rlang, tidyeval, dplyr, ggplot and non-syntactic column names

gistfile1.txt

## I have a data set something like this.
mtc <- mtcars %>%
  mutate(cyl                       = factor(cyl),
         gear                      = factor(gear),
         `Q17_anx/worry about abc` = gear,
         `Q17_anx about abc`       = gear)

## An admittedly ugly function (on various levels / from diverse perspectives)
## in which I'm trying baby steps for tidy evaluation.
ppp <- function(dfr, q, by = NULL, n.resp = length(unique(dfr$cyl))) {
  grp <- rlang::syms(q)
  response <- rlang::ensym(q)

  ## count answered and skipped.
  skipped <- dfr %>% filter(is.na(!!!grp)) %>% count(!!!grp) %>% pull(n)
  N <- n.resp - skipped

  if (!is.null(by)) {
    grp <- rlang::syms(list(q, by))
  }

  d <- dfr %>%
    filter(!is.na(!!response)) %>%
    count(!!!grp)

  if (!is.null(by)) {
    b <- rlang::ensym(by)
    d <- d %>%
      group_by(!!b) %>%
        mutate(N = sum(n))
  } else {
    d <- d %>% mutate(N = sum(n))
  }

  d <- d %>%
    mutate(perc = n/N)
  N <- sum(d$n)

  p <- d %>%
    ggplot(mapping = aes(x = !!response, y = perc, colour = !!response, fill = !!response)) +
      geom_col() +
      labs(x = '', y = '') +
      scale_y_continuous(limits = range(0, 1), breaks = seq(0, 1, 0.10), labels = scales::percent) +
      scale_x_discrete(limits = rev(levels(d[[q]]))) +
      coord_flip() +
      guides(fill = FALSE, colour = FALSE) +
      theme_bw() +
      theme(strip.background  =  ggplot2::element_rect(colour  =  NA, fill  =  NA))
  if (!is.null(by)) p <- p + facet_wrap(as.formula(paste('~', by)))

  return(p)
}

## Then I'm calling the function like this.
mtc %>% ppp(q = 'cyl', by = 'gear') # Works
mtc %>% ppp(q = 'cyl', by = 'Q17_anx/worry about abc') # ERROR (unexpected symbol)
mtc %>% ppp(q = 'cyl', by = 'Q17_anx about abc') # ERROR ibid

## Error for the last above call:
## Error in parse(text = x, keep.source = FALSE) : 
##  <text>:1:11: unexpected symbol
##1: ~ Q17_anx about
##              ^

On twitter @_lionelhenry said

You need facet_wrap(vars(!!by)).

If your function is meant to take strings, I wouldn't use NSE in its interface at all. Just use sym() and syms() to transform to symbols. Or pass the raw strings directly to .data[[, that's a bit safer:

facet_wrap(vars(.data[[by]]))