Hoofdstuk 7 - Formal Specifications

Question 1

What does it mean when we say that a formal specification is (a) consistent, (b) complete and (c) unambiguous ?

Answer 1

◦ (a) A formal specification is consistent when there are no contradictions in the specification
◦ (b) A formal specification is complete when all properties are defined in terms of known concepts
◦ (c) A formal specification is unambiguous when misinterpretations are impossible, it could also be minimal.

Question 2

What does it mean for a state-chart to be (a) consistent, (b) complete, and (c) unambiguous ?

Answer 2

◦ (a) A state-chart is consistent when every state is reachable from the initial state and the final state is reachable from every other state.
◦ (b) A state-chart is complete when every event (state pair) has a transition
◦ (c) A state-chart is unambiguous (deterministic) when the same event does not appear on more than one transition leaving a given state.

Question 3

What does it mean for an algebraic specification to be (a) consistent and (b) complete ?

Answer 3

◦ (a) An algebraic specification is consistent when it is never possible to deduce contradictions
◦ (b) An algebraic specification is complete when all query expressions can be reduced

Question 4

Why is an UML class diagram a semi-formal specification?

Answer 4

Because it is based on a notation with precise syntax but loose semantics

Question 5

Can you give 3 arguments against formal methods? Can you counter them?

Answer 5

◦ High cost of specification but better verifiability
◦ Requires highly trained staff but only simple math and logic is required
◦ Customers don’t understand but depends on customers and specification style, prototypes can be generated

Question 6

What’s the distinction between a semi-formal and a formal specification?

Answer 6

The semantics are not loose

Question 7

Why is it necessary to complement sequence diagrams with state charts?

Answer 7

A state-chart allows to specify all valid (also all illegal) scenario’s