Методы и Интерфейсы
Лекция 3

Фёдор Короткий

* Methods

Пример вызова метода:

  const day = 24 * time.Hour
  fmt.Println(day.Seconds()) // "86400"

Пример определения метода:

  func (c Celsius) String() string { return fmt.Sprintf("%g°C", c) }

* Methods

.play geometry/point.go

- Специальный аргумент называется _receiver_
- Мы *не*используем* `this` или `self`
- _receiver_ обычно называют по первой букве в имени типа

* Method call

.play geometry/point_test.go /func/,/^}/

* Path

  // A Path is a journey connecting the points with straight lines.
  type Path []Point

  // Distance returns the distance traveled along the path.
  func (path Path) Distance() float64 {
      sum := 0.0
      for i := range path {
          if i > 0 {
              sum += path[i-1].Distance(path[i])
          }
      }
      return sum
  }

* Path

  perim := Path{
      {1, 1},
      {5, 1},
      {5, 4},
      {1, 1},
  }
  fmt.Println(perim.Distance()) // "12"

* Pointer Receiver

  func (p *Point) ScaleBy(factor float64) {
      p.X *= factor
      p.Y *= factor
  }

* Nil receiver

  type IntList struct {
      Value int
      Tail *IntList
  }

  func (list *IntList) Sum() int {
      if list == nil {
          return 0
      }
      return list.Value + list.Tail.Sum()
  }

* Embedding

  type Point struct{ X, Y float64 }
  type ColoredPoint struct {
      Point
      Color color.RGBA
  }

Methods

  red := color.RGBA{255, 0, 0, 255}
  blue := color.RGBA{0, 0, 255, 255}
  var p = ColoredPoint{Point{1, 1}, red}
  var q = ColoredPoint{Point{5, 4}, blue}
  fmt.Println(p.Distance(q.Point)) // "5"
  p.ScaleBy(2)
  q.ScaleBy(2)
  fmt.Println(p.Distance(q.Point)) // "10"

* Embedding != Inheritance

  var p = ColoredPoint{Point{1, 1}, red}
  var q = ColoredPoint{Point{5, 4}, blue}

  p.Distance(q) // compile error: cannot use q (ColoredPoint) as Point

* Lock

  var cache = struct {
      sync.Mutex
      mapping map[string]string
  } {
      mapping: make(map[string]string),
  }

  func Lookup(key string) string {
      cache.Lock()
      v := cache.mapping[key]
      cache.Unlock()
      return v
  }

* Method Values

  p := Point{1, 2}
  q := Point{4, 6}

  distanceFromP := p.Distance
  fmt.Println(distanceFromP(q))
  var origin Point
  fmt.Println(distanceFromP(origin))

  scaleP := p.ScaleBy
  scaleP(2)
  scaleP(3)
  scaleP(10)

* Method Values

  type Rocket struct { /* ... */ }
  func (r *Rocket) Launch() { /* ... */ }

  r := new(Rocket)
  time.AfterFunc(10 * time.Second, func() { r.Launch() })

  time.AfterFunc(10 * time.Second, r.Launch)

* Method Expression

  p := Point{1, 2}
  q := Point{4, 6}
  distance := Point.Distance  // method expression
  fmt.Println(distance(p, q)) // "5"
  fmt.Printf("%T\n", distance) // "func(Point, Point) float64"
  scale := (*Point).ScaleBy
  scale(&p, 2)
  fmt.Println(p) // "{2 4}"
  fmt.Printf("%T\n", scale) // "func(*Point, float64)"

* Encapsulation

  type Counter struct { n int }

  func (c *Counter) N() int { return c.n }
  func (c *Counter) Increment() { c.n++ }
  func (c *Counter) Reset() { c.n = 0 }

Сетеры/гетеры для простых типов не приветствуются.

  type Point { x, y int }

  func (p *Point) X() int { return x }
  func (p *Point) Y() int { return y }
  func (p *Point) SetX(x int) { p.x = x }
  func (p *Point) SetY(y int) { p.y = y }

* Interfaces

  func Fprintf(w io.Writer, format string, args ...interface{}) (int, error)

  func Printf(format string, args ...interface{}) (int, error) {
      return Fprintf(os.Stdout, format, args...)
  }

  func Sprintf(format string, args ...interface{}) string {
      var buf bytes.Buffer
      Fprintf(&buf, format, args...)
      return buf.String()
  }

* Interfaces

  // Writer is the interface that wraps the basic Write method.
  type Writer interface {
      // Write writes len(p) bytes from p to the underlying data stream.
      // It returns the number of bytes written from p (0 <= n <= len(p))
      // and any error encountered that caused the write to stop early.
      // Write must return a non-nil error if it returns n < len(p).
      // Write must not modify the slice data, even temporarily.
      //
      // Implementations must not retain p.
      Write(p []byte) (n int, err error)
  }

* Interfaces

  type ByteCounter int

  func (c *ByteCounter) Write(p []byte) (int, error) {
      *c += ByteCounter(len(p)) // convert int to ByteCounter
      return len(p), nil
  }

  var c ByteCounter
  c.Write([]byte("hello"))
  fmt.Println(c) // "5", = len("hello")

* Stringer

  package fmt

  // The String method is used to print values passed
  // as an operand to any format that accepts a string
  // or to an unformatted printer such as Print.
  type Stringer interface {
      String() string
  }

* Interface types

  package io

  type Reader interface {
      Read(p []byte) (n int, err error)
  }

  type Closer interface {
      Close() error
  }

  type ReadWriter interface {
      Reader
      Writer
  }

  type ReadWriteCloser interface {
      Reader
      Writer
      Closer
  }

* Interface satisfaction

  var w io.Writer
  w = os.Stdout // OK: *os.File has Write method
  w = new(bytes.Buffer) // OK: *bytes.Buffer has Write method
  w = time.Second // compile error: time.Duration lacks Write method

* MethodSet

  type IntSet struct { /* ... */ }
  func (*IntSet) String() string

  var s IntSet
  var _ = s.String() // OK: s is a variable and &s has a String method

  var _ fmt.Stringer = &s // OK
  var _ fmt.Stringer = s // compile error: IntSet lacks String method

* interface{}

  var any interface{}
  any = true
  any = 12.34
  any = "hello"
  any = map[string]int{"one": 1}
  any = new(bytes.Buffer)

* Interface satisfaction

  // *bytes.Buffer must satisfy io.Writer
  var _ io.Writer = (*bytes.Buffer)(nil)

* Interface values

  var w io.Writer
  w = os.Stdout
  w = new(bytes.Buffer)
  w = nil

  var x interface{} = time.Now()

* Nil pointer

  var buf *bytes.Buffer
  if debug {
      buf = new(bytes.Buffer) // enable collection of output
  }
  f(buf) // NOTE: subtly incorrect!

  // If out is non-nil, output will be written to it.
  func f(out io.Writer) {
      // ...do something...
      if out != nil {
          out.Write([]byte("done!\n"))
      }
  }

* Nil pointer fix

  var buf io.Writer
  if debug {
      buf = new(bytes.Buffer) // enable collection of output
  }
  f(buf) // OK

* Type Assertions

Concrete type:

  var w io.Writer
  w = os.Stdout
  f := w.(*os.File) // success: f == os.Stdout
  c := w.(*bytes.Buffer) // panic: interface holds *os.File, not *bytes.Buffer

Interface type:

  var w io.Writer
  w = os.Stdout
  rw := w.(io.ReadWriter) // success: *os.File has both Read and Write
  w = new(ByteCounter)
  rw = w.(io.ReadWriter) // panic: *ByteCounter has no Read method

* Type assertion

  var w io.Writer
  var rw io.ReadWriter

  w = rw // io.ReadWriter is assignable to io.Writer
  w = rw.(io.Writer) // fails only if rw == nil

Check type

  var w io.Writer = os.Stdout
  f, ok := w.(*os.File) // success: ok, f == os.Stdout
  b, ok := w.(*bytes.Buffer) // failure: !ok, b == nil

* Type switches

  func sqlQuote(x interface{}) string {
      if x == nil {
          return "NULL"
      } else if _, ok := x.(int); ok {
          return fmt.Sprintf("%d", x)
      } else if _, ok := x.(uint); ok {
          return fmt.Sprintf("%d", x)
      } else if b, ok := x.(bool); ok {
          if b {
              return "TRUE"
          }
          return "FALSE"
      } else if s, ok := x.(string); ok {
          return sqlQuoteString(s) // (not shown)
      } else {
          panic(fmt.Sprintf("unexpected type %T: %v", x, x))
      }
  }

* Type switch

  switch x.(type) {
  case nil: // ...
  case int, uint: // ...
  case bool: // ...
  case string: // ...
  default: // ...
  }

  switch x := x.(type) { /* ... */ }

* Type switch

  func sqlQuote(x interface{}) string {
      switch x := x.(type) {
      case nil:
          return "NULL"
      case int, uint:
          return fmt.Sprintf("%d", x) // x has type interface{} here.
      case bool:
          if x {
              return "TRUE"
          }
          return "FALSE"
      case string:
          return sqlQuoteString(x) // (not shown)
      default:
          panic(fmt.Sprintf("unexpected type %T: %v", x, x))
      }
  }

* panic, defer, recover

  switch s := suit(drawCard()); s {
  case "Spades": // ...
  case "Hearts": // ...
  case "Diamonds": // ...
  case "Clubs": // ...
  default:
      panic(fmt.Sprintf("invalid suit %q", s)) // Joker?
  }

  func Reset(x *Buffer) {
      if x == nil {
          panic("x is nil") // unnecessary!
      }
      x.elements = nil
  }

* defer

  func main() {
      f(3)
  }

  func f(x int) {
      fmt.Printf("f(%d)\n", x+0/x) // panics if x == 0
      defer fmt.Printf("defer %d\n", x)
      f(x - 1)
  }

Output:

  f(3)
  f(2)
  f(1)
  defer 1
  defer 2
  defer 3

* recover

  func Parse(input string) (s *Syntax, err error) {
      defer func() {
          if p := recover(); p != nil {
              err = fmt.Errorf("internal error: %v", p)
          }
      }()
      // ...parser...
  }

* errors

  type error interface {
      Error() string
  }

  type Unwrapper interface {
      Unwrap() error
  }

Unwrap

  errors.Unwrap(fmt.Errorf("... %w ...", ..., err, ...)) == err

* errors.Is & errors.As

Is

  if errors.Is(err, os.ErrExist)

  if err == os.ErrExist

As

  var perr *os.PathError
  if errors.As(err, &perr) {
      fmt.Println(perr.Path)
  }

  if perr, ok := err.(*os.PathError); ok {
      fmt.Println(perr.Path)
  }