evaluate expressions (with a keyword)

eval_string.R

#' Convert strings to R objects
#'
#' @param x a string
#' @details It is particularly designed for the YAML strings that can be
#'   converted into R code. Strings to be evaluated should start with *`'r'`*
#'   letter and be surrounded with backticks. If this is not the case, it will
#'   return string as is. It is advised to keep all string in double quotes,
#'   otherwise unquoted strings will be converted by the yaml reader.
#' @noRd
eval_string <- function(x) {
  # beginning pattern:
  if (grepl("^(`r\\s+)", x)) {
    # remove backtick, r and space beginning and backtick at the end:
    eval(parse(text = gsub("(^`r\\s+)|(`$)", "", x)))
  } else {
    # return string unevaluated:
    x
  }
}