foreach wrapper function that parallelizes any user-supplied expressions

foreach-expr-sketch.R

# Get operator (parallel or serial)
# Idea from <https://github.com/tidymodels/tune/blob/c4a1d891ac77e64086a417ab75ad822178cd87ee/R/parallel.R>
get_operator <- function() {
  is_par <- foreach::getDoParWorkers() > 1
  if (is_par) {
    res <- foreach::`%dopar%`
  } else {
    res <- foreach::`%do%`
  }
  res
}

#' Parallelize arbitrary user-supplied expression
#'
#' @param expr R expression.
#' @param n Number of iterations.
#' @param seed Initial random seed.
#' @param counter Character string. Name of the loop counter variable. Default is `"i"`.
fpar <- function(expr, n, seed, counter = "i") {
  `%op%` <- get_operator()
  expr <- substitute(expr)
  foreach::foreach(
    i = seq_len(n),
    # .packages = ...,
    # .export = ...,
    # .errorhandling = ...,
    .combine = "c"
  ) %op% {
    assign(counter, value = i)
    set.seed(seed + i - 1)
    eval(expr)
  }
}

# User code will look like this

library(doParallel)
registerDoParallel(parallel::detectCores())

fpar(
  {
    x <- exp(log(index))
    y <- rnorm(1, sd = 0.1)
    x + y
  },
  n = 10,
  seed = 42,
  counter = "index"
)