Declarative Specification of Indentation Rules
A Tooling Perspective on Parsing and Pretty-Printing Layout-Sensitive Languages

Luís Eduardo de Souza Amorim (TU Delft)

In layout-sensitive languages, the indentation of an expression or
statement can influence how a program is parsed. While some
layout-sensitive languages (e.g., Python) have wide adoption in
practice, there is little support for software language engineers
in building tools for layout-sensitive languages. As a result,
parsers, pretty-printers, program analyses, or refactorings often
need to be handwritten, which decreases the maintainability and
extensibility of these tools. Even state-of-the-art language
workbenches have yet little support for layout-sensitive languages,
restricting the development and prototyping of such languages.

To ease dealing with layout-sensitive languages in practice, we
introduce a novel approach to support declarative specifications
of layout-sensitive languages using layout declarations. Layout
declarations are high-level specifications of indentation rules
that occur in layout-sensitive languages, abstracting from low-level
technicalities. We show how to automatically derive an efficient
layout-sensitive generalized parser and a corresponding pretty-printer
from a language specification with layout declarations. We validate
our approach in a case-study using a syntax definition for the
Haskell programming language, investigating the performance of the
generated parser and the correctness of the generated pretty-printer
when considering a corpus of 22191 Haskell files.