Is there a way to return to R from lua_shell() while keeping the lua state intact?

[SugarRayLua]

Hi, thanks for making a great R-lua bridge! There aren't many in existence... :-)

Your lua_shell function is really helpful, as it is easier just to open the lua_shell to do some lua programming while in R instead of passing everything in a lua() call. However, it doesn't seem that one can then easily get back to R from the shell; it would be helpful if I could do some programming in the lua_shell and then return to R and then perhaps return the variables or selected functions from luajit that I programmed when in the lua_shell. Is that possible or might that be possible in a future version of luajr? The only way for now that I could figure out to get out of lua_shell is by the standard lua os.exit() method which then unfortunately returns me to the bash command line and closes my R session.

[nicholasdavies]

Hello!

If you just hit return on an empty line, the Lua shell should return you to R leaving the Lua state intact (as it operates on the default state, unless you pass in an alternative state). I didn't realise I hadn't documented this, sorry! Let me know if that isn't working for you.

Thanks,

Nick

[SugarRayLua]

Hi, Nick, yes, thanks, hitting return works, perfectly! Being able to go in and out of the lua shell is very handy :-)

Two follow up questions:

1. Some R-Lua bridges allow referencing variables in the other language without returning them: e.g.

Lua> C = 5
R> test = luajr.c
R> test 
# would hope to print "5"

It doesn't look like one can do that right now in luajr; instead it looks like one always needs to do instead:

Lua> C = 5
R> test = lua("return C")
R> test
# prints "5"

Is that correct or is there another more efficient way to reference Lua values in R session?

2. I'm not quite sure what the advantage is to creating R object in Lua with luajr versus just creating them in R (or versus creating similar Lua objects and passing them to R via lua() function)? E.g.:

Create R List in R:

R> test = list(pkgName = "luajr", command = "shell")
R> test
# prints $pkgname [1] "luajr" [2] "shell"

Create R List in Lua via a Lua table and pass to R:

Lua> test = {pkgName = "luajr", command = "shell"}
R> test2 = lua("return test")
R> test2
# prints same as in straight R example above

Create list object in lua using luajr module:
I'm actually not sure how one initializes a list with named keys and values but it seems more complicated then the two examples above, and it is not clear to me whether once properly initialized, R can directly reference the R list that was created with luajr module. For simpler array type list objects, I understand to do:

Lua> test = luajr.list
Lua> test[1] = 5

But don't quite understand how to set up a key, value list like "test" in the above examples. The documentation indicates one uses ListName(attr,x), but I can't figure out how to make that work.

Thus, the bottom line question is what would be the advantage of creating R objects inside LuaJit, and if there is an advantage, it would be helpful if you could provide an example of how to create a key, value list in Lua and answer as to whether that list created in Lua could be referenced directly from R.

Thanks again,

Sincerely,

Mike

[nicholasdavies]

1. Yes, that's correct.
2. I don't know. Whether there is an advantage to doing this either way would depend on what you're doing, I think.
3. You need to use test = luajr.list() with parentheses, as it's a function. You then get/set values within Lua using e.g. test.x = 1 or test["foo"] = "hello". When this gets passed back to R, it will be an R list with entries named "x" and "foo". The v(attr) or v(attr,x) can be used to get/set other attributes you want the R list to have when it is passed back to R, such as the "class" attribute.

[SugarRayLua]

Thanks, Nick, I think I understand better now (part of the problem was leaving off the ()).

It looks like then by defining objects as R objects using the luajr module in lua, one is assured that when they get passed to R via the "lua('return obj')" function that one is getting the specific type of R object that one wants from lua rather than having the lua('return obj') function determine the best R object to convert the lua object it is passed to.

It also still seems that the answer to #1 still holds true: even though one has created a R list in lua as per your example ("test = luajr.list(); test.x = 1") R won't know about the variable "test" until still passed to R via a "test = lua('return test')" function call.

I think I now have enough information I need to use luajr successfully 😊