`R_HOME` is more for introspection than configuration

[jennybc]

I'm pretty intrigued by this project! I saw it in use during Hadley's recent data science hangout.

So I came to look at the README so I can try it out.

I've done a lot of work on R runtime discovery in Positron and one thing I read caught my eye:

https://github.com/posit-dev/mcp-repl#r-interpreter-which-r-installation-is-used

To force a specific R installation, set R_HOME in the environment that launches mcp-repl.

We've occasionally seen folks trying to use R_HOME like this in Positron's issues and it generally leads to frustration. While trying to understand those situations, I've had to read up on what R_HOME is designed to do and I think this is basically fighting against the intent of R_HOME:

posit-dev/positron#12278 (comment)

TL;DR

My impression is that R_HOME is intended to be a env var you can consult once you've launched and R version to ask it "where do you live?" It's not something one sets and then expects that to drive the usage of a specific R installation.

I just thought this might be worth some 🤔

[t-kalinowski]

Thanks!

One nice thing of using R_HOME is that mcp-repl, when used from R in something like ellmer, will this way default to using the same R installation as it was called from (which is usually what people will want).

But that is perhaps a little too magical, and it might be better if instead users were encouraged to do something explicit like mcp-repl --interperter {R.home("bin/R")}.

I agree, this is not the intended usage R_HOME.

I'll put it on the roadmap to transition to using an envvar like MCP_REPL_R instead.

[jennybc]

One nice thing of using R_HOME is that mcp-repl, when used from R in something like ellmer, will this way default to using the same R installation as it was called from (which is usually what people will want).

Ah, yeah, if you're sort of running R within R, I see how this use of R_HOME could come up. I'm sure I don't fully grasp the typical or possible contexts.

Yeah, the QUARTO_R env var is a good analogy, which is how MCP_REPL_R feels.