A Constructive Formalization of the Catch and Throw Mechanism

The catch/throw mechanism is a programming construct for non-local exit. In the practical programming, this mechanism plays an important role when programmers handle exceptional situations. In this paper we give a constructive formalization which captures the mechanism in the proofs-as-programs notion. We introduce a modi ed version of LJ equipped with inference rules corresponding to the operations of catch and throw. Then we show that we can actually extract programs which make use of the catch/throw mechanism from proofs under a certain realizability interpretation. Although the catch/throw mechanism provides only a restricted access to the current continuation, the formulation remains constructive in contrast to the works due to Gri n and Murthy on more powerful facilities such as call/cc (call-with-current-continuation) of Scheme.