added type assertion example (#567)

* added type assertion example

* simplified example

examples/interfaces/interfaces.go

@@ -51,6 +51,15 @@ func measure(g geometry) {
 	fmt.Println(g.perim())
 }
 
+// Type assertion can be performed to explicitly check the runtime type of the value.
+// It allows the access of fields and methods belonging to the specific type.
+// See [`switch` example](switch) for an alternative approach to handle type assertion.
+func detectCircle(g geometry) {
+	if c, ok := g.(circle); ok {
+		fmt.Println(c.radius)
+	}
+}
+
 func main() {
 	r := rect{width: 3, height: 4}
 	c := circle{radius: 5}
@@ -61,4 +70,10 @@ func main() {
 	// these structs as arguments to `measure`.
 	measure(r)
 	measure(c)
+
+	// `detectCircle` takes structs that satisfy the `geometry` interface
+	// if the struct is of type `circle`, it prints out the radius.
+	detectCircle(r) // doesn't print anything.
+	detectCircle(c)
+
 }

examples/interfaces/interfaces.hash

@@ -1,2 +1,2 @@
-d4ea9541521cfee94107ba9331d0dabb1f9f16c1
-XJASG4MxBQr
+9a362e2c9aed98013fa9b91af81d6cc373979db6
+tfsLP7R0dtH

examples/interfaces/interfaces.sh

@@ -5,6 +5,7 @@ $ go run interfaces.go
 {5}
 78.53981633974483
 31.41592653589793
+5
 
 # To learn more about Go's interfaces, check out this
 # [great blog post](https://jordanorelli.tumblr.com/post/32665860244/how-to-use-interfaces-in-go).