Active bindings in R are much like properties in other languages: They look like a variable, but querying or setting the value triggers a function call. They can be created in R via makeActiveBinding(), but with this API the function used to compute or change the value of a binding cannot take additional arguments. The bindr package faciliates the creation of active bindings that are linked to a function that receives the binding name, and an arbitrary number of additional arguments.
You can install bindr from GitHub with:
# install.packages("devtools")
devtools::install_github("krlmlr/bindr")
For illustration, the append_random() function is used. This function appends a separator (a dash by default) and a random letter to its input, and talks about it, too.
set.seed(20161510)
append_random <- function(x, sep = "-") {
message("Evaluating append_random(sep = ", deparse(sep), ")")
paste(x, sample(letters, 1), sep = sep)
}
append_random("a")
#> Evaluating append_random(sep = "-")
#> [1] "a-h"
append_random("X", sep = "+")
#> Evaluating append_random(sep = "+")
#> [1] "X+k"
In this example, we create an environment that contains bindings for all lowercase letters, which are evaluated with append_random(). As a result, a dash and a random letter are appended to the name of the binding:
library(bindr) env <- create_env(letters, append_random) ls(env) #> [1] "a" "b" "c" "d" "e" "f" "g" "h" "i" "j" "k" "l" "m" "n" "o" "p" "q" "r" "s" #> [20] "t" "u" "v" "w" "x" "y" "z" env$a #> Evaluating append_random(sep = "-") #> [1] "a-s" env$a #> Evaluating append_random(sep = "-") #> [1] "a-h" env$a #> Evaluating append_random(sep = "-") #> [1] "a-c" env$c #> Evaluating append_random(sep = "-") #> [1] "c-o" env$Z #> NULL
Bindings can also be added to existing environments:
populate_env(env, LETTERS, append_random, "+") #> Evaluating append_random(sep = "-") #> Evaluating append_random(sep = "-") #> Evaluating append_random(sep = "-") #> Evaluating append_random(sep = "-") #> Evaluating append_random(sep = "-") #> Evaluating append_random(sep = "-") #> Evaluating append_random(sep = "-") #> Evaluating append_random(sep = "-") #> Evaluating append_random(sep = "-") #> Evaluating append_random(sep = "-") #> Evaluating append_random(sep = "-") #> Evaluating append_random(sep = "-") #> Evaluating append_random(sep = "-") #> Evaluating append_random(sep = "-") #> Evaluating append_random(sep = "-") #> Evaluating append_random(sep = "-") #> Evaluating append_random(sep = "-") #> Evaluating append_random(sep = "-") #> Evaluating append_random(sep = "-") #> Evaluating append_random(sep = "-") #> Evaluating append_random(sep = "-") #> Evaluating append_random(sep = "-") #> Evaluating append_random(sep = "-") #> Evaluating append_random(sep = "-") #> Evaluating append_random(sep = "-") #> Evaluating append_random(sep = "-") env$a #> Evaluating append_random(sep = "-") #> [1] "a-q" env$Z #> Evaluating append_random(sep = "+") #> [1] "Z+c"
Both named and unnamed arguments are supported:
create_env("binding", paste, "value", sep = "-")$binding
#> [1] "binding-value"
A parent environment can be specified for creation:
env2 <- create_env("a", identity, .enclos = env)
env2$a
#> a
env2$b
#> NULL
get("b", env2)
#> Evaluating append_random(sep = "-")
#> [1] "b-t"
The bindings by default have access to the calling environment:
create_local_env <- function(names) {
paste_with_dash <- function(...) paste(..., sep = "-")
binder <- function(name, append) paste_with_dash(name, append)
create_env(names, binder, append = "appending")
}
env3 <- create_local_env("a")
env3$a
#> [1] "a-appending"
All bindings are read-only:
env3$a <- NA #> Error: Binding is read-only. env3$a <- NULL #> Error: Binding is read-only.
Existing variables or bindings are not overwritten:
env4 <- as.environment(list(a = 5)) populate_env(env4, list(quote(b)), identity) ls(env4) #> [1] "a" "b" populate_env(env4, letters, identity) #> Error in populate_env(env4, letters, identity): Not creating bindings for existing variables: b, a
Active bindings must be R functions. To interface with C++ code, one must bind against an exported Rcpp function, possibly with rng = false if performance matters. The bindrcpp package uses bindr to provide an easy-to-use C++ interface for parametrized active bindings, and is the recommended way to interface with C++ code. In the remainder of this section, an alternative using an exported C++ function is shown.
The following C++ module exports a function change_case(to_upper = FALSE), which is bound against in R code later.
#include <Rcpp.h>
#include <algorithm>
#include <string>
using namespace Rcpp;
// [[Rcpp::export(rng = FALSE)]]
SEXP change_case(Symbol name, bool to_upper = false) {
std::string name_string = name.c_str();
std::transform(name_string.begin(), name_string.end(),
name_string.begin(), to_upper ? ::toupper : ::tolower);
return CharacterVector(name_string);
}
Binding from R:
env <- create_env(list(as.name("__ToLower__")), change_case)
populate_env(env, list(as.name("__tOuPPER__")), change_case, TRUE)
ls(env)
#> [1] "__ToLower__" "__tOuPPER__"
env$`__ToLower__`
#> [1] "__tolower__"
get("__tOuPPER__", env)
#> [1] "__TOUPPER__"
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