Condition handling

Colin Fay


warnings and messages

The stop_if(), warn_if() and message_if() are easy to use functions that send an error, a warning or a message if a condition is met. Each function has its counterpart with _not that returns a message if the condition is not met.

stop_if_not() is quite the same as assert_that() from the {assertthat} package, except that it can takes mappers. It is not the same as base stopifnot(), as it doesn’t take a list of expression.

These functions are also flexible as you can pass base predicates (is.numeric, is.character…), a custom predicate built with mappers, or even your own predicate function.

You can either choose a custom message or just let the built-in messages be printed:

x <- 12
# Stop if .x is numeric
stop_if(.x = x, 
        .p = is.numeric)

y <- "20"
# stop if .x is not numeric
stop_if_not(.x = y, 
            .p = is.numeric, 
            msg = "y should be numeric")
a  <- "this is not numeric"
# Warn if .x is charcter
warn_if(.x = a, 
        .p = is.character)

b  <- 20
# Warn if .x is not equal to 10
warn_if_not(.x = b, 
        .p = ~ .x == 10 , 
        msg = "b should be 10")

c <- "a"
# Message if c is a character
message_if(.x = c, 
           .p = is.character, 
           msg = "You entered a character element")

# Build more complex predicates
d <- 100
message_if(.x = d, 
           .p = ~ sqrt(.x) < 42, 
           msg = "The square root of your element must be more than 42")

# Or, if you're kind of old school, you can still pass classic functions

e <- 30
message_if(.x = e, 
           .p = function(vec){
             return(sqrt(vec) < 42)
           msg = "The square root of your element must be more than 42")

If you need to call a function that takes no argument at .p (like curl::has_internet()), use this function as .x.

stop_if(.x = curl::has_internet(), msg = "You shouldn't have internet to do that")

warn_if(.x = curl::has_internet(), 
            msg = "You shouldn't have internet to do that")

message_if(.x = curl::has_internet(), 
            msg = "Huray, you have internet \\o/")

If you don’t specify a .p, the default test is isTRUE().

a <-$Ozone)
message_if_any(a, msg = "NA found")

In function

That can come really handy inside a function:

my_fun <- function(x){
  stop_if_not(.x = curl::has_internet(), 
              msg = "You should have internet to do that")
          msg =  "x is not a character vector. The output may not be what you're expecting.")
  paste(x, "is the value.")


none, all, any

stop_if(), warn_if() and message_if() all have complementary tests with _all, _any and _none, which combine the if_* and the warn_*, stop_* and message_* seen before. They take a list as first argument, and a predicate. They test if any, all or none of the elements validate the predicate.

stop_if_any(iris, is.factor, msg = "Factors here. This might be due to stringsAsFactors.")

warn_if_none(1:10, ~ .x < 0, msg = "You need to have at least one number under zero.")

message_if_all(1:100, is.numeric, msg = "That makes a lot of numbers.")


on_error() behaves as on.exit() except it happens only when there is an error in the function.

y <- function(x){
  on_error(~ print("ouch"))
[1] 2.484907
Error in log(x) : non-numeric argument to mathematical function
[1] "ouch"