On Wed, Jun 1, 2011 at 7:02 PM, Justin Greene wrote:

...

Right now, you can apply whatever validation you want to an individual fields and display the error messages inline. For complex scenarios, I'm not quite certain what an interface (both programming interface and UI) would look like for assigning these error messages inline. Short of that, it seems outside the scope of the form package itself to validate such interactions. For example, you could always write code along the lines of:

((res, form), enctype) <- runFormPost ... case res of   FormSuccess x ->       case complexTest x of           Left e -> showFormWithErrMsg form e           Right y -> onSuccess y   _ -> showFormWithoutErrMsg form

My only comment about this is that it creates a bad user experience.  If my form type binding is not successful on all fields then this complex validation scenario will not fire.  This makes it so the user has to get past one set of error messages, and then move on to the next set.  Ideally they would receive all error messages at once that are possible to validate at the time.  While it may not be easy in haskell to accomplish this, it's definitely something that needs to be considered imo.

An example form: CheckBox FirstName Date of Birth

If a user selects the checkbox then the first name is now required. However, they entered an incorrectly formatted date of birth.  We have enough information to fire to the complex validator in this scenario at the same time as showing an error about an improper date for the date of birth field.  The above example wouldn't do this though, it would first show an error about the date of birth being invalid, and then after that is fixed it would show the error about the first name being required.

I realize haskell makes this scenario challenging, so it may not be worth your time to optimize for it, but I think any advanced forms framework should consider it.

I would love to help design this api, but my hobby time is going to be all used up for a long while (trying to bootstrap a startup in my spare time). I will say that I think template haskell is probably the only way to actually fulfill these types of requirements in haskell (though I could easily be wrong about that).

I think the solution here is to create a combined field for checkbox + name field. In the Applicative case, this would constrain you to having both widgets display in the same table cell/div, but I think this is acceptable: Applicatives are meant for "standard" layouts. If you want something more custom, you'll end up with a Monadic form providing a "FormResult (Bool, Text)". Obviously, the library isn't going to do *exactly* what you're looking for in every case (unless you're in the fortunate position like me of having written the library), but I think there's enough power available to cover even complicated cases in a safe way. I think TH isn't necessary here, though its usage may allow for significantly more concise definitions. Michael