Ability to check if bridgestan sources are already available about bridgestan

[WardBrian]

The message here is probably a bit misleading, because it doesn't just look for a BridgeStan, it looks for a BridgeStan instance of the matching version if $BRIDGESTAN is not set manually, so it will download a new one if you update your install

get_bridgestan_path() is not really set up to be used in this way -- it is the function that triggers the download when it is not found, so it wouldn't really help here. We can think of alternatives to this design

[avehtari]

I was expecting the same behavior as in cmdstanr::cmdstan_path() https://mc-stan.org/cmdstanr/reference/set_cmdstan_path.html

[WardBrian]

We have an intentionally different behavior than cmdstan, where the get_bridgestan_path() function always returns a path, downloading it if necessary.
This is in part because a given version of bridgestan is only ever really intended to work with a single version of the C++ sources, so we know which version we should download. CmdStanR/Py don't really have that notion

[avehtari]

Ah, that is not clear from the documentation, which is not mentioning get_bridgestan_path() at all, but advises to make the download separately, and when I use the proposed git clone option, I wanted to check whether the path points to a correct directory, I tried using get_bridgestan_path(), but then it downloaded the latest release and missed that I had the latest version via git.

[WardBrian]

Oh, that's just because I forgot to update the docs after the downloading feature was ported to R in #172.

Here's what we say in Python, which is now true for R as well:

The first time you compile a model, the BridgeStan source code will be downloaded and placed in ~/.bridgestan/. If you prefer to use a source distribution of BridgeStan, consult the following section.
...
To use the BridgeStan source you’ve manually downloaded instead of one the package will download for you, you must use set_bridgestan_path() or the $BRIDGESTAN environment variable.

I will update R's page to say something similar

[WardBrian]

@avehtari I've opened a PR with doc changes -- are you also hoping for any behavior changes, or is just properly explaining the behavior enough to close this?

[avehtari]

Function like bridgestan_path() similar to cmdstan_path() would be helpful to check that the environment variable BRIDGESTAN is working. If setting that environment variable did not work for some reason, another (unnecessary) installation is made to ~/.bridgestan.

[WardBrian]

@avehtari would changing the definition of get_bridgestan_path() to accept a download argument (default TRUE) line up with what you were expecting?

[avehtari]

Sounds like yes