Unwinding scope in `source()`

source-unwind.R

# Prototype of `source()` as currently implemented
source2 <- function(file, env = parent.frame()) {
  exprs <- parse(file)
  for (expr in exprs) {
    eval(expr, env)
  }
}

# Prototype of `source()` using an evaluation strategy where the
# unwinding scope is the whole file rather than each line
source3 <- function(file, env = parent.frame()) {
  exprs <- parse(file)

  cb <- function() {
    for (expr in exprs) {
      # Using `delayedAssign()` as an interface to the C-level function
      # `Rf_eval()`. Unlike the R-level `eval()`, this doesn't create
      # an unwinding scope.
      eval(bquote(delayedAssign("do", .(expr), eval.env = env)))
      do
    }
  }

  # Here we use `eval()` to create an unwinding scope for `env`.
  # We call ourselves back immediately once the scope is created. 
  eval(as.call(list(cb)), env)
  invisible()
}

# Let's try these functions on the following script
file <- tempfile()
cat(file = file, "
  on.exit(cat('on exit!\\n'))
  cat('now!\\n')
  return()
  cat('never run!\\n')
")

# Whereas `source()` and `source2()` have linewise unwinding scope,
# `source3()` has filewise unwinding scope

source(file)
#> on exit!
#> now!
#> never run!

source2(file)
#> on exit!
#> now!
#> never run!

source3(file)
#> now!
#> on exit!