What/Where are the attributes of a function object?

Question

By playing around with a function in R, I found out there are more aspects to it than meets the eye.

Consider ths simple function assignment, typed directly in the console:

f <- function(x)x^2

The usual "attributes" of f, in a broad sense, are (i) the list of formal arguments, (ii) the body expression and (iii) the environment that will be the enclosure of the function evaluation frame. They are accessible via:

> formals(f)
$x
> body(f)
x^2
> environment(f)
<environment: R_GlobalEnv>

Moreover, str returns more info attached to f:

> str(f)
function (x)  
 - attr(*, "srcref")=Class 'srcref'  atomic [1:8] 1 6 1 19 6 19 1 1
  .. ..- attr(*, "srcfile")=Classes 'srcfilecopy', 'srcfile' <environment: 0x00000000145a3cc8>

Let's try to reach them:

> attributes(f)
$srcref
function(x)x^2

This is being printed as a text, but it's stored as a numeric vector:

> c(attributes(f)$srcref)
[1]  1  6  1 19  6 19  1  1

And this object also has its own attributes:

> attributes(attributes(f)$srcref)
$srcfile


$class
[1] "srcref"

The first one is an environment, with 3 internal objects:

> mode(attributes(attributes(f)$srcref)$srcfile)
[1] "environment"
> ls(attributes(attributes(f)$srcref)$srcfile)
[1] "filename"      "fixedNewlines" "lines" 
> attributes(attributes(f)$srcref)$srcfile$filename
[1] ""
> attributes(attributes(f)$srcref)$srcfile$fixedNewlines
[1] TRUE
> attributes(attributes(f)$srcref)$srcfile$lines
[1] "f <- function(x)x^2" ""

There you are! This is the string used by R to print attributes(f)$srcref.

So the questions are:

1. Are there any other objects linked to f? If so, how to reach them?

2. If we strip f of its attributes, using attributes(f) <- NULL, it doesn't seem to affect the function. Are there any drawbacks of doing this?

Answer 1

As far as I know, srcref is the only attribute typically attached to S3 functions. (S4 functions are a different matter, and I wouldn't recommend messing with their sometimes numerous attributes).

The srcref attribute is used for things like enabling printing of comments included in a function's source code, and (for functions that have been sourced in from a file) for setting breakpoints by line number, using utils::findLineNum() and utils::setBreakpoint().

If you don't want your functions to carry such additional baggage, you can turn off recording of srcref by doing options(keep.source=FALSE). From ?options (which also documents the related keep.source.pkgs option):

‘keep.source’: When ‘TRUE’, the source code for functions (newly defined or loaded) is stored internally allowing comments to be kept in the right places. Retrieve the source by printing or using ‘deparse(fn, control = "useSource")’.

Compare:

options(keep.source=TRUE)
f1 <- function(x) {
    ## This function is needlessly commented
    x
}

options(keep.source=FALSE)
f2 <- function(x) {
    ## This one is too
    x
}

length(attributes(f1))
# [1] 1
f1
# function(x) {
#     ## This function is needlessly commented
#     x
# }

length(attributes(f2))
# [1] 0
f2
# function (x) 
# {
#     x
# }

Answer 2

I jst figured out an attribute that compiled functions (package compiler) have that is not available with attributes or str. It's the bytecode.

Example:

require(compiler)

f <- function(x){ y <- 0; for(i in 1:length(x)) y <- y + x[i]; y }

g <- cmpfun(f)

The result is:

> print(f, useSource=FALSE)
function (x) 
{
    y <- 0
    for (i in 1:length(x)) y <- y + x[i]
    y
}

> print(g, useSource=FALSE)
function (x) 
{
    y <- 0
    for (i in 1:length(x)) y <- y + x[i]
    y
}
<bytecode: 0x0000000010eb29e0>

However, this doesn't show with normal commands:

> identical(f, g)
[1] TRUE
> identical(f, g, ignore.bytecode=FALSE)
[1] FALSE
> identical(body(f), body(g), ignore.bytecode=FALSE)
[1] TRUE
> identical(attributes(f), attributes(g), ignore.bytecode=FALSE)
[1] TRUE

It seems to be accessible only via .Internal(bodyCode(...)):

> .Internal(bodyCode(f))
{
    y <- 0
    for (i in 1:length(x)) y <- y + x[i]
    y
}

> .Internal(bodyCode(g))
<bytecode: 0x0000000010eb29e0>