How understanding and restructuring differ from compiling -a rewriting perspective

Syntactic and semantic analysis are established topics in the area of compiler construction. Their application to the understanding and restructuring of large software systems reveals, however that they have various shortcomings that need to be addressed. In this paper we study these shortcomings and propose several solutions. First, grammar recovery and grammar composition are discussed as well as the symbiosis of lexical syntax and context-free syntax. Next, it is shown how a relational calculus can be defined by way of term rewriting and how a fusion of term rewriting and this relational calculus can be obtained to provide semantics-directed querying and restructuring. Finally, we discuss how the distance between concrete syntax and abstract syntax can be minimized for the benefit of restructuring. In particular we pay attention to origin tracking, a systematic technique to maintain a mapping between the output and the input of the rewriting process. Along the way, opportunities for further research are indicated.