[R] environments: functions within functions

[Sarah Goslee]

Hi,

I ran into a problem with S3 method dispatch and scoping while trying
to use functions from the mixR package within my own functions. I know
enough to find the problem (I think!), but not enough to fix it
myself. The problem isn't really a package-specific problem, so I'm
starting here, and will file an issue with the maintainer once I have
a solution.

Detailed explanation below, but briefly, the S3 methods in this
package use match.call() and then eval() to select the correct
internal method. This works fine from the command line, but if the
method is called from within another function, the use of
environment() within eval() means that the objects passed to the
wrapper function are no longer visible within the eval() call.

I have a two-part question:
A. How do I get around this right now?
B. What would the correct approach be for the package authors?

library(mixR)

# first example from ?mixfit
## fitting the normal mixture models
set.seed(103)
x <- rmixnormal(200, c(0.3, 0.7), c(2, 5), c(1, 1))
data <- bin(x, seq(-1, 8, 0.25))
fit1 <- mixfit(x, ncomp = 2)  # raw data
rm(x, data)
###

# simple function
funworks <- function(x) {
    print(x)
}

###

# almost identical simple function
funfails <- function(thisx) {
    print(thisx)
}

###

funworks(fit1)
funfails(fit1)

#######

The explanation as I understand it...

print called on this object gets passed to print.mixfitEM(), which is:


function (x, digits = getOption("digits"), ...)
{
    family <- x$family
    mc <- match.call()
    mc$digits <- digits
    fun.name <- paste0("print", family)
    mc[[1]] <- as.name(fun.name)
    eval(mc, environment())
}


Working through the calls, when eval() is called from within funfails(), mc is
printnormal(x = thisx, digits = 7)
and the calling environment does not contain thisx.

In funworks(), it's
printnormal(x = x, digits = 7)

and x is found.

So, I can get around the problem by naming my argument x, as in
funworks(), but that's unsatisfying. Is there something else I can do
to get my functions to work?

And what's the correct way to do what print.mixfitEM() is doing, so
that it works regardless? I poked around for a while, but didn't find
a clear (to me!) answer.

Thanks,
Sarah

-- 
Sarah Goslee (she/her)
http://www.numberwright.com