It is more the other way around ML predates Modula-2, and the module system like ideas were already present in Mesa and UCSD Pascal. :)

It's been a while since I used UCSD Pascal (in high school, on Apple II boxes with an extra hardware card), so I don't recall the details of the module system there but I'm pretty sure it did not back then permit separate compilation of definitions and implementation files as Modula-2 did (.def and .mod in Modula-2, respectively).

I also saw some ML/SML at uni in the early 1990s but no mention of modules then (this is not to say you are wrong, but I question how visible any work in that space was that may have existed). Now Mesa I never got my hands on, nor do I know anyone that did.

Modula-2 was popular e.g. on the Atari ST 520 series, I still have the handbook on my bookshelf next to me (the vendor of the compiler I had was called "Application Systems Heidelberg", founded in Germany in 1985 they now seem to have gone into gaming). This was a decent 32-bit compiler, and with 1 MB RAM you could write great applications when C compilers at the time were still insufficient in their ability to provide proper type checking support (pre-ANSI/ISO standardization).

Reddit has a discussion on Modula-2 and its family tree here: https://www.reddit.com/r/programming/comments/1cuam2p/what_e...