We describe a facility for improving optimization of Haskell programs using rewrite rules. Library authors can use rules to express domain-specific optimizations that the compiler cannot discover for itself. The compiler can also generate rules internally to propagate information obtained from automated analyses. The rewrite mechanism is fully implemented in the released Glasgow Haskell Compiler. Our system is very simple, but can be effective in optimizing real programs. We describe two practical applications involving short-cut deforestation, for lists and for rose trees, and document substantial performance improvements on a range of programs. 1 Introduction Optimising compilers perform program transformations that improve the efficiency of the program. However, a compiler can only use relatively shallow reasoning to guarantee the correctness of its optimisations. In contrast, the programmer has much deeper information about the program and its intended behaviour. For example, a programmer may know that

Abstract. We present a method of specifying standard imperative program optimisations as a rewrite system. To achieve this we have extended the idea of matching sub-terms in expressions with simple patterns to matching blocks in a control flow graph. In order to express the complex restrictions on the applicability of these rewrites we add temporal logic side conditions. The combination of these features allows a flexible, high level, yet executable specification of many of the transformations found in optimising compilers. 1