In Go, when a type implements an interface, it must implement all the methods defined in that interface. Your geometry interface specifies two methods: area() and perim(). However, the circle type only implements the area() method—it does not implement perim().

So when you call detectShape(cir1), Go expects cir1 to fully satisfy the geometry interface, but it doesn't, because it lacks the perim() method. This results in a compilation error.

Your first call (fmt.Print(cir1.area(), " ")) works just fine because you're directly calling the method implemented in circle, without involving the interface.

To fix this issue, you need to provide an implementation for perim() in circle, like so :

func (c circle) perim() float32 {
    return 2 * math.Pi * c.radius
}

Once you add this, circle will fully satisfy geometry, and detectShape(cir1) will execute properly.