Later I got into programming language theory, and took another look at MUMPS from that perspective. As a programming language, it’s truly terrible in ways that languages like COBOL and FORTRAN are not. Just as one example, “local” variables have an indefinite lifetime and are accessible throughout a process, i.e. they’re not scoped to functions. But you can dynamically hide/shadow and delete them. It would be hard to design a less tractable way of managing variables if you tried.
MUMPS’ value proposition was how it handled persistent data as a built-in part of the language. In that sense it was a precursor to systems like dBASE, which were eventually supplanted by SQL databases. MUMPS was a pretty good persistent data management system coupled with a truly terrible programming language.