The higher level question is whether the Haskell specification fully and correctly specifies the desired behavior. In my 30-odd years of experience in Mil/Aerospace, I have never seen a fully and correctly specified set of requirements that could be transliterated into correct executable code. If nothing else, they all have had implicit assumptions. That is the Achilles heel of the IBM "Master Programmer" method, reborn as "outsourcing".

Since the specification is executable Haskell, that implies they wrote Haskell test programs to show that the specification implements the desired behavior.

Writing a program to verify the specification that another program implements... and then claiming a formal proof of correctness of the system is now recursive. Turtles all the way down.