Archive for the ‘macros’ Category

Announcement: clj-decline – validation sucks.

Saturday, March 12th, 2011

I pushed a new validation library for clojure to github yesterday. Check out clj-decline.

Why another validation library?

Well, why does validation suck so much?

Of course, dealing with user input is annoying anyway. But validation libraries always seem to want to do things in just the wrong way for the project you’re working on.

Let me count the ways:

  • They do too much. Session management? XSS detection? Javascript validation? Form definitions? Bollocks. It will never work with the code I’ve already got running in production and I’m not going to drag in all of it just to get the core validation functions, assuming I can even use them in my app.
  • Also, validation does not mean “force this input into another type”. That’s not validation. Stick it somewhere else.
  • They assume too much. If you can only validate a single value in a map, you’re useless. I need to check if a frobniz has either two wheebles or an odd number of crinks, and don’t try to stop me.
  • I might need to validate something that isn’t a map. Maybe I want to check two maps. Maybe I need to check a single string or a file upload.
  • They still assume too much. This may come as a shock, but not everyone speaks English on this planet. I have to support multiple languages in the same web app. Give me more options than pre-defined strings for errors, you lousy piece of American imperialist software! People of the earth, throw off your shackles and your “reality” TV shows!


  • Macros, macros, macros all over the place. Yes, macros are cool, but no, I don’t want to stick every validation in a named var. If I wanted that, I could (def some-name (make-validation …)) so I don’t need your macro anyway. I want to use closures that validate for this specific user and now you’ve stopped me.
  • Don’t be passive agressive. Validation is a user-centric feature. If I wanted to tell the user only their first error, I’d use exceptions. Don’t force the user to submit their form twenty times until they’ve fixed all their mistakes. Give them as much information as possible so they know what’s going on.

    So, how does clj-decline fix all that?

    It doesn’t. It just stays away from most of the above. clj-decline is simple. It validates arguments and returns errors. Everything else is up to the user or some other library. It’s completely functional, has no macros, no built-in predicates, nothing binds it to a web framework or anything else, and errors / messages can be anything you like. The only decision I made is that errors are grouped by key (which can also be anything you like).