What are Formal Methods

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What are Formal Methods?

• SE 420, Formal Methods
• Mark Ardis, RIT

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What are formal methods? (1/2)

• Formal methods are methods that use formal
  language.
• Formal methods use formal logic.

3
What are formal methods? (2/2)

• Formal methods use formal specification
  languages. That is, they use languages that have
  a mathematically-defined static semantics.
• Formal methods employ analysis that is
  mathematically sound.

4
When are formal methods most useful?

• When you have to get it right.
• When you cannot test it adequately, and you have
  to get it right.
• Examples
• safety-critical applications
• aviation
• medical equipment
• economically-critical applications
• Stock Exchange
• ATMs

5
Why did Jonathan Jacky use formal methods?

• "I learned Z ... when I got tired of programming
  by trial and error."
• "I write large programs for a serious purpose."
• "We needed a complete and unambiguous description
  of what our system would do."

6
Can you compile Z specifications?

• No, that would not make sense.
• a Z specification is a design, not an
  implementation
• it describes what must be true, not how to do it
• But, you can
• check them for type errors with tools like ZTC
• review them (just like code inspection)
• prove assertions about them formally
• refine them into code by hand (just like refining
  a UML design into code)

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Cartoon of the Day
8
6 Fallacies about Software

• Software is too complicated to get completely
  right.
• The only way to know whether software works is to
  test it.
• Users don't know what they want---only trial and
  error will lead to the right solution.
• Everything has bugs in it, so there is no point
  in making anything better.
• It costs too much to make highly-reliable
  software.
• Computing is still too immature to be reliable.

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7 Myths About Formal Methods

• Hall work for Praxis Systems, a company that uses
  formal methods in their work.

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Myth 1 Formal methods can guarantee that
software is perfect.
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Myth 2 Formal methods work by proving that
programs are correct.
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Myth 3 Only highly critical systems benefit from
their use.
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Myth 4 They involve complex mathematics.
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Myth 5 They increase the cost of development.
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Myth 6 They are incomprehensible to clients.
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Myth 7 Nobody uses them for real projects.