A new approach to specifying and handling exceptions

An operation generally exhibits different patterns of behavior over different parts of its domain. Depending upon the context, such behavior may either be conceived of as "normal" or as an "exception." Thus, the behavior of an operation Is quite naturally characterized by the set of partial operations that characterize its (different) behavior on Its subdomains, and exceptions essentially serve to extend (modify) the normal behavior of an operation. In this milieu, we consider the issues of specifying and "handling" exceptional conditions that might occur during the execution of an operation. We argue that one of the important features that an exception handling mechanism need possess is to enable the behavior of an operation be altered over part of Its domain i.e. that of being able o incrementally modify t h e semantics of t h e partial operations that serve to characterize an operation. Surprisingly, this requirement arises out of a pragmatic consideration ? that of providing for the existence of a library of subroutines. We propose a general mechanism for specifying exceptions and their handlers that does not compromise the possibility of efficient implementations. Examples of application of the method are presented In the context of abstract algebraic) data type specifications, using skeletal specifications of a Stack and an error-correcting parser for a context free grammar. The major advantage of the proposed mechanism over conventional appoaches is that of completely avoiding the problem of "dynamic context propagation."