Exception Handlers as Extensible Cases

Exceptions are an indispensable part of modern programming languages. They are, however, handled poorly, especially by higher-order languages such as Standard ML and Haskell: in both languages a well-typed program can unexpectedly fail due to an uncaught exception. In this paper, the authors propose a technique for type-safe exception handling. Their approach relies on representing exceptions as sums and assigning exception handlers polymorphic, extensible row types. Based on this representation, they describe an implicitly typed external language EL where well-typed programs do not raise any unhandled exceptions.