Using the Handle Abstraction

Our haskellings program is starting to get a bit more complicated, so it would be nice to write some unit tests for it. But this is very difficult to do for a command line application! To set this up, we'll start by refactoring our program to use the Handle abstraction. This allows the program to work regardless of whether it's using the command line or file for its input. So the user will see the command line version, but our tests will be able to use files! We'll also use the process of Compile Driven Development to systematically refactor our program to use these new configuration elements!