Code example for StackOverflow question on `foreach` and `R6` classes (https://stackoverflow.com/questions/57349958).

CodeExample.R

# # # Scope of the issue:
# 
# - without the following line `object$parent_env <- environment()` (i.e., line 54 below), it throws an error: 
#     - `Error in { : task 1 failed - "object 'Work' not found"`.
#
# - I would like to know:
#     1. what are some potential pitfalls when assigning the `parent_env` inside `foreach` 
#     2. why does it work in the first place

# Minimal reproducible example:

Work <- R6::R6Class("Work",

    public = list(
        values = NULL,


        initialize = function() {
            self$values <- "some values"
        }
    )
)

# Now, the following `Task` class uses the `Work` class in the constructor.

Task <- R6::R6Class("Task",
    private = list(
        ..work = NULL
    ),


    public = list(
        initialize = function(time) {
            private$..work <- Work$new()
            Sys.sleep(time)
        }
    )
)

# In the `Factory` class, the `Task` class is created and the `foreach` is implemented in `..m.thread()`.

Factory<- R6::R6Class("Factory",

    private = list(
        ..warehouse = list(),


        ..m.thread = function(object, amount, ...) {
            cluster <- parallel::makeCluster(parallel::detectCores() -  1)
            doParallel::registerDoParallel(cluster)

            private$..warehouse <- foreach::foreach(1:amount, .export = ls(parent.env(self$.__enclos_env__))) %dopar% {
                # What exactly does `environment()` encapsulate in this context?
                object$parent_env <- environment()

                # Instantiate the class that uses `Work` in the constructor.
                object$new(...) 
            }

            parallel::stopCluster(cluster)
        }
    ),


    public = list(
        initialize = function(object, ..., amount = 10) {
            private$..m.thread(object = object, amount = amount, ...)
        }
    )
)

# Then, it is called as:

library(foreach)

x = Factory$new(Task, time = 2, amount = 10)

# End of file.