subset deconstruction

subset2.r

SUBSET2 <- function(dat, ex) {
  ex <- substitute(ex)
  frames <- rev(sys.frames())
  for(f in frames) {
    message("---------------------------------------------")
    cat("Vars: ", paste(ls(f), sep = ","), "\n")

    call <- substitute(substitute(x, f), list(x = ex))
    ex <- eval(call)
    cat("Expr: ", deparse(ex), "\n")
  }
  dat[eval(ex, dat, parent.frame()),]
}

# In most circumstances it works ok
SUBSET2(mtcars, cyl == 4)$cyl
x <- 4
SUBSET2(mtcars, cyl == x)$cyl

# But because it evaluates in every frame, it's possible to
# create situations that don't work
f <- function() {
  cyl <- 4
  g()
}

g <- function() {
  SUBSET2(mtcars, cyl == 4)$cyl
}
f()