Question

I want to create a function that will retry an expression if it fails. Here’s my working version:

retry <- function(.FUN, max.attempts=3, sleep.seconds=1) {
  x <- NULL
  if(max.attempts > 0) {
    f <- substitute(.FUN)
    x <- try(eval(f))
    if(class(x) == "try-error") {
      Sys.sleep(sleep.seconds)
      return(suppressWarnings(retry(.FUN, max.attempts-1)))
    }
  }
  x
}

retry(stop("I'm here"))

If I remove the suppressWarnings() function above, then I get a set of warnings on each recursive call. Does anyone know what I’m doing wrong that would cause that?

Here’s an example that can be run repeatedly:

retry({ tmp <- function() { if(rnorm(1) < 0) stop("I'm here") else "success" }; tmp() })

Answer 1

I’m not sure if I can describe the cause exactly, but I’ve isolated the problem and can fix it. The basic problem is the recursion: retry(.FUN, max.attempts-1) – when the recursive call calls substitute(.FUN) it’s going to have go up a level of the call stack to figure out what the value of .FUN is – it has to restart the evaluation of a promise (the delayed execution of function arguments) a level up.

A fix is to just do the substitution once:

retry <- function(.FUN, max.attempts = 3, sleep.seconds = 0.5) {
  expr <- substitute(.FUN)
  retry_expr(expr, max.attempts, sleep.seconds)
}

retry_expr <- function(expr, max.attempts = 3, sleep.seconds = 0.5) {
  x <- try(eval(expr))

  if(inherits(x, "try-error") && max.attempts > 0) {
    Sys.sleep(sleep.seconds)
    return(retry_expr(expr, max.attempts - 1))
  }

  x
}

f <- function() {
  x <- runif(1)
  if (x < 0.5) stop("Error!") else x
}

retry(f())

To create functions that you can use flexibly, I highly recommend minimising the use of substitute. In my experience, you’re usually best off having one function that does the substitution, and another that does all the work. This makes it possible to use the function when called from another function:

g1 <- function(fun) {
  message("Function starts")
  x <- retry(fun)
  message("Function ends")
  x
}
g1(f())
# Function starts
# Error in eval(expr, envir, enclos) : object 'fun' not found
# Error in eval(expr, envir, enclos) : object 'fun' not found
# Error in eval(expr, envir, enclos) : object 'fun' not found
# Error in eval(expr, envir, enclos) : object 'fun' not found
# Function ends

g2 <- function(fun) {
  message("Function starts")
  expr <- substitute(fun)
  x <- retry_expr(expr)
  message("Function ends")
  x
}
g2(f())
# Function starts
# Error in f() : Error!
# Function ends
# [1] 0.8079241