ocaml-decimal VS merlin

Re: your test not working, I'm not sure how to diagnose that as I don't know the details of your project. The only thing I can say, is here's a project that I work on and it is set up so the dune test command runs the tests: https://github.com/yawaramin/ocaml-decimal/

We need more context to be able to do that. Without it, the best we can do is very general pointers. E.g. here is an OCaml library that is basically a port of a Python one: https://github.com/yawaramin/ocaml-decimal

Merlin: Context sensitive completion for OCaml in Vim and Emacs\ (0 comments)

Instead of searching functions based on their type structure (like Hoogle), you could search for functions that "consume"/"produce" values of given types (like OCaml's Merlin). I think Rust already computes variance of type constructors, so such a tool just would have to obtain this information.

However, after posting this question I stumbled upon this Github issue where they say it isn't supposed to work out of the box and you're supposed to bind a key to it by editing your .emacs file. Turns out the default .emacs file binds auto-complete to "backtab" which means Shift+Tab but that didn't work. I did eventually discover that I can get some kind of completion by binding backtab to completion-at-point like this:

Self-advertising: I partcipated to the writing of Merlin: A Language Server for OCaml (Experience Report), which explains the overall design of Merlin, a language server for OCaml. A key idea of Merlin are that classic lexing-parsing-typing pipelines can easily be adapted to be incremental for a Language Server, especially when they are using immutable data structures.