Use character string as function argument

A more natural way to do what you want is to use do.call. For example,

R> l[["trim"]] = 0
R> l[["na.rm"]] = FALSE
R> l[["x"]] = 1:10
##Or l <- list(trim = 0, na.rm = FALSE, x = 1:10)
R> do.call(mean, l)
 [1] 5.5

If for some reason you really want to use a myoptions string, you could always use strsplit to coarce it into a list form. For example,

R> y = "trim=0, na.rm=FALSE"
R> strsplit(y, ", ")
[[1]]
[1] "trim=0"      "na.rm=FALSE" 
R> strsplit(y, ", ")[[1]][1]
[1] "trim=0"

You can use eval and parse:

foo <- eval(parse(text = paste("mean(x,", myoptions, ")")))

Here's a third answer that both uses parse, alist and do.call. My motivation for this new answer, is in the case where arguments are passed interactively from a client-side as chars. Then I guess, there is no good way around not using parse. Suggested solution with strsplit, cannot understand the context whether a comma , means next argument or next argument within an argument. strsplit does not understand context as strsplit is not a parser.

here arguments can be passed as "a=c(2,4), b=3,5" or list("c(a=(2,4)","b=3","5")

#' convert and evaluate a list of char args to a list of arguments
#'
#' @param listOfCharArgs a list of chars 
#'
#' @return
#' @export
#'
#' @examples
#' myCharArgs = list('x=c(1:3,NA)',"trim=0","TRUE")
#' myArgs = callMeMaybe(myCharArgs)
#' do.call(mean,myArgs)
callMeMaybe2 = function(listOfCharArgs) {
  CharArgs = unlist(listOfCharArgs)
  if(is.null(CharArgs)) return(alist())
    .out = eval(parse(text = paste0("alist(",
      paste(parse(text=CharArgs),collapse = ","),")")))
}

myCharArgs = list('x=c(1:3,NA)',"trim=0","TRUE")
myArgs = callMeMaybe2(myCharArgs)
do.call(mean,myArgs)
 [1] 2