Data::TreeValidator - Easy validation and transformation of scalar tree structures


    use Data::TreeValidator::Sugar qw( branch leaf );
    use Data::TreeValidator::Constraints qw( required );

    my $validator = branch {
        name => branch {
            first_name => leaf( constraints => [ required ] ),
            last_name => leaf( constraints => [ required ] ),
        age => leaf

    my $result = $validator->process({
        name => {
            first_name => 'Oliver',
            last_name => 'Charles',
        age => 21

    my $clean = $result->clean;


There exist a plethora of form libraries on CPAN, but this takes a different approach. Data::TreeValidator takes the extremely simplistic approach that a form is nothing more than a tree, that is given questionable data. The process of validating a form can be thought of in 2 stages: constraints and transformations.


Constraints constraint data to match certain values. In the synopsis above, the required constraint is applied to the first_name and last_name nodes, meaning that these must be passed a true string (not undef, and not an empty string) in order to be valid.

Constraints can do a lot more than this however, as a constraint is just a function. You could pass a function that verifies something is an integer, another than verifies that the integers are within bounds, and another that is closure with access to your database handle, in order to guarantee uniqueness of an attribute.

Constraints are chained and applied in order.


After all constraints pass, the input data is then chained through a series of transformations. Transformations allow to ensure you get data back in a consistent for you expect. For example, you could apply transformations on a text input to make sure it has no leading or trailing whitespace, then another to ensure that the string is in Title Case.

Transformations are essentially mapping functions, which take data of one type, and return data in another type (which may, or may not be the same).

Transformations are also applied in order, and all transformations are composed together, so that input flows from one into the next.


You will probably be most intrested in the following documentation:

Data::TreeValidator::Branch, Data::TreeValidator::Leaf

The essentials for specifying the structure of your validation tree.


A branch that can repeat it's input

Data::TreeValidator::Constraints, Data::TreeValidator::Transformations

Useful constraints and transformations you may wish to make use of.


Syntatic sugar to ease the creation of validation specifications.


Why do we need another way to validate data? I have a few presonal issues with the philosophy behind the other form libraries on CPAN.

Mixed responsibility

I do not think it is the form validations responsibility to handle the view of the form itself. A form, in my eyes, should be a specification for how to constrain data, and how to transform it. It is not a system for saying how the HTML should look.

This is not to say I'm against helpers to perform rendering a view to input data, I just do not believe it should be part of the same distribution.

Over specialization

LIkewise, most of the form systems seem to be overly specializing, with field types that map to HTML input controls. I suppose in practice, this makes sense, but again - the form system does not have to be matched to HTML, it should be a level above that. Furthermore, most of the problems I've ran into with form systems have required a change to the form system itself, and have not been something I can fix. I like to think that the architecture Data::TreeValidator has can extend to most circumstances, but time will tell.

Excessive state

A validator in Data::TreeValidator is fundamentally immutable. The act of calling process creates a special result object that takes the given input, and an optional given initialization object, but does not change state as a result of the call. This is a huge advantage in my opinion, as it allows us to fully cache a form at application startup, rather than generating them on request.

Other form libraries do allow this, to an extent, but I've found them limiting. Once I start extending form with my own parameters, I've found it very difficult to inject these at the time of processing, rather than the point of instantiation. Data::TreeValidator takes a different approach where you pass in extra parameters at process time, not construction.


Oliver Charles


This software is copyright (c) 2012 by Oliver Charles <>.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.