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Ambiguity

Ambiguity. Common signs on escalators in the UK What do they mean? if I carry two pairs of new shoes I just bought? if I have no shoes? if I have no dog?. Shoes Must Be Worn. Dogs Must Be Carried. Formalisms to the Rescue?. Use the Predicate Calculus:

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Ambiguity

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  1. Ambiguity • Common signs on escalators in the UK • What do they mean? • if I carry two pairs of new shoes I just bought? • if I have no shoes? • if I have no dog? Shoes Must Be Worn Dogs Must Be Carried CSC 402 Fall 2002

  2. Formalisms to the Rescue? • Use the Predicate Calculus: For All x (OnEscalator(x) -> Exists y such that (PairOfShoes(y) AND IsWearing(x,y)) and for the second sign we’ll have: For all x ((On Escalator(x) AND IsDog(x)) -> IsCarried(x)) CSC 402 Fall 2002

  3. Do Dogs Have to Wear Shoes? • The formalism did not automatically remove ambiguity • do dogs have to wear shoes? • what counts as a pair of shoes? • what counts as “wearing” shoes? • there are ways to fix this, but it shows formalisms are not magic • Did you notice the signs read “must” ? • this is known as “optative” mood (the language of requirements, normative concerns) • but the formalism is in the “indicative” mood CSC 402 Fall 2002

  4. Refutability • To “refute” an assertion is to demonstrate that it is wrong • not merely a competing assertion, to really show it incorrect • All respectable scientific theories are refutable • this supports peer review for correctness • A theory that is not refutable will not be taken seriously • Software Requirements must be refutable! (Science in what we do?) • A domain description of the system’s environment or domain claims to describe the way things are • it should be written to invite counterexamples! • A requirement claims to describe how things ought to be when the system is installed • the customer should be able to look at it and say, “NO, that isn’t what I want.” • OR, “yes, this is the right requirement, but your product has failed to meet it.” CSC 402 Fall 2002

  5. Designations • A designation singles out some sort of phenomena and tells you how to recognize it and gives it a name • note: a “definition” can be neither true nor false • All designations in your requirements document must be reliably and unambiguously recognizable • you should never be able to weasel out of a refutation by saying, “it all depends on what you mean by payment.” (fill in your favorite observable your document) • if this does happen, you need to fix the designation • You proceed to write requirements by describing relationships among the designated phenomena, the entities and data items of interest. • you will run the risk of refutation because your readers can use your designations to pin you down and give a counterexample. CSC 402 Fall 2002

  6. Your reader can say something like, “This is a motor vehicle, right? And here are the roadwheels as you’ve defined them. Your description says all motor vehicles have an even number of roadwheels. But here, your user has a motor vehicle with 3 roadwheels. So you’re wrong. CSC 402 Fall 2002

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