The Shape Interface

An interface in Go is a list of method signatures:

type Shape interface {
	Area() float64
	Perimeter() float64
}

A type satisfies it by having those methods. There is no implements, no registration, and nothing in Rect mentions Shape. The compiler checks the match where the value is used - assigned to a Shape variable, passed to a Shape parameter, appended to a []Shape - and that is the only place the relationship exists.

That inversion matters: the interface belongs to the code that consumes it, not to the types that satisfy it. You can write an interface for a type you did not write, in your own package, after the fact.

Two values, one variable

An interface value holds two things: the concrete type, and the value of that type. var s Shape = Rect{2, 3} records "this is a Rect" beside the Rect itself. Calling s.Area() looks up Rect's Area and runs it, so the same line of code does different work depending on what is in there.

The zero value of an interface is nil: no type, no value. Calling a method on it panics, so a function that returns "nothing" as an interface returns nil and callers check for it.

Asking what is inside

A type assertion recovers the concrete value, and the comma-ok form asks rather than insists:

r, ok := s.(Rect)      // ok is false if s does not hold a Rect

The same syntax works with another interface on the right, which asks whether the value has that method set as well:

if n, ok := s.(Named); ok {
	fmt.Println(n.Name())
}

Without , ok, a failed assertion panics. Use the two-value form unless you can prove the assertion holds.

Small interfaces

The interfaces you meet in the standard library are one or two methods: io.Reader, io.Writer, fmt.Stringer, error. Small interfaces are easy to satisfy by accident, which is exactly the point - a type written years ago fits a function written today. Start with the smallest set of methods your function actually calls.

Task

Implement Area and Perimeter on Rect and Circle, Name on Circle, then the four functions that work through the interface: TotalArea, Largest, Describe and CountAtLeast.

Hints

• math.Pi is the constant; Circle{R: 2} has an area of 4 * math.Pi.
• Largest starts with var best Shape. The zero value of an interface is nil, so an empty slice returns nil without a special case.
• Describe needs if n, ok := s.(Named); ok - a plain s.(Named) would panic on a Rect, which has no Name method.
• %.2f formats both numbers. Circle{R: 1} describes as circle: area 3.14, perimeter 6.28.
• CountAtLeast is inclusive: an area exactly equal to min counts.