deepMerge.R

# "Deep"-merges named lists, preferring values from y
deepMerge <- function(x, y) {
  if (is.null(y)) return(x)
  if (is.null(x)) return(y)
  if (is.list(x) && is.list(y)) {
    Reduce(function(merged, name) {
      merged[[name]] <- deepMerge(x[[name]], y[[name]])
      merged
    }, union(names(x), names(y)), init = x)
  } else y
}

identical(
  deepMerge(
    list(x = 1, y = 2),
    list(z = 3)
  ),
  list(x = 1, y = 2, z = 3)
)

# They aren't actually identical because reasons but you get the idea.
identical(
  deepMerge(
    list(x = 1, y = list(q = 2, z = 10)),
    list(z = 3, y = list(q = 5, x = 23))
  ),
  list(
    x = 1,
    z = 3,
    y = list(
      q = 5,
      z = 10,
      x = 23
    )
  )
)