Dependency Injection

• by having familiarity with the io.Writer interface
• we were able to use the bytes.Buffer in our tests

• and the use other writers in the standard library to use our function in a command line or web server

• test code

• if you can't test a function easily, it's usually because of dependencies being hard wired into a function or global state
• dependency injection motivates you to inject a dependency via an interface to mock out the resource which enables testing
• separate concerns
• decouple where data goes from how it is generated
• allow code ot be reusable