RANT

** 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!
 
  Ahem.

* 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).

Enjoy!

Joost.