jsrepl

A REPL for R that works in a browser, communicating with R using Javascript via the Rserve package.

One modification is needed in the inst/Rserv.conf file before installing. After installing the package, try

jsrepl::start_repl()

from a terminal.

Bugs / missing features

• The console.input enable flag works for readLines(), debug(), etc., but not readline().

• Output without newlines are not actually sent immediately, unless flush.console() is called explicitly.

for (i in 1:5) { 
    Sys.sleep(0.5)
    cat("=")
    ## flush.console()
}

• Progress bars created using txtProgressBar() work

• Not surprisingly, terminal escape sequences don't work. To get output that contains escape sequences (without the desirable effect), try setting options(cli.ansi = TRUE, cli.num_colors" = 256), and then try to print a tibble. One way to deal with this could be to recognize escape codes and start tags, and hope we will always get matching closing tags. Another, probably better, option is to use a HTML terminal emulator like https://xtermjs.org/

• run.Rserve() currently starts a new session for every new connection, e.g., if we reload the web page. It would be nice to have the option of having a common / single session that all clients will connect to.

• HTML help does not work out of the box. Can be made to work by setting suitable port options, and making sure the R httpd server does not start on its own.

• Default pager is not useful (unless we handle stdout OOB messages specially); probably could just set options(pager) to write to file / show / unlink.

Blocking issues

There are three somewhat separate issues.

Persistence of connection

Should two users connecting to Rserve get the same session (with same state in terms of global environment, packages, etc.) or different sessions?

Both scenarios have their uses. Rserve thinks that a new session is "better", and does that on Linux / Mac, but not on Windows because of OS limitations. As far as I can tell, there is no way to get persistence on Linux / Mac. Note that just disabling fork/vfork detection in configure is not enough (FIXME Is there supposed to be a way that does not work?)

Status check:

• Linux: not persistent by default, no way to change

• Mac: not persistent by default, no way to change

• Windows: TODO

Printing messages while evaluating

It is relatively easy to have R evaluate a command and print the return value. It is less easy to handle user interaction while a command is being evaluated.

This is the main selling point of Rserve over simpler alternatives like httpuv (along with security, but that's not really our concern because we expect to run locally). Rserve achieves this using out-of-band messages. As a test case, consider:

for (i in 1:5) { 
    Sys.sleep(1)
    cat("Step: ", i, fill = TRUE)
}

[There is an issue described above with flush.console() when messages don't have a newline, which has an impact on the next issue, but that's more of an R-side problem.]

Unfortunately, while this works on Linux and Mac, it does not work on Windows. This is a major deal-breaker.

Status check:

• Linux: Works

• Mac: Works

• Windows: TODO

Prompts and input

Out-Of-Band communication during evaluation can be in both directions. This is analogous to modal dialogs in a GUI. The most important use-case is debugging, but for a simple example, consider

menu(c("A", "B"))

There are two parts of this: showing a prompt, and accepting input that gets send back to R. We consider these separately.

Prompts

This mostly works if OOB messaging works, but the prompt (Selection: ) does not get printed. This is really probably related to lack of newlines, but none of these show up on Mac:

readline(prompt = "What is your favorite color? ")
readline(prompt = "What is your favorite color?\n ")

Compare this with the following, which works as well as can be expected.

readline.fc <- function(prompt) {
    cat(prompt)
    flush.console()
    readline("")
}
readline.fc(prompt = "What is your favorite color? ")

In other words, it's really a sub-issue of the previous issue, but nonetheless, it's something that needs to be fixed if this is to become usable.

Status check:

• Linux: Prompts do not get shown

• Mac: Prompts do not get shown

• Windows: TODO

Inputs

The more important feature is the ability to accept user input during this kind of interaction and send the response to R for further processing. As with OOB messages, this works on Linux / Mac but not on Windows.

Status check:

• Linux: Input is accepted and sent to R for processing

• Mac: Input is accepted and sent to R for processing

• Windows: TODO