Code Style Sheets: CSS for Code

Program text is rendered using impoverished typographic styles. Beyond choice of fonts and syntax-highlighting colors, code editors and related tools utilize very few text decorations. These limited styles are, furthermore, applied in monolithic fashion, regardless of the programs and tasks at hand. We present the notion of code style sheets for styling the textual representation of programs. Motivated by analogy to cascading style sheets (CSS) for styling HTML documents, code style sheets provide mechanisms for defining rules to select and style abstract syntax trees (ASTs). Technically, code style sheets generalize notions from CSS over untyped HTML trees to a programming-language setting with algebraic data types (e.g. ASTs). Practically, code style sheets allow ASTs to be styled granularly, based on semantic information -- such as the structure of abstract syntax, static type information, and corresponding run-time values -- as well as design choices on the part of authors and readers of a program. In this paper, we design and implement a code style sheets system for a subset of Haskell -- the rich syntactic and semantic structure of Haskell provide a fertile first setting in which to explore the notion of code style sheets. We illustrate several use cases involving code presentation and visualization tasks.