R’s eval vs get

I’ve been fighting damn scoping issues lately and finally sat down to figure out what works and when. Documenting some of them here

The lessons I’ve learned from this:

1. eval(a, envir = ... ) DOES NOT WORK!
2. Instead, do eval(str2lang("a"), envir = ...)
3. eval.parent(a) DOES NOT WORK!
4. eval.parent(str2lang("a")) works and SKIPS GENERIC!
5. get("a", envir = ...) seems to always work as expected

So to summarize, I think get is always a safer option than eval. In addition, get is much faster:

> f1 <- function() {
+ eval.parent(str2lang("a"))
+ }
> f2 <- function() {
+ get("a", envir = sys.frame(0))
+ }
> a <- 1
> microbenchmark:::microbenchmark(f1(), f2(), times = 10000)
Unit: nanoseconds
 expr  min   lq      mean median   uq     max neval
 f1() 1353 1558 1854.5325   1640 1763 1019547 10000
 f2()  533  656  808.4462    697  738  800074 10000

Obviously this doesn’t cover more advanced situations, like evaluating full calls, but I’m still working on puzzling all that out.

Here’s the sample code I used to evaluate this:

foo <- function(x) {
  a <- "in generic"
  UseMethod("foo")
}

foo.numeric <- function(x) {
  a <- "in specific"
  print(sys.calls())

  # Itself
  print(paste("a:",
               a
              ))
  # Eval
  print(paste("eval(a):",
               eval(a)
        ))
  print(paste('eval(str2lang("a")):',
               eval(str2lang("a"))
        ))

  # eval.parent
  print(paste("eval.parent(a):",
               eval.parent(a)
              ))
  print(paste('eval.parent(str2lang("a")):',
               eval.parent(str2lang("a"))
              ))

  # eval with sys.frames
  print(paste("eval(a, envir = sys.frame(0)):",
               eval(a, envir = sys.frame(0))
              ))
  print(paste('eval(str2lang("a"), envir = sys.frame(0)):',
               eval(str2lang("a"), envir = sys.frame(0))
              ))
  print(paste("eval(a, envir = sys.frame(1)):",
               eval(a, envir = sys.frame(1))
              ))
  print(paste('eval(str2lang("a"), envir = sys.frame(1)):',
               eval(str2lang("a"), envir = sys.frame(1))
              ))

  # get
  print(paste('get("a"):',
               get("a")
              ))

  # get with sys.frames
  print(paste('get("a", envir = sys.frame(0)):',
               get("a", envir = sys.frame(0))
              ))
  print(paste('get("a", envir = sys.frame(1)):',
               get("a", envir = sys.frame(1))
              ))
}

a <- "in global"
foo(1)

[[1]]
foo(1)

[[2]]
UseMethod("foo")

[1] "a: in specific"
[1] "eval(a): in specific"
[1] "eval(str2lang(\"a\")): in specific"
[1] "eval.parent(a): in specific"
[1] "eval.parent(str2lang(\"a\")): in global"
[1] "eval(a, envir = sys.frame(0)): in specific"
[1] "eval(str2lang(\"a\"), envir = sys.frame(0)): in global"
[1] "eval(a, envir = sys.frame(1)): in specific"
[1] "eval(str2lang(\"a\"), envir = sys.frame(1)): in generic"
[1] "get(\"a\"): in specific"
[1] "get(\"a\", envir = sys.frame(0)): in global"
[1] "get(\"a\", envir = sys.frame(1)): in generic"

This work is licensed under CC BY-NC 4.0