Go Interfaces

type Shape interface { # define interface
    Area() float64 # method signature
    Perimeter() float64
}

type Rectangle struct { # define struct
    width, height float64
}
func (r Rectangle) Area() float64 { # implement method
    return r.width * r.height
}
func (r Rectangle) Perimeter() float64 {
    return 2 * (r.width + r.height)
}

var s Shape # declare interface variable
s = Rectangle{width: 10, height: 5} # assign concrete type
area := s.Area() # call interface method

var i interface{} # can hold any type
i = 42 # assign int
i = "hello" # assign string
value := i.(string) # type assertion
value, ok := i.(int) # safe type assertion

switch v := i.(type) { # check type
case int:
    fmt.Println("Integer:", v)
case string:
    fmt.Println("String:", v)
default:
    fmt.Println("Unknown type")
}