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The Computer Journal 1989 32(4):305-311; doi:10.1093/comjnl/32.4.305
© 1989 by British Computer Society
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A Proposal for Making Eiffel Type-safe

W. R. Cook *

Hewlett-Packard Laboratories, 1501 Page Mill Road, P.O. Box 10490, Palo Alto, CA 94303-0969, USA

Statically type-correct Eiffel programs may produce run-time errors because (1) attributes may be redeclared during inheritance, invalidating assignments in the superclass, (2) a formal method argument type may be restricted in violation of the contravariance of function types, and (3) two applications of a generic class are assumed to conform if the actual arguments conform. The third problem is solved by case analysis on the variance of generic parameters. Declaration by association provides a solution to the first two problems, but is suffers from additional difficulties. Type attributes, or generic parameters with default values, are suggested as a replacement for most cases of declaration by association. The special association type used to express type recursion cannot be explained using type attributes, and its appears to be a truly novel construct for typing object-oriented programs. One consequence of this construct is that Eiffel's conformance hierarchy is a proper subset of its inheritance hierarchy.

Received April 1989.

* Hewlett-Packard Laboratories, 1501 Page Mill Road, P.O. Box 10490, Palo Alto, CA 94303-0969, USA


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