This is a meditation on processing data from HTML forms. There is a page at the Perl 5 wiki on that subject - it is a more practical list of available form processing libraries, here I wanted to ponder a bit more theoretically on that subject (and perhaps later update that other page).
What form processors do:
- Parsing the CGI parameters and constructing internal version of them. This might be basic (only parsing), through constructing a simple data structure (for hierarchical params - like param.subparam etc) to constructing objects (for dates).
- Saving the values to the persistance layer (database)
- Loading values from the persistance layer for point 5
- Generating HTML (with error indicators etc).
- Should the HTML be generated? I think it is useful as a 'first stage' approach. The logic of adding the error messages and inserting appriopriate error CSS classes is quite complicated so this saves a lot of work. But there should be also left a path for upgrading to using external form templates with the library - so that later, when the project grows the designers can use some more fancy constructs.
- Should validation be done on the internal representation (objects) or on the key value pairs from the first parsing? If it is done on internal representation - then how errors are supposed to be propagated to the HTML generator (which works on the key value pairs)?
- How to interface with specialized data validating libraries?
- When form should be saved to the database? Obviously only when it has valid parameters - but some people also check the HTTP method and other things.
- When values should be loaded from the database? Only if form is not submitted - but I also like to pass values to a form from a link - so I would like if it would not overwrite it with values from the db.