Parsing Fortran-77 with proprietary extensions

TL;DR

This paper presents a method for parsing old Fortran-77 code with proprietary extensions to facilitate migration to modern Fortran, using a combination of island grammar and regular parsing techniques.

Contribution

It introduces a novel parsing approach combining island grammar and regular parser to handle proprietary extensions in Fortran-77 for migration purposes.

Findings

Successfully parsed Fortran-77 with proprietary extensions

Built an abstract syntax tree for Esope code

Facilitated code migration to modern Fortran

Abstract

Far from the latest innovations in software development, many organizations still rely on old code written in "obsolete" programming languages. Because this source code is old and proven it often contributes significantly to the continuing success of these organizations. Yet to keep the applications relevant and running in an evolving environment, they sometimes need to be updated or migrated to new languages or new platforms. One difficulty of working with these "veteran languages" is being able to parse the source code to build a representation of it. Parsing can also allow modern software development tools and IDEs to offer better support to these veteran languages. We initiated a project between our group and the Framatome company to help migrate old Fortran-77 with proprietary extensions (called Esope) into more modern Fortran. In this paper, we explain how we parsed the Esope language with a combination of island grammar and regular parser to build an abstract syntax tree of the code.