Tuesday, October 13, 2009

HTML::FormHandler - a few notes

This is not an introduction to HTML::FormHandler - the docs there are very good, but I wanted to write down the main ideas I had when contributing to the design of it.

Let's look at the functions of a form processor:

  1. processing the input into internal representation
  2. checking the values
  3. saving the internal representation into the persistence layer and loading values from it
  4. rendering the form (with input, error messages, etc)

The 'best practice' of rendering the HTML view of an application page is to do it in the templates. Unfortunately there is just too much logic needed for the correct rendering of forms (mostly with the display of errors) to leave that for the templates mini language (like that of TT) - that's why this needs to be done by the form processor. But because of the diversity of the rendering requirements it also needs to be easily replaceable, ideally part by part. Then the application could start with a crude but working HTML rendering, introduced with minimal effort from the programmer since it is built into the processor and later gradually refined and replaced.

CGI.pm and after it Catalyst and other libraries parse the url-encoded and body parameters into a hash of scalar values but internally we would like to manipulate objects like DateTime instead of lists of values for year, month and day. In HFH this conversion is done in two phases - first the input is turn into a deeper structure built of hashes and arrays - then then the processor goes recursively from the leaves and at each node applies a conversion if it is defined for that node.

There is always a question when we should do the checks - sometimes it is easier do do them on the input data sometimes on the internal values. A good example is DateTime - we can check if the day field is a number between 1 and 31, but we also need to check if the whole date is correct and the business rules could add another requirement that the day of the week is Monday or some other operation that is most convenient on the DateTime object. In FormHandler all of this is possible:

How this works? As explained above the processor starts with the leave nodes - and checks the day, month and year fields (here they don't define any transformations only checks, but they could). Then it goes to the parent node date_time and there it has the hash of values from the leave nodes ready for the next transformation and checks - it is passed to:

sub { DateTime->new( shift ) }

If that subroutine triggers an exception, for example when someone passes

{ year => 2009, month => 2, day => 30 }

to it - then the error is recorded with the message This is not a correct date. If it works - the value returned from it, a DateTime object, is then passed to the next check, and later presumably saved to the database. The checks and transformations are taken from a single list - so you can interweave them as you need.
Instead of transform and check in the apply attribute we can also use the Moose types and coercions and also we can mix these two styles (for example coerce some value using Moose types and then check it again). If someone needs even more power - he can also write his own field class with it's own validation method - and do both validation and transformations in there, that method would receive the same hash of child values.

To save the full data structure of values that can contain many inter-related records HTML::FormHandler::Model::DBIC uses
DBIx::Class::ResultSet::RecursiveUpdate. I separated it so that HFH is not related to the hacks I made there :) Or maybe I hoped that perhaps other form processors could reuse my module. Using it you can update a company record together with all it's related addresses in one form.

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