I thought that was the point of the section- that you may be able to tell something is wrong with the example, but the compiler can't. Thus by programming in that fashion you're not taking advantage of the power of the compiler.
The improved way below where the Just a is unwrapped with a case allows the compiler to see whether or not x is valid.
The improved way below where the Just a is unwrapped with a case allows the compiler to see whether or not x is valid.