I saw that there were a few threads opened 1-2 years ago asking about a REPL.
I wanted to use one and created a quick and dirty one awhile ago, but recently fixed it up a bit more.
rescript-repl - npm (npmjs.com)
I still need to clean things up and test some more and I have some rough blog posts detailing how I thought about structuring things. Nonetheless, it is functional at the moment and so I just wanted to put it out there for now.
Any constructive feedback is welcome.