Keep <- function(f, xs) Filter(Negate(is.null), Map(f, xs))

# Takes a data frame and filters it by Shiny input values. Each input's name
# must correspond to a column name in the data frame.
# Example:
# output$plot <- renderPlot({
#   mtcars %>%
#     filterer(input$gear, input$carb) %>%
#     ggplot(aes(mpg, disp)) + geom_line()
# })
filterer <- function(tbl, ...) {
  quos(...) %>%
    Keep(function(input_name) {
      val <- eval_tidy(input_name)
      if (val != "All") quo(!!call_args(input_name)[[2]] == !!val)
    }, .) %>%
    filter(tbl, !!!.)
}