Play around with active bindings

activebindings.R

mab <- function(obj, expr, envir = .GlobalEnv) {
    makeActiveBinding(obj, 
                      function(e) { 
                        if(missing(e)) {
                           eval(parse(text = force(expr)), envir = envir)
                        } else {
                           makeActiveBinding(force(obj), function(expr) eval(parse(text = force(e)), envir = force(envir)), force(envir))
                        }
                      }, 
                      env = envir)
}

e <- new.env(parent = baseenv())
e$a <- 1:3
mab("b", "a*2", envir = e)
e$b
rm(a, envir = e)
e$b
e$a <- 5:1
e$b
e$b <- "a*3"
e$b
e$b <- "a+0.5"
e$b