type switch

switch.go

package main

import "fmt"

type Fetchable interface {
  URL() string
}

type Completable interface {
  HasCompleted()
}

type Foo struct{}

type Bar struct{}

func (f Foo) URL() string {
  return "example.cm/foo"
}

func (b Bar) URL() string {
  return "example.com/bar"
}

func (s Bar) HasCompleted() {
  fmt.Println("Bar has completed")
}

func PresentURL(f Fetchable) {
  fmt.Println("URL: ", f.URL())

  switch o := f.(type) {
  case Completable:
    o.HasCompleted()
  default:
    fmt.Println("This item is not completable")
  }
}

func main() {
  f, b := Foo{}, Bar{}

  PresentURL(f)
  PresentURL(b)
}