[semigroupoid]

Of course compiling successfully does not imply a correct program with the desired behavior. Nobody claims that GHC is able to verify the behavioral correctness of your program and it also can't tell you if your function actually sorts a list or if it does not. However, if you claim in your definition of "sort" that it takes a list and returns a list, then GHC can verify if that is actually true. That is the power of types. They are no magic things that write correct programs for you.