CODEWITHSUNDEEP
reval
crayola
crayola

Reputation: 1678

Pass unevaluated commands to a function in R

I am a bit of an R novice, and I am stuck with what seems like a simple problem, yet touches pretty deep questions about how and when things get evaluated in R.

I am using Rserve quite a bit; the typical syntax to get things evaluated remotely is a bit of a pain to type repeatedly:

RSeval(connection, quote(try(command)))

So I would like a function r which does the same thing with just the call: 

r(command)

My first, naive, bound to fail attempt involved: 

r <- function(command) {
    RSeval(c, quote(try(command)))
}

You've guessed it: this sends, literally, try(command) to my confused Rserve daemon. I want command to be partially evaluated, if that makes any sense -- i.e. replaced by what I typed as an argument, but without evaluating it locally.

I looked for solutions to this, browsed throught the documentation for quote, substitute, eval, call, etc.. but I was not able to find something that worked. Either command gets evaluated locally, or not at all.

This is not a big problem, I can type the whole damn quote(try()) thing all the time; but at this point I am mostly curious as to how to get this to work!

EDIT: More explanations as to what I want to do.

Upvotes: 4

Views: 1649

Answers (2)

Greg Snow
Greg Snow

Reputation: 49650

You probably want to use the substitute command, it can give you the argument unevaluated that you can build into the call.

Upvotes: 5

aL3xa
aL3xa

Reputation: 36110

I'm not sure if I understood you correctly - would eval(parse(text = command)) do the trick? Notice that command is a character, so you can easily pass it as a function argument. If I'm getting the point...

Anyway, evaluating user-specified commands is potentially malicious, therefore not recommended. You should either install AppArmor and tweak it (which is not an easy one), or drop the whole evaluation thing...

Upvotes: 0

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