Can macros be used in R?

Question

I wrote several macros in R using the defmacro function from the gtools package:

 #' IfLen macro #' #' Check whether a object has non-zero length, and #' eval expression accordingly. #' #' @param df An object which can be passed to \code{length} #' @param body1 If \code{length(df)} is not zero, then this clause is evaluated, otherwise, body2 is evaluated. #' @param body2 See above. #' @importFrom gtools defmacro #' #' @examples #' ifLen(c(1, 2), { print('yes!') }, {print("no!")}) #' #' @author kaiyin #' @export ifLen = gtools::defmacro(df, body1, body2 = {}, expr = { if(length(df) != 0) { body1 } else { body2 } }) #' IfLet macro #' #' Eval expression x, assign it to a variable, and if that is TRUE, continue #' to eval expression1, otherwise eval expression2. Inspired by the clojure #' \code{if-let} macro. #' #' @param sym_str a string that will be converted to a symbol to hold value of \code{x} #' @param x the predicate to be evalueated, and to be assigned to a temporary variable as described in \code{sym_str} #' @param body1 expression to be evaluated when the temporary variable is TRUE. #' @param body2 expression to be evaluated when the temporary variable is FALSE. #' @importFrom gtools defmacro #' #' @examples #' ifLet("..temp..", TRUE, {print(paste("true.", as.character(..temp..)))}, #' {print(paste("false.", as.character(..temp..)))}) #' #' @author kaiyin #' @export ifLet = gtools::defmacro(sym_str, x, body1, body2={}, expr = { stopifnot(is.character(sym_str)) stopifnot(length(sym_str) == 1) assign(sym_str, x) if(eval(as.symbol(sym_str))) { body1 } else { body2 } }) #' IfLetLen macro #' #' Similar to ifLet, but conditioned on whether the length of #' the result of \code{eval(x)} is 0. #' #' #' @param x the predicate to be evalueated, and to be assigned to a temporary var called \code{..temp..} #' @param body1 expression to be evaluated when \code{..temp..} is TRUE. #' @param body2 expression to be evaluated when \code{..temp..} is FALSE. #' @importFrom gtools defmacro #' #' @examples #' ifLetLen("..temp..", 1:3, {print(paste("true.", as.character(..temp..)))}, #' {print(paste("false.", as.character(..temp..)))}) #' #' @author kaiyin #' @export ifLetLen = gtools::defmacro(sym_str, x, body1, body2={}, expr = { stopifnot(is.character(sym_str)) stopifnot(length(sym_str) == 1) assign(sym_str, x) ifLen(eval(as.symbol(sym_str)), { body1 }, { body2 }) }) 

Is there a danger of using them?

Answer 1

Of course you have to be careful. Macros do not have their own environment.

A simple example is f (x, y) = (x + y) ^ 2:

 m1 <- defmacro(x, y, expr = { x <- x + y x*x }) 

Executing m1 will change the input variables in the calling environment:

 > x <- 1 > y <- 2 > m1(x, y) [1] 9 > x [1] 3 > y [1] 2 

Normal R functions behave differently:

 f1 <- function(x, y) { x <- x + y x*x } > x <- 1 > y <- 2 > f1(x, y) [1] 9 > x [1] 1 > y [1] 2