How can I decorate a function in R?

Question

I'm trying to instrument some functions in R, by replacing them with my own versions which record some information and then call the original versions. My trouble is, how do I construct the call such that it exactly replicates the original arguments.

First, we'll set up an environment

   env <- new.env()

One way I could do this is by getting the call and modifying it. However, I don't know how to do this with a primitive function.

## Option 1: get the call and modify it
env$`[` <- function(x, i, j, ...) {
    my.call <- match.call()
    ## This isn't allowed.
    my.call[[1]] <- as.name(.Primitive("["))

    record.some.things.about(i, j)

    eval(my.call, envir = parent.frame())
}

Alternatively, I could just get the arguments and use do.call. However, I haven't worked out how to extract the arguments correctly.

## Option 2: do a call with the arguments
env$`[` <- function(x, i, j, ...) {
    ## This outputs a list, so if 'i' is missing, then 'j' ends up in the 'i' position.
    ## If I could get an alist here instead, I could keep 'j' in the right position.
    my.args <- match.call()[-1]

    record.some.things.about(i, j)

    do.call(
        .Primitive("["),
        my.args,
        envir = parent.frame()
    )
}

Then, if we've done things right, we can eval some expression which uses [ inside the enviroment we've constructed, and our instrumented function will be called instead:

## Should return data.frame(b = c(4, 5, 6))
evalq(
    data.frame(a = c(1, 2, 3), b = c(4, 5, 6))[, "b"],
    envir = env
)

Can anyone help/advise?

Possibly relevant: http://stackoverflow.com/questions/23124096/r-decorator-to-change-both-input-and-output — nrussell, Mar 02 '16 at 12:58
What I tend to do to fix some defaults is something along the following lines: Say I want `table` to always include missing values, then in the beginning of my script, I use `table <- function(x, ...) base:::table(x, ..., useNA="ifany")`. Whatever you want to record about `i` and `j`, you could probably construct something along similar lines. — coffeinjunky, Mar 02 '16 at 13:14
Can you explain more plainly what you want to achieve? If I understand correctly, you either want to write a method for `[` or want to substitute `[` with your own function. However, your function name is confusing me. — Roland, Mar 02 '16 at 13:30
Now you have muddled this even more. You assign a function to an environment. Why? — Roland, Mar 02 '16 at 13:36
I would really advise you to follow a different path. Create an S3 class and define a method for `[`. — Roland, Mar 02 '16 at 13:40
Roland: I don't understand why S3 would help? This doesn't have anything to do with object orientation or method dispatch as far as I can tell. I want to intercept *all uses* of [, not just those for some particular object. — GlennS, Mar 02 '16 at 13:43

score 4 · Accepted Answer · answered Mar 02 '16 at 13:57

Use trace:

trace(`[.data.frame`, quote({print("hello!")}), at = 1) 
iris[1,]
#Tracing `[.data.frame`(iris, 1, ) step 1 
#[1] "hello!"
#  Sepal.Length Sepal.Width Petal.Length Petal.Width Species
#1          5.1         3.5          1.4         0.2  setosa

score 0 · Answer 2 · answered Mar 02 '16 at 13:52

Can't you just capture everything for original function args with triple dots arg, that gets passed on to the original function?

sum <- function(..., na.rm = TRUE) {
  print("my sum") # here is where you can "record some info"
  base::sum(..., na.rm = na.rm) # then call original function w/ ...
}

base::sum(5,5, NA)
##[1] NA

# your function
sum(5,5, NA)
##[1] "my sum"
##[1] 10

How can I decorate a function in R?

2 Answers2