Procedures

Preconditions

Files:     [Slides]     [Code examples]

Postconditions

Files:     [Slides with solutions]     [Code examples with solutions]

Notice that some slides have been corrected after class.

What you should have learned after completing chapter 4.2:

• The semantics of recursive procedures can be formalized in turns of higher-order fixed points.
• We reason modularly about procedure calls, that is, we rely solely on the procedure contract without inspecting its implementation.
• For partial correctness reasoning, this yields a simple approach: try to proof the specification for the procedure and use the contracts whenever we encounter a procedure call.
• In contrast to loops, we cannot strengthen procedure contracts whenever we encounter a new client. It is thus very important to support framing: properties that are not affected by a call should be preserved.
• Similarly to loops, we use variants to prove that a procedure terminates.
• You should be able to automate modular reasoning about procedures and termination proofs with variants by encoding these concepts into PL0.