Type assertions in Go

When working with interfaces in Go, every once-in-awhile you'll need access to the underlying type of an interface value. You can cast an interface to its underlying type using a type assertion.

type shape interface {
	area() float64
}

type circle struct {
	radius float64
}

// "c" is a new circle cast from "s"
// which is an instance of a shape.
// "ok" is a bool that is true if s was a circle
// or false if s isn't a circle
c, ok := s.(circle)

Assignment

Implement the getExpenseReport function.

• If the expense is an email then it should return the email's toAddress and the cost of the email.
• If the expense is an sms then it should return the sms's toPhoneNumber and its cost.
• If the expense has any other underlying type, just return an empty string and 0.0 for the cost.