Eliminating Spurious Error Messages Using Exceptions, Polymorphism, and Higher-Order Functions

Author:Ramsey, Norman, Department of Computer ScienceUniversity of Virginia

Many language processors make assumptions after detecting an error. If the assumptions are invalid, processors may issue a cascade of error messages in which only the first represents a true error in the input; later messages are side effects of the original error. Eliminating such spurious error messages requires keeping track of values within the compiler that are not available because of a previously detected error. Examples include symbol-table entries, types, and intermediate code. This paper presents a discipline for tracking unavailable values and avoiding cascading error messages. The discipline itself is unsurprising, but it is both formalized and implemented in terms of a type constructor and combinators expressed in Standard ML, and the ML type rules enforce the discipline. The type constructor distinguishes intermediate results that are unavailable because of a previously detected error. The combinators transform ordinary functions, which assume all intermediate results are available, into functions that silently propagate the unavailability of intermediate results. ML's type system guides the application of the combinators; if the compiler writer does not account for a potentially unavailable value, the source code of the compiler does not type-check. The techniques presented exploit several features of Standard ML, including exceptions, higher-order functions, parametric polymorphism, and static type checking. Using these features enables the ML type system to ensure that the error-tracking discipline is applied consistently, relieving the programmer of that burden.

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Source Citation:

Ramsey, Norman. "Eliminating Spurious Error Messages Using Exceptions, Polymorphism, and Higher-Order Functions." University of Virginia Dept. of Computer Science Tech Report (1997).

University of Virginia, Department of Computer Science
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