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course/5-interfaces/exercises/1-interfaces/readme.md

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+# Interfaces in Go
+
+Interfaces are collections of method signatures. A type "implements" an interface if it has all of the methods of the given interface defined on it.
+
+In the following example, a "shape" must be able to return its area and perimeter. Both `rect` and `circle` fulfill the interface.
+
+```go
+type shape interface {
+  area() float64
+  perimeter() float64
+}
+
+type rect struct {
+    width, height float64
+}
+func (r rect) area() float64 {
+    return r.width * r.height
+}
+func (r rect) perimeter() float64 {
+    return 2*r.width + 2*r.height
+}
+
+type circle struct {
+    radius float64
+}
+func (c circle) area() float64 {
+    return math.Pi * c.radius * c.radius
+}
+func (c circle) perimeter() float64 {
+    return 2 * math.Pi * c.radius
+}
+```
+
+When a type implements an interface, it can then be used as the interface type.
+
+## Assignment
+
+The `birthdayMessage` and `sendingReport` structs have already implemented the `getMessage` methods. The `getMessage` method simply returns a string, and any type that implements the method can be considered a `message`.
+
+First, add the `getMessage()` method as a requirement on the method interface.
+
+Second, complete the `sendMessage` function. It should print a message's `message`, which it obtains through the interface method. Notice that your code doesn't need to worry *at all* about whether a specific message is a `birthdayMessage` or a `sendingReport`!