A Shinyvalidate Hack

Lately I've been coding a Shiny web application (in R) in which users are confronted by several forms. Since their inputs go into a database, some amount of input validation seems prudent (to put it mildly). I came across the shinyvalidate package, which is proving very useful. You can specify a variety of rules for input fields (starting with whether they are required and moving on to various filters on what is allowed). If the input fails to meet the validation rules, messages are printed in red under the input controls (something you've probably seen before if you ever tried to submit a web form with something missing or glaringly incorrect).

The remainder of this post probably won't make much sense unless you have some experience with shinyvalidate. (I'm not promising it will make sense if you do.) I ran into a bit of a snag with a few input fields where the validation rules involved a disjunction. In one case the fields was the URL of an organization's web site, and in the other it was their contact email address. Both fields are "required" in my application, with required in quotes because it is possible for an organization not to have a web site and/or not to have a contact email address. (For instance, some organizations have web sites but no email address, expecting people to use a contact form on their home page.) The approach I'm taking is to allow "none" as a valid response for both web and email addresses if there is none. So I want to enforce the rule that the input value is either "none" or a valid address.

The shinyvalidate package provides mechanisms for enforcing conjunctions of rules (satisfy all of these requirements) but apparently does not have an explicit mechanism for enforcing disjunctions (either this or that). It does let you create custom functions to validate inputs, but I wanted to use their built-in web (sv_url()) and email (sv_email()) functions so that I would not have to go down the regular expression rabbit hole. That meant writing a function that combined a simple test (x != "none", where x is the value submitted in a text field of the form) with their functions.

It took me a while to figure out that while sv_url and sv_email take a few arguments (including an optional customized error message), they do not actually take the form input (what I'm calling x) as an argument. Instead,  the output value of sv_url() or sv_email() is itself a function with the form input as its sole argument. Once that sank in, making the custom validators was trivial. Here is my URL validator.

urlOrNone <- function(x) {
  if (x != "none") sv_url("Please enter a valid URL, including the http/https prefix.")(x)
} 

If x either equals "none" or satisfies the sv_url validator, the return value is NULL, which allows the input to pass validation. Otherwise, the user gets the custom error message and the input is disallowed.