Return of the Clocks

Last time, Chris left you with a blank drawing window, and now it’s time to turn it into a clock. This lesson we use

• the time library to get the time and display it as a string;
• a TVar from the stm package to give the application some mutable state;
• some basic concurrency primitives to start a thread that will advance the clock; and
• the pango package to set the font.

Now we have a clock! Along the way, we talk about the drawing process and the concept of invalidating areas of the window in order to repaint them.

This lesson is all about the Bifunctor laws and property testing them. Since the Bifunctor laws are similar to the Functor laws, we also spend a good portion of this lesson familiarizing ourselves with aspects of the hedgehog testing library, especially ranges and the concept of shrinking.