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A Semantics for Imprecise Exceptions

Simon Peyton Jones Microsoft Research Cambridge 4pm Tuesday 23 February 1999 Room 2511, JCMB, King's Buildings In addition to this seminar, on Wednesday Simon will give the Informatics Colloquium Scripting COM components in Haskell. This is at 4pm in the David Hume Tower, Lecture Theatre C. Contact Samson Abramsky for further details.

Some modern superscalar microprocessors provide only imprecise exceptions. That is, they do not guarantee to report the same exception that would be encountered by a straightforward sequential execution of the program. In exchange, they offer increased performance or decreased area (which amount to much the same thing).

This performance/precision tradeoff has not so far been much explored at the programming language level. In this paper we propose a design for imprecise exceptions in the lazy functional programming language Haskell. We discuss various simpler designs, and conclude that imprecision is essential if the language is still to enjoy its current rich algebra of transformations. We sketch a precise semantics for the language extended with exceptions.

From the functional programming point of view, the paper shows how to extend Haskell with exceptions without crippling the language or its compilers. From the point of view of the wider programming language community, we pose the question of whether precision and performance can be traded off in other languages too.

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