Merge branch 'reflect_lecture' into 'master'

Reflect lecture See merge request slon/shad-go-private!29
2020-04-16 15:06:20 +00:00 · 2020-04-16 15:06:20 +00:00 · 8ddace7b75
commit 8ddace7b75
parent ce8a26a0ea 0bb71917f1
9 changed files with 924 additions and 0 deletions
--- a/lectures/08-reflect/display/display.go
+++ b/lectures/08-reflect/display/display.go
@ -0,0 +1,92 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 333.
+
+// Package display provides a means to display structured data.
+package display
+
+import (
+	"fmt"
+	"reflect"
+	"strconv"
+)
+
+//!+Display
+
+func Display(name string, x interface{}) {
+	fmt.Printf("Display %s (%T):\n", name, x)
+	display(name, reflect.ValueOf(x))
+}
+
+// OMIT
+
+// formatAtom formats a value without inspecting its internal structure.
+// It is a copy of the the function in gopl.io/ch11/format.
+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:
+		if v.Bool() {
+			return "true"
+		}
+		return "false"
+	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"
+	}
+}
+
+//!+display
+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))
+		}
+
+		// OMIT
+	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))
+	}
+}
+
+// OMIT
--- a/lectures/08-reflect/display/display_test.go
+++ b/lectures/08-reflect/display/display_test.go
@ -0,0 +1,259 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+package display
+
+import (
+	"io"
+	"net"
+	"os"
+	"reflect"
+	"sync"
+	"testing"
+
+	"gopl.io/ch7/eval"
+)
+
+// NOTE: we can't use !+..!- comments to excerpt these tests
+// into the book because it defeats the Example mechanism,
+// which requires the // Output comment to be at the end
+// of the function.
+
+func Example_expr() {
+	e, _ := eval.Parse("sqrt(A / pi)")
+	Display("e", e)
+	// Output:
+	// Display e (eval.call):
+	// e.fn = "sqrt"
+	// e.args[0].type = eval.binary
+	// e.args[0].value.op = 47
+	// e.args[0].value.x.type = eval.Var
+	// e.args[0].value.x.value = "A"
+	// e.args[0].value.y.type = eval.Var
+	// e.args[0].value.y.value = "pi"
+}
+
+func Example_slice() {
+	Display("slice", []*int{new(int), nil})
+	// Output:
+	// Display slice ([]*int):
+	// (*slice[0]) = 0
+	// slice[1] = nil
+}
+
+func Example_nilInterface() {
+	var w io.Writer
+	Display("w", w)
+	// Output:
+	// Display w (<nil>):
+	// w = invalid
+}
+
+func Example_ptrToInterface() {
+	var w io.Writer
+	Display("&w", &w)
+	// Output:
+	// Display &w (*io.Writer):
+	// (*&w) = nil
+}
+
+func Example_struct() {
+	Display("x", struct{ x interface{} }{3})
+	// Output:
+	// Display x (struct { x interface {} }):
+	// x.x.type = int
+	// x.x.value = 3
+}
+
+func Example_interface() {
+	var i interface{} = 3
+	Display("i", i)
+	// Output:
+	// Display i (int):
+	// i = 3
+}
+
+func Example_ptrToInterface2() {
+	var i interface{} = 3
+	Display("&i", &i)
+	// Output:
+	// Display &i (*interface {}):
+	// (*&i).type = int
+	// (*&i).value = 3
+}
+
+func Example_array() {
+	Display("x", [1]interface{}{3})
+	// Output:
+	// Display x ([1]interface {}):
+	// x[0].type = int
+	// x[0].value = 3
+}
+
+func Example_movie() {
+	//!+movie
+	type Movie struct {
+		Title, Subtitle string
+		Year            int
+		Color           bool
+		Actor           map[string]string
+		Oscars          []string
+		Sequel          *string
+	}
+	//!-movie
+	//!+strangelove
+	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",
+			"Brig. Gen. Jack D. Ripper":  "Sterling Hayden",
+			`Maj. T.J. "King" Kong`:      "Slim Pickens",
+		},
+
+		Oscars: []string{
+			"Best Actor (Nomin.)",
+			"Best Adapted Screenplay (Nomin.)",
+			"Best Director (Nomin.)",
+			"Best Picture (Nomin.)",
+		},
+	}
+	//!-strangelove
+	Display("strangelove", strangelove)
+
+	// We don't use an Output: comment since displaying
+	// a map is nondeterministic.
+	/*
+		//!+output
+		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
+		//!-output
+	*/
+}
+
+// This test ensures that the program terminates without crashing.
+func Test(t *testing.T) {
+	// Some other values (YMMV)
+	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
+
+	var w io.Writer = os.Stderr
+	Display("&w", &w)
+	// Output:
+	// Display &w (*io.Writer):
+	// (*&w).type = *os.File
+	// (*(*(*&w).value).file).fd = 2
+	// (*(*(*&w).value).file).name = "/dev/stderr"
+	// (*(*(*&w).value).file).nepipe = 0
+
+	var locker sync.Locker = new(sync.Mutex)
+	Display("(&locker)", &locker)
+	// Output:
+	// Display (&locker) (*sync.Locker):
+	// (*(&locker)).type = *sync.Mutex
+	// (*(*(&locker)).value).state = 0
+	// (*(*(&locker)).value).sema = 0
+
+	Display("locker", locker)
+	// Output:
+	// Display locker (*sync.Mutex):
+	// (*locker).state = 0
+	// (*locker).sema = 0
+	// (*(&locker)) = nil
+
+	locker = nil
+	Display("(&locker)", &locker)
+	// Output:
+	// Display (&locker) (*sync.Locker):
+	// (*(&locker)) = nil
+
+	ips, _ := net.LookupHost("golang.org")
+	Display("ips", ips)
+	// Output:
+	// Display ips ([]string):
+	// ips[0] = "173.194.68.141"
+	// ips[1] = "2607:f8b0:400d:c06::8d"
+
+	// Even metarecursion!  (YMMV)
+	Display("rV", reflect.ValueOf(os.Stderr))
+	// Output:
+	// Display rV (reflect.Value):
+	// (*rV.typ).size = 8
+	// (*rV.typ).ptrdata = 8
+	// (*rV.typ).hash = 871609668
+	// (*rV.typ)._ = 0
+	// ...
+
+	// a pointer that points to itself
+	type P *P
+	var p P
+	p = &p
+	if false {
+		Display("p", p)
+		// Output:
+		// Display p (display.P):
+		// ...stuck, no output...
+	}
+
+	// a map that contains itself
+	type M map[string]M
+	m := make(M)
+	m[""] = m
+	if false {
+		Display("m", m)
+		// Output:
+		// Display m (display.M):
+		// ...stuck, no output...
+	}
+
+	// a slice that contains itself
+	type S []S
+	s := make(S, 1)
+	s[0] = s
+	if false {
+		Display("s", s)
+		// Output:
+		// Display s (display.S):
+		// ...stuck, no output...
+	}
+
+	// a linked list that eats its own tail
+	type Cycle struct {
+		Value int
+		Tail  *Cycle
+	}
+	var c Cycle
+	c = Cycle{42, &c}
+	if false {
+		Display("c", c)
+		// Output:
+		// Display c (display.Cycle):
+		// c.Value = 42
+		// (*c.Tail).Value = 42
+		// (*(*c.Tail).Tail).Value = 42
+		// ...ad infinitum...
+	}
+}
--- a/lectures/08-reflect/format/format.go
+++ b/lectures/08-reflect/format/format.go
@ -0,0 +1,41 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 332.
+
+// Package format provides an Any function that can format any value.
+//!+
+package format
+
+import (
+	"reflect"
+	"strconv"
+)
+
+// Any formats any value as a string.
+func Any(value interface{}) string {
+	return formatAtom(reflect.ValueOf(value))
+}
+
+// formatAtom formats a value without inspecting its internal structure.
+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"
+	}
+}
+
+// OMIT
--- a/lectures/08-reflect/format/format_test.go
+++ b/lectures/08-reflect/format/format_test.go
@ -0,0 +1,24 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+package format_test
+
+import (
+	"fmt"
+	"testing"
+	"time"
+
+	"gopl.io/ch12/format"
+)
+
+func Test(t *testing.T) {
+	// The pointer values are just examples, and may vary from run to run.
+	//!+time
+	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"
+	//!-time
+}
--- a/lectures/08-reflect/lecture.slide
+++ b/lectures/08-reflect/lecture.slide
@ -0,0 +1,276 @@
+reflect
+Лекция 7
+
+Короткий Фёдор
+
+* Reflection
+
+- Механизм для чтения и изменения значений, не зная их реальный тип момент компиляции.
+- Пример использования `reflect`: `fmt`, `encoding/json`.
+- Эти пакеты используют `reflect` в реализации, но *не*светят* reflect в интерфейсе.
+
+* Why Reflection?
+
+Иногда нам нужно API, которое работает со значениями, которые не
+объединены общим интерфейсом.
+
+  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 "???"
+      }
+  }
+
+* reflect.Type and reflect.Value
+
+- Пакет `reflect` определяет два основных типа: `reflect.Type` и `reflect.Value`.
+- Единственная реализация `reflect.Type` - _type_descriptor_.
+
+  func TypeOf(interface{}) Type
+
+- Пример:
+
+  t := reflect.TypeOf(3)  // a reflect.Type
+  fmt.Println(t.String()) // "int"
+  fmt.Println(t)          // "int"
+
+- `reflect.TypeOf` всегда возвращает конкретный тип.
+
+  var w io.Writer = os.Stdout
+  fmt.Println(reflect.TypeOf(w)) // "*os.File"
+
+- `Sprintf` формат `%T` использует `TypeOf` внутри.
+
+  fmt.Printf("%T\n", 3) // "int"
+
+* reflect.Type and reflect.Value
+
+- `reflect.Value` хранит значение любого типа.
+
+  v := reflect.ValueOf(3)
+  fmt.Println(v)          // "3"
+  fmt.Printf("%v\n", v)   // "3"
+  fmt.Println(v.String()) // NOTE: "<int Value>"
+
+- Метод `.Type()` возвращает тип значения, хранящегося внутри `Value`.
+
+  t := v.Type()           // a reflect.Type
+  fmt.Println(t.String()) // "int"
+
+- Метод `.Interface()` - обратная операция к `reflect.ValueOf`.
+
+  v := reflect.ValueOf(3) // a reflect.Value
+  x := v.Interface()      // an interface{}
+  i := x.(int)            // an int
+  fmt.Printf("%d\n", i)   // "3"
+
+* Разница между reflect.Value и interface{}
+
+- Оба хранят любое значение.
+- `interface{}` - хранит значение, но не даёт доступа к нему. Можно достать конкретный тип, через _type_assertion_.
+- `reflect.Value` - предоставляет доступ к значению внутри, вне зависимости от типа этого значения.
+
+* format.Any
+
+.code format/format.go /func Any/,/OMIT/
+
+* format.Any
+
+  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"
+
+* Debug Display
+
+  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",
+          // ...
+      },
+      // ...
+  }
+
+* Debug Display
+
+  Display("strangelove", strangelove)
+
+Вывод:
+
+  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
+
+* Debug Display
+
+  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
+
+* Debug Display
+
+.code display/display.go /func Display/,/OMIT/
+
+* Debug Display
+
+.code display/display.go /func display/,/OMIT/
+
+* Debug Display
+
+.code display/display.go /case reflect.Ptr/,/OMIT/
+
+* Setting Variables
+
+- Некоторые выражения являются переменными: `x`, `x.f[1]`, `*p`.
+- Некоторые выражения не являются переменными: `x`+`1`, `f(2)`.
+- Переменные имеют _адрес_. Значение переменной можно изменить.
+
+  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
+
+- `CanAddr`
+
+  fmt.Println(a.CanAddr()) // "false"
+  fmt.Println(b.CanAddr()) // "false"
+  fmt.Println(c.CanAddr()) // "false"
+  fmt.Println(d.CanAddr()) // "true"
+
+* Setting Variables
+
+  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"
+
+- Или напрямую
+
+  d.Set(reflect.ValueOf(4))
+  fmt.Println(x) // "4"
+
+- Или через специальный метод
+
+  d := reflect.ValueOf(&x).Elem()
+  d.SetInt(3)
+  fmt.Println(x) // "3"
+
+- Нельзя менять _не_адресуемое_ значение.
+
+  x := 2
+  b := reflect.ValueOf(x)
+  b.Set(reflect.ValueOf(3)) // panic: Set using unaddressable value
+
+* Setting Variables
+
+Приватные поля можно читать, но нельзя менять.
+
+  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"
+
+* Struct Tags
+
+.code search/main.go /func search/,/^}/
+
+* Struct Tags
+
+.code params/params.go /func Unpack/,/OMIT/
+
+.code params/params.go /Build map of/,/OMIT/
+
+* Struct Tags
+
+.code params/params.go /Update/,/^}/
+
+* Struct Tags
+
+.code params/params.go /func populate/,/^}/
+
+* Method Set
+
+.code methods/methods.go /func Print/,/^}/
+
+  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
+
+* Interface
+
+- Результат `ValueOf` - всегда конкретный тип.
+
+  var w io.Writer
+  t := reflect.TypeOf(w) // t == ???
+
+  var v interface{} = w  // v == nil 
+  t = reflect.TypeOf(v)  // t == ???
+
+- Чтобы получить `reflect.Type` равный интерфейсу, нужно использовать промежуточный указатель.
+
+  var w io.Writer
+  ptrT := reflect.TypeOf(&w) // ptrT == *io.Writer
+  t    := ptrT.Elem()        // t == io.Writer
+
+* Заключение
+
+- `reflect` создаёт хрупкий код. Там, где была бы ошибка компилятора, возникает `panic` во время исполнения.
+- Поведение `reflect` кода нужно документировать отдельно, и оно не ясно из типов аргументов.
+- `reflect` работает медленнее, чем специализация под конкретный тип.
+
--- a/lectures/08-reflect/methods/methods.go
+++ b/lectures/08-reflect/methods/methods.go
@ -0,0 +1,29 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 351.
+
+// Package methods provides a function to print the methods of any value.
+package methods
+
+import (
+	"fmt"
+	"reflect"
+	"strings"
+)
+
+//!+print
+// Print prints the method set of the value x.
+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"))
+	}
+}
+
+//!-print
--- a/lectures/08-reflect/methods/methods_test.go
+++ b/lectures/08-reflect/methods/methods_test.go
@ -0,0 +1,49 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+package methods_test
+
+import (
+	"strings"
+	"time"
+
+	"gopl.io/ch12/methods"
+)
+
+func ExamplePrintDuration() {
+	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
+}
+
+func ExamplePrintReplacer() {
+	methods.Print(new(strings.Replacer))
+	// Output:
+	// type *strings.Replacer
+	// func (*strings.Replacer) Replace(string) string
+	// func (*strings.Replacer) WriteString(io.Writer, string) (int, error)
+}
+
+/*
+//!+output
+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)
+//!-output
+*/
--- a/lectures/08-reflect/params/params.go
+++ b/lectures/08-reflect/params/params.go
@ -0,0 +1,94 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 349.
+
+// Package params provides a reflection-based parser for URL parameters.
+package params
+
+import (
+	"fmt"
+	"net/http"
+	"reflect"
+	"strconv"
+	"strings"
+)
+
+//!+Unpack
+
+// Unpack populates the fields of the struct pointed to by ptr
+// from the HTTP request parameters in req.
+func Unpack(req *http.Request, ptr interface{}) error {
+	if err := req.ParseForm(); err != nil {
+		return err
+	}
+
+	// OMIT
+
+	// 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)
+	}
+
+	// OMIT
+
+	// 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
+}
+
+//!-Unpack
+
+//!+populate
+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
+}
+
+//!-populate
--- a/lectures/08-reflect/search/main.go
+++ b/lectures/08-reflect/search/main.go
@ -0,0 +1,60 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 348.
+
+// Search is a demo of the params.Unpack function.
+package main
+
+import (
+	"fmt"
+	"log"
+	"net/http"
+)
+
+//!+
+
+import "gopl.io/ch12/params"
+
+// search implements the /search URL endpoint.
+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 main() {
+	http.HandleFunc("/search", search)
+	log.Fatal(http.ListenAndServe(":12345", nil))
+}
+
+/*
+//!+output
+$ go build gopl.io/ch12/search
+$ ./search &
+$ ./fetch 'http://localhost:12345/search'
+Search: {Labels:[] MaxResults:10 Exact:false}
+$ ./fetch 'http://localhost:12345/search?l=golang&l=programming'
+Search: {Labels:[golang programming] MaxResults:10 Exact:false}
+$ ./fetch 'http://localhost:12345/search?l=golang&l=programming&max=100'
+Search: {Labels:[golang programming] MaxResults:100 Exact:false}
+$ ./fetch 'http://localhost:12345/search?x=true&l=golang&l=programming'
+Search: {Labels:[golang programming] MaxResults:10 Exact:true}
+$ ./fetch 'http://localhost:12345/search?q=hello&x=123'
+x: strconv.ParseBool: parsing "123": invalid syntax
+$ ./fetch 'http://localhost:12345/search?q=hello&max=lots'
+max: strconv.ParseInt: parsing "lots": invalid syntax
+//!-output
+*/