With a well-designed page, this error will not occur during Web browsing, so no user will ever see the page showing the precondition violation. The resulting, corrected page will only call the component when the values are valid, i.e. they satisfy the the preconditions on the Eiffel features. They provide built-in documentation . They support the testing and debugging process. For a general introduction to Design by Contract and its application in Eiffel see the introductory paper on Design by Contract.
Contracts are particularly powerful for the kind of distributed systems made possible by the Web.
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