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Belina, Hogrefe (edits Reed)

6.3 Error terms

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Sometimes an operator should only operate on a subset of the elements of a sort. For the other elements, the result of the operator should be undefined, or the application of the operator should not be allowed to these elements. In SDL-88 the user has the possibility to specify a forbidden application of an operator. For this purpose the special term Error! is implicitly included in any sort.

A classical example here is again the division operator. The application of the operator to zero should not be allowed:

FOR ALL x, z IN Real
(x/0) == Error!);

Error! means: “Application of the operator to this element is not allowed, and if done, the future behaviour of the system is undefined".

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