shapes: add Shape interface

shapes/shapes.go

@@ -2,6 +2,10 @@ package shapes
 
 import "math"
 
+type Shape interface {
+	Area() float64
+}
+
 type Rectangle struct {
 	Width  float64
 	Height float64

shapes/shapes_test.go

@@ -13,23 +13,24 @@ func TestPerimeter(t *testing.T) {
 }
 
 func TestArea(t *testing.T) {
-	t.Run("rectangles", func(t *testing.T) {
-		rectangle := Rectangle{30.5, 20.5}
-		got := rectangle.Area()
-		exp := 625.25
+	checkArea := func(t testing.TB, shape Shape, exp float64) {
+		t.Helper()
+		got := shape.Area()
 
 		if got != exp {
 			t.Errorf("got %.2f expected %.2f", got, exp)
 		}
+	}
+
+	t.Run("rectangles", func(t *testing.T) {
+		rectangle := Rectangle{30.5, 20.5}
+		exp := 625.25
+		checkArea(t, rectangle, exp)
 	})
 
 	t.Run("circles", func(t *testing.T) {
 		circle := Circle{10}
-		got := circle.Area()
 		exp := 314.1592653589793
-
-		if got != exp {
-			t.Errorf("got %.2f expected %.2f", got, exp)
-		}
+		checkArea(t, circle, exp)
 	})
 }