Principled parsing for indentation-sensitive languages: revisiting landin's offside rule

Abstract

Several popular languages, such as Haskell, Python, and F#, use the indentation and layout of code as part of their syntax. Because context-free grammars cannot express the rules of indentation, parsers for these languages currently use ad hoc techniques to handle layout. These techniques tend to be low-level and operational in nature and forgo the advantages of more declarative specifications like context-free grammars. For example, they are often coded by hand instead of being generated by a parser generator.