It creates a Schroedinger type which has two data constructors: an Opened constructor which takes a Probable object — that is, whose Live or Dead state is known — and an Unopened constructor which takes a random generator, and an object without a particular state, such as a Cat.

… If, however, you bind a function to an Opened box with a Live thing, it will apply the function to what’s in the box — in this case, the Cat itself.

… Here is the meat of this example, it’s reason for being, all contained within this one line: If you bind a function to an Unopened box, it gets bound in turn to an Opened box containing a Cat whose fate has been decided by the dice.

… This is fairly linear: it gets a random generator from the operating system, then creates an Unopened box and returns it, which gets printed. print does its work by calling show on the Schroedinger type, since it was derived from Show earlier.