# function to compute summary stat
compute_box_value <- function(data, vars, funs) {
  f = funs_(funs)
  result <- data %>%
    summarize_at(.cols = vars, .funs = f)
}

# simple user defined function that gets count of rows with certain values of x
equals <- function(x, test_value) {
  sum(x %in% test_value)
}

x <- data.frame(value = sample(1:5, 10, TRUE))

vars <- c("value")

# this works
print(compute_box_value(x, vars, "mean(., na.rm = TRUE)"))

# this works
summarize_at(x, vars, .funs = "equals", test_value = 1)

# this doesn't work (error: couldn't find function equals)
print(compute_box_value(x, vars, "equals(., test_value = 1)"))