Semantics - Directed Compilation of Non - Linear Patterns 1 Olivier

This paper describes the automatic derivation of compiled patterns and of a pattern compiler by partial evaluation. Compiling a pattern is achieved by specializing a pattern matching program with respect to the pattern. Generating a pattern compiler is achieved by specializing the specializer with respect to the pattern matching program, i.e., by self-applying the partial evaluator. The compiled patterns and the compiler are semantics-based because they are obtained using meaning-preserving transformations upon the de nitional pattern matching program and the partial evaluator. The results are unexpectedly good: not only all are the operations depending on the pattern (syntax analysis, resolution of cross-references due to the non-linearity) performed at compile time, but whereas the general pattern matcher builds the substitution environment incrementally and for nothing in case of failure, compiled patterns perform all the structural and equality tests rst, and build the result only if the match succeeds. This non trivial runtime staging has been obtained automatically, which is remarkable because staging in general is known to be an art. This example stresses continuation-passing style as a convenient style for writing general programs. This style makes it possible to circumvent the approximations of binding time analysis (which is an essential requirement for e cient self-application), and in addition, to stage residual programs automatically. These observations have been con rmed by later experiments.