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question about an R idiom: eval()ing a quoted block

3 messages · Ben Bolker, Duncan Murdoch, Peter Dalgaard

Original
Ben Bolker
# 
In a few places in the R source code, such as the $initialize element 
of `family` objects, and in the body of power.t.test() (possibly other 
power.* functions), sets of instructions that will need to be run later 
are encapsulated by saving them as an expression and later applying 
eval(), rather than as a function. This seems weird to me; the only 
reason I can think of for doing it this way is to avoid having to pass 
back multiple objects and assign them in the calling environment (since 
R doesn't have a particularly nice form of Python's tuple-unpacking idiom).

   Am I missing something?

  cheers
    Ben


https://github.com/r-devel/r-svn/blob/eac72e66a4d2c2aba50867bd80643b978febf5a3/src/library/stats/R/power.R#L38-L52

https://github.com/r-devel/r-svn/blob/master/src/library/stats/R/family.R#L166-L171
Duncan Murdoch
#
On 11/07/2023 6:01 p.m., Ben Bolker wrote:
Those examples are very old (the second is at least 20 years old).  It 
may be they were written by someone who was thinking in S rather than in 
R.

As far as I recall (but I might be wrong), S didn't have the same 
scoping rules for accessing and modifying local variables in a function 
from a nested function.

Duncan Murdoch
8 days later
Peter Dalgaard
# 
In the case of power.t.test, there is a situation where you want to create 4 different functions with the same function body, in order to pass them to uniroot(). I think that at the time, it just seemed convenient to express that idea with a quote-eval (macro-like) construction, rather than figure out how to do it with functions. I suppose that it could have been done neatly enough along the lines of

g <- function(x1, x2, x3, x4)
{
  f <- function(x1, x2, x3, x4) {...body...}
  if (is.null(x1)
     x1 <- uniroot(function(x1) {target - f(x1, x2, x3, x4)}, ....)$root
  ....
}

This adds an extra layer of function call, but probably makes the byte compiler happier!

You might also use the ... argument to uniroot() as in

x1 <- uniroot(f, x2=x2, x3=x3, x4=x4, ..other args..)$root

but that gets a bit clunky.

-pd