Basic insurance against bad args to .Fortran() calls

useful-ensurers.R

eval({
  ensure_nonnull <- ensures_that(!is.null(.) ~ "Must not be NULL" )
  ensure_nonempty <- ensures_that(length(.) > 0 ~ "Must not be empty", +ensure_nonnull )
  ensure_finite <- ensures_that(all(is.finite(.)) ~ "Must be finite", +ensure_nonempty )
  ensure_positive <- ensures_that(all(. > 0) ~ "Must be positive", +ensure_finite )
  ensure_nonnegative <- ensures_that(all(. >= 0) ~ "Must be positive or zero", +ensure_finite )
  ensure_unique <- ensures_that(length(unique(.)) == 1)
})

# Wrapped in `eval({...})` b/c otherwise RStudio will complain about syntax