Exception Handlers in Functional Programming Languages

| Constructs for expressing exception handling can greatly help to avoid clutter in code by allowing the programmer to separate the code to handle unusual situations from the code for the normal case. Handling exceptions in functional languages has received less attention mainly due to the inherent connict between the control ow oriented approach of exception handlers and the functional style of evaluation. This has (i) resulted in loss of refer-ential transparency, (ii) made non-strict constructs behave as strict functions, and most importantly, (iii) forced an order of evaluation on the subexpressions in a function. In this paper, we propose a new approach to embed exception handlers in functional languages. The proposed approach discards the conventional view of treating exceptions as a means of eeecting a control transfer; instead, exceptions are used to change the state of an object. The two types of exceptions, Terminate and Resume, are treated diierently in this paper. A Terminate exception, when raised, is viewed as shielding the input object. On the other hand, a Resume exception designates the input object as curable and requires the immediate application of a handler function. This approach enables the clean semantics of functions raising exceptions (1) without associating any implementation restriction and (ii) without loss of the referential transparency and the commutativity properties of functions.