellers",
        "Pres. Merkin Muffley":       "Peter Sellers",
        "Gen. Buck Turgidson":        "George C. Scott",
        // ...
    },
    // ...
}

Display("strangelove", strangelove)

Display strangelove (display.Movie):
strangelove.Title = "Dr. Strangelove"
strangelove.Subtitle = "How I Learned to Stop Worrying and Love the Bomb"
strangelove.Year = 1964
strangelove.Color = false
strangelove.Actor["Gen. Buck Turgidson"] = "George C. Scott"
strangelove.Actor["Brig. Gen. Jack D. Ripper"] = "Sterling Hayden"
strangelove.Actor["Maj. T.J. \"King\" Kong"] = "Slim Pickens"
strangelove.Actor["Dr. Strangelove"] = "Peter Sellers"
strangelove.Actor["Grp. Capt. Lionel Mandrake"] = "Peter Sellers"
strangelove.Actor["Pres. Merkin Muffley"] = "Peter Sellers"
strangelove.Oscars[0] = "Best Actor (Nomin.)"
strangelove.Oscars[1] = "Best Adapted Screenplay (Nomin.)"
strangelove.Oscars[2] = "Best Director (Nomin.)"
strangelove.Oscars[3] = "Best Picture (Nomin.)"
strangelove.Sequel = nil

Display("os.Stderr", os.Stderr)
// Output:
// Display os.Stderr (*os.File):
// (*(*os.Stderr).file).fd = 2
// (*(*os.Stderr).file).name = "/dev/stderr"
// (*(*os.Stderr).file).nepipe = 0

func Display(name string, x interface{}) {
    fmt.Printf("Display %s (%T):\n", name, x)
    display(name, reflect.ValueOf(x))
}

func display(path string, v reflect.Value) {
    switch v.Kind() {
    case reflect.Invalid:
        fmt.Printf("%s = invalid\n", path)
    case reflect.Slice, reflect.Array:
        for i := 0; i < v.Len(); i++ {
            display(fmt.Sprintf("%s[%d]", path, i), v.Index(i))
        }
    case reflect.Struct:
        for i := 0; i < v.NumField(); i++ {
            fieldPath := fmt.Sprintf("%s.%s", path, v.Type().Field(i).Name)
            display(fieldPath, v.Field(i))
        }
    case reflect.Map:
        for _, key := range v.MapKeys() {
            display(fmt.Sprintf("%s[%s]", path,
                formatAtom(key)), v.MapIndex(key))
        }

    case reflect.Ptr:
        if v.IsNil() {
            fmt.Printf("%s = nil\n", path)
        } else {
            display(fmt.Sprintf("(*%s)", path), v.Elem())
        }
    case reflect.Interface:
        if v.IsNil() {
            fmt.Printf("%s = nil\n", path)
        } else {
            fmt.Printf("%s.type = %s\n", path, v.Elem().Type())
            display(path+".value", v.Elem())
        }
    default: // basic types, channels, funcs
        fmt.Printf("%s = %s\n", path, formatAtom(v))
    }
}

x := 2                   // value    type   variable?
a := reflect.ValueOf(2)  // 2        int    no
b := reflect.ValueOf(x)  // 2        int    no
c := reflect.ValueOf(&x) // &x       *int   no
d := c.Elem()            // 2        int    yes

fmt.Println(a.CanAddr()) // "false"
fmt.Println(b.CanAddr()) // "false"
fmt.Println(c.CanAddr()) // "false"
fmt.Println(d.CanAddr()) // "true"

x := 2
d := reflect.ValueOf(&x).Elem()   // d refers to the variable x
px := d.Addr().Interface().(*int) // px := &x
*px = 3                           // x = 3
fmt.Println(x)                    // "3"

d.Set(reflect.ValueOf(4))
fmt.Println(x) // "4"

d := reflect.ValueOf(&x).Elem()
d.SetInt(3)
fmt.Println(x) // "3"

x := 2
b := reflect.ValueOf(x)
b.Set(reflect.ValueOf(3)) // panic: Set using unaddressable value

stdout := reflect.ValueOf(os.Stdout).Elem() // *os.Stdout, an os.File var
fmt.Println(stdout.Type())                  // "os.File"
fd := stdout.FieldByName("fd")
fmt.Println(fd.Int()) // "1"
fd.SetInt(2)          // panic: unexported field

fmt.Println(fd.CanAddr(), fd.CanSet()) // "true false"

func search(resp http.ResponseWriter, req *http.Request) {
    var data struct {
        Labels     []string `http:"l"`
        MaxResults int      `http:"max"`
        Exact      bool     `http:"x"`
    }
    data.MaxResults = 10 // set default
    if err := params.Unpack(req, &data); err != nil {
        http.Error(resp, err.Error(), http.StatusBadRequest) // 400
        return
    }

    // ...rest of handler...
    fmt.Fprintf(resp, "Search: %+v\n", data)
}

func Unpack(req *http.Request, ptr interface{}) error {
    if err := req.ParseForm(); err != nil {
        return err
    }

    // Build map of fields keyed by effective name.
    fields := make(map[string]reflect.Value)
    v := reflect.ValueOf(ptr).Elem() // the struct variable
    for i := 0; i < v.NumField(); i++ {
        fieldInfo := v.Type().Field(i) // a reflect.StructField
        tag := fieldInfo.Tag           // a reflect.StructTag
        name := tag.Get("http")
        if name == "" {
            name = strings.ToLower(fieldInfo.Name)
        }
        fields[name] = v.Field(i)
    }

    // Update struct field for each parameter in the request.
    for name, values := range req.Form {
        f := fields[name]
        if !f.IsValid() {
            continue // ignore unrecognized HTTP parameters
        }
        for _, value := range values {
            if f.Kind() == reflect.Slice {
                elem := reflect.New(f.Type().Elem()).Elem()
                if err := populate(elem, value); err != nil {
                    return fmt.Errorf("%s: %v", name, err)
                }
                f.Set(reflect.Append(f, elem))
            } else {
                if err := populate(f, value); err != nil {
                    return fmt.Errorf("%s: %v", name, err)
                }
            }
        }
    }
    return nil
}

func populate(v reflect.Value, value string) error {
    switch v.Kind() {
    case reflect.String:
        v.SetString(value)

    case reflect.Int:
        i, err := strconv.ParseInt(value, 10, 64)
        if err != nil {
            return err
        }
        v.SetInt(i)

    case reflect.Bool:
        b, err := strconv.ParseBool(value)
        if err != nil {
            return err
        }
        v.SetBool(b)

    default:
        return fmt.Errorf("unsupported kind %s", v.Type())
    }
    return nil
}

func Print(x interface{}) {
    v := reflect.ValueOf(x)
    t := v.Type()
    fmt.Printf("type %s\n", t)

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

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

type MyType struct{}

func (m MyType) ValueMethod() {} // value receiver
func (m *MyType) PtrMethod()  {} // pointer receiver

func printReceiverInfo(x interface{}) {
    t := reflect.TypeOf(x)
    fmt.Printf("t = %v, NumMethod = %d\n", t, t.NumMethod())
    fmt.Printf("IsPtr = %v\n", t.Kind() == reflect.Ptr)

    for i := 0; i < t.NumMethod(); i++ {
        method := t.Method(i)
        receiverType := method.Func.Type().In(0)
        fmt.Printf("  %s: In(0) = %v, IsPtr = %v\n",
            method.Name, receiverType, receiverType.Kind() == reflect.Ptr)
    }
    fmt.Println()
}

t = main.MyType, NumMethod = 1
IsPtr = false
  ValueMethod: In(0) = main.MyType, IsPtr = false

t = *main.MyType, NumMethod = 2
IsPtr = true
  PtrMethod: In(0) = *main.MyType, IsPtr = true
  ValueMethod: In(0) = *main.MyType, IsPtr = true

type MyType struct{}

func (m MyType) ValueMethod() {}  // value receiver
func (m *MyType) PtrMethod() {}   // pointer receiver

t := reflect.TypeOf(MyType{})
pt := reflect.PtrTo(t) // *T

fmt.Println(t.NumMethod())  // 1 (только ValueMethod)
fmt.Println(pt.NumMethod()) // 2 (ValueMethod + PtrMethod)

var w io.Writer
t := reflect.TypeOf(w) // t == ???

var v interface{} = w  // v == nil 
t = reflect.TypeOf(v)  // t == ???

var w io.Writer
ptrT := reflect.TypeOf(&w) // ptrT == *io.Writer
t    := ptrT.Elem()        // t == io.Writer