match.fun provide error with functions defined inside functions

Question

I get error when try to apply match.fun to the functions define within other functions.

x <- matrix(rnorm(10*100), nrow=100) # data sample
descStats <- function(x, stats = c("n", "min", "max", "srange", "mean", "median", "sd")) {
  n <- function(x, ...) sum(!is.na(x), ...)
  srange <- function(x, ...) max(x, ...) - min(x, ...)
  fun <- function(x) {
    result <- vapply(stats, function(z) match.fun(z)(x, na.rm=TRUE), FUN.VALUE=numeric(1))
  }
  if (is.vector(x)) {
    result <- fun(x)
  }
  if (is.matrix(x) || is.data.frame(x)) {
    result <- t(apply(x, 2, fun))
  }
  return(result)
}
descStats(x)
## Error in get(as.character(FUN), mode = "function", envir = envir) : 
##   object 'n' of mode 'function' was not found

If I define n and srange outside of descStats function it works fine.

n <- function(x, ...) sum(!is.na(x), ...)
srange <- function(x, ...) max(x, ...) - min(x, ...)
descStats2 <- function(x, stats = c("n", "min", "max", "srange", "mean", "median", "sd")) {
  fun <- function(x) {
    result <- vapply(stats, function(z) match.fun(z)(x, na.rm=TRUE), FUN.VALUE=numeric(1))
  }
  if (is.vector(x)) {
    result <- fun(x)
  }
  if (is.matrix(x) || is.data.frame(x)) {
    result <- t(apply(x, 2, fun))
  }
  return(result)
}
descStats2(x)
##         n       min      max   srange        mean      median        sd
##  [1,] 100 -2.303839 2.629366 4.933205  0.03711611  0.14566523 1.0367947
##  [2,] 100 -1.968923 2.169382 4.138305 -0.03917503  0.02239458 0.9048509
##  [3,] 100 -2.365891 2.424077 4.789968 -0.08012138 -0.23515910 1.0438133
##  [4,] 100 -2.740045 2.127787 4.867832  0.03978241  0.15363449 0.9778891
##  [5,] 100 -1.598295 2.603525 4.201820  0.23796616  0.16376239 1.0428915
##  [6,] 100 -1.550385 1.684155 3.234540 -0.11114479 -0.09264598 0.8260126
##  [7,] 100 -2.438641 3.268796 5.707438  0.02948100 -0.05594740 1.0481331
##  [8,] 100 -1.716407 2.795340 4.511747  0.22463606  0.16296613 0.9555129
##  [9,] 100 -2.359165 1.975993 4.335158 -0.33321888 -0.17580933 0.9784788
## [10,] 100 -2.139267 2.838986 4.978253  0.15540182  0.07803265 1.0149671

Another way it's use eval(call(FUN, args)). For instance.

descStats3 <- function(x, stats = c("n", "min", "max", "srange", "mean", "median", "sd")) {
  n <- function(x, ...) sum(!is.na(x), ...)
  srange <- function(x, ...) max(x, ...) - min(x, ...)
  fun <- function(x) {
    result <- vapply(stats, function(z) eval(call(z, x, na.rm=TRUE)), FUN.VALUE=numeric(1))
  }
  if (is.vector(x)) {
    result <- fun(x)
  }
  if (is.matrix(x) || is.data.frame(x)) {
    result <- t(apply(x, 2, fun))
  }
  return(result)
}
descStats3(x)
##         n       min      max   srange        mean      median        sd
##  [1,] 100 -2.303839 2.629366 4.933205  0.03711611  0.14566523 1.0367947
##  [2,] 100 -1.968923 2.169382 4.138305 -0.03917503  0.02239458 0.9048509
##  [3,] 100 -2.365891 2.424077 4.789968 -0.08012138 -0.23515910 1.0438133
##  [4,] 100 -2.740045 2.127787 4.867832  0.03978241  0.15363449 0.9778891
##  [5,] 100 -1.598295 2.603525 4.201820  0.23796616  0.16376239 1.0428915
##  [6,] 100 -1.550385 1.684155 3.234540 -0.11114479 -0.09264598 0.8260126
##  [7,] 100 -2.438641 3.268796 5.707438  0.02948100 -0.05594740 1.0481331
##  [8,] 100 -1.716407 2.795340 4.511747  0.22463606  0.16296613 0.9555129
##  [9,] 100 -2.359165 1.975993 4.335158 -0.33321888 -0.17580933 0.9784788
## [10,] 100 -2.139267 2.838986 4.978253  0.15540182  0.07803265 1.0149671
identical(descStats2(x), descStats3(x))
## [1] TRUE

Why descStats not work?

Answer 1

It's relatively easy (and illustrative) to write your own version of match.fun. I've called my function fget to indicate that it's a version of get specifically designed for functions, and hence obeys the regular scoping rules for functions. (If you're not sure what they are, think about this code: c <- 10; c(c, 5))

#' Find a function with specified name.
#'
#' @param name length one character vector giving name
#' @param env environment to start search in.
#' @examples
#' c <- 10
#' fget("c")
fget <- function(name, env = parent.frame()) {
  if (identical(env, emptyenv())) {
    stop("Could not find function called ", name, call. = FALSE)
  }

  if (exists(name, env, inherits = FALSE) && is.function(env[[name]])) {
    env[[name]]
  } else {
    fget(name, parent.env(env))
  }
}

The implementation is as a straightforward recursive function: the base case is the emptyenv(), the eventual ancestor of every environment, and for each environment along the stack of parents, we check to see that both an object called name exists, and that it is a function.

It works in the simple test case provided by @nograpes because the environment defaults to the calling environment:

fun <- function(x) {
  n <- sum
  fget('n')(x)
}
fun(10)
# [1] 10