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NAME ^

Brick::Tutorial - How to use Brick

SYNOPSIS ^

DESCRIPTION ^

Brick is a way to organize business rules to validate data. It's easy to validate values by themselves, but the validation of relationships is much trickier.

Making a brick

The name "Brick" comes from the terminology for the validation routines, each of which is a brick. Think of the building block toys the come from Denmark and rhyme with "Say Go". Every time I create a brick, I'll add it to the "bucket", which keeps track of all the bricks.

Each brick represents one part of a validation, and it should be as targeted as possible. A brick is just a anonymous subroutine that follows an interface.

The brick returns it result in one of three ways.

Returns true

If the brick returns a true value, it means that the data passed its condition.

Returns false

If the brick returns a false value (but didn't die), that means that the data did not pass the condition, but it's not a failure. This is for "selectors".

dies with a reference

If a brick dies with a reference, it's like an exception. The brick uses an anonymous hash as the argument to die. That hash contains the name of the brick, a message about why the brick failed, and perhaps a description of the brick.

diess with a string

If a brick dies with a string, there's a programming problem. This isn't part of the interface, so if this happens, I've messed up somehow.

Every brick has access to all of the input data. Since I'm concerned about complex relationships, I don't want to limit the effect of any particular piece. With maximum flexibility, I have maximum power.

        # every brick gets all the input
        $brick->( \%input );

To create a brick, I need an subroutine to do the validation. In this case, I want to ensure that the input hash has a key named 'cat'. If it does I return true, and die with a hash reference otherwise. I'll talk about that die later.

        my $sub = sub {
                my $input = shift;
                
                return 1 if exists $input->{cat};
                
                die { 
                        handler      => 'Cat key check',
                        failed_field => 'cat'
                        message      => "The input didn't have a field named 'cat'",
                        };
                }

The brick doesn't do me much good until I add it to the bucket, though. My call to add_to_brick returns the anonymous subroutine, but also keeps track of it with a name and a description, as well as the details about which line of code it comes from and many other details.

        $brick = $bucket->add_to_bucket( {
                name        => 'cat key checker',
                description => "The input didn't have a field named 'cat'",
                code        => $sub
                } );

The bucket has two major functions: it keeps track of the relationships between bricks so I can "explain" a business rule (discussed later), and so I can easily debug what I've done. Since I'm going to be making a lot of closures, I want to know where they came from in the code. I'd go crazy without being able to use the bucket to help me keep track of things. More on that coming up.

I'm going to make a lot of bricks, and some of them will be almost the same. Instead of checking for the 'cat' key, I want to do the same thing but for the key 'dog'. I need a brick factory. The factory gets a $setup hash as an argument. I put the anonymous subroutine inline with the my call to add_to_bucket. Everywhere that I had the literal 'cat' before I now have a variable, $setup-{field}>, which came from the input to _input_key_exists.

        my $cat_brick = $bucket->_input_key_exists( { field => 'cat' } );
        
        my $dog_brick = $bucket->_input_key_exists( { field => 'dog' } );

Somewhere I defined the _input_key_exists method so it shows up in the Bucket class:

        package Brick::Bucket;
        
        sub _input_key_exists
                {
                my( $bucket, $setup ) = @_;
                
                $bucket->add_to_bucket( {
                        name        => '$setup->{field} key checker',
                        description => "The input didn't have a field named '$setup->{field}'",
                        code        => sub {
                                my $input = shift;
                                
                                return 1 if exists $input->{ $setup->{field} };
                                
                                die { 
                                        handler => 'Cat key check',
                                        message => "The input didn't have a field named '$setup->{field}'",
                                        };
                                },
                        } );
                
                }

Every time I call _input_key_exists I get a new brick, because it's always a new closure (closing over $setup). The factory automatically adds the brick to the bucket.

Composing bricks

I'll build a business rule from several bricks. When I want to test a business rule, I run all of the bricks and look at their return value. Instead of keeping track of a bunch of bricks, though, I'll compose them into larger structures so I only have to remember one thing. A composer simply creates a new brick based on the ones I give it. The bucket will keep track of the relationships for me.

I create another factory that puts the bricks together. Inside _cat_and_dog_exists, I create the bricks for 'cat' and 'dog', and use them as arguments to __compose_satisfy_all, which creates a new brick that only returns true when both $cat_brick and $dog_brick return true.

        sub _cat_and_dog_exists
                {
                my( $bucket, $setup );
                
                my $cat_brick = $bucket->_input_key_exists( 
                        { %$setup, field => 'cat' } );
                
                my $dog_brick = $bucket->_input_key_exists( 
                        {  %$setup, field => 'dog' } );
                
                
                $bucket->__compose_satisfy_all( $cat_brick, $dog_brick );
                }

A composer can decide when it should return true, though. In the previous example both bricks had to return true, but if I only need at least one of them to be true, I can use a different composer, perhaps __compose_satisfy_any, in which case only one of the bricks needs to pass:

        sub _either_cat_and_dog_exists
                {
                my( $bucket, $setup );
                
                my $cat_brick = $bucket->_input_key_exists( 
                        { %$setup, field => 'cat' } );
                
                my $dog_brick = $bucket->_input_key_exists( 
                        {  %$setup, field => 'dog' } );
                
                
                $bucket->__compose_satisfy_any( $cat_brick, $dog_brick );
                }

Brick comes with several composers in Brick::Composers, but you can also create your own if those don't work for you. The composer is really just another factory to create bricks.

Selectors

Selectors are a special sort of brick that doesn't die. When it fails, it just returns 0 (not just false, but specifically 0). I can use these with composers to decide if I want to continue with the rest of the bricks in that composition.

The composer __compose_pass_or_stop can use a selector to stop processing. It won't die, so it doesn't fail. It doesn't keep going, either, so it effectively prunes the validation to exclude those bricks that don't apply to the situation.

The composer __compose_pass_or_skip usually composes bricks made with __compose_pass_or_stop. Once one thing stops processing, it moves onto the next brick.

See the example in Brick::Selector.

Filters

Filters are a special sort of brick that always returns true. I use them to affect the input data before I start to validate it.

Making constraints ^

A constraint is a business rule. It's made up of bricks, but it also has some extra glue to connect the input data to the validation routines. The constraints are the end of the line for composition. They are the bits that actually run the bricks and pass the input data to them.

Constraints should be public subroutines (so no leading underscores) whose name reflects what it does.

        sub check_cat_and_dog
                {
                my( $bucket, $setup );
                
                my $brick = $bucket->_either_cat_and_dog_exists( $setup );
                
                my $constraint = $brick->__make_constraint( $brick, $setup );
                }

I don't need to call the constraint subroutines myself. Brick will automatically do that when it constructs a profile.

Making profiles ^

A profile is a collection of constraints to apply to input data. The constraints essentially give an assortment of bricks a name and represent a business rule. The profile represents all of the business rules put together.

In data, the profile is a list of anonymous arrays. Each of the anonymous arrays specify three things:

A label

The label can be anything. It reminds you which profile element you're working with.

A constraint name

The constraint name refers to the method to use for that business rule. This has to be the name of an existing method (or a code reference returned by can()).

An anonymous setup hash

The last element is a hash of setup information for the bricks. This is the $setup variable seen in the examples.

Here's a simple profile.

        my @Profile = (
                #  label          #method name       #setup
                [ cat_and_dog  => check_cat_and_dog  => {}  ]
                );

To apply the profile, I pass it along with the input hash to apply:

        use Brick;
        
        my $Brick = Brick->new;
        
        my $profile = $Brick->profile_class->new( \@Profile );
        
        $Brick->apply( $profile, \%input );

Before I apply a profile, I might want to use lint to check it for errors. It's a class method since it hasn't created an object yet:

        $Brick->profile_class->lint( \@Profile );

I can dump the profile in a handy text format with explain to see if it does what I want:

        $profile->explain;

SOURCE AVAILABILITY ^

This source is part of a SourceForge project which always has the latest sources in SVN, as well as all of the previous releases.

        svn co https://brian-d-foy.svn.sourceforge.net/svnroot/brian-d-foy brian-d-foy

If, for some reason, I disappear from the world, one of the other members of the project can shepherd this module appropriately.

AUTHOR ^

brian d foy, <bdfoy@cpan.org>

COPYRIGHT ^

Copyright (c) 2007, brian d foy, All Rights Reserved.

You may redistribute this under the same terms as Perl itself.

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