I definitely agree with Perrin. The most effective, usable system
is going to be a framework that allows you to glue together concise,
targeted bits of code. You can't beat the power of Perl for scripting
complicated interactions between little bits of data.
We use XML::Comma for managing
web-site user profiles, mailing list systems, content archives,
etc. We haven't done quite this kind of shopping cart, but it's a
pretty good fit.
In Comma, you write document definitions that specify data
structures, constrain the contents of the parts of those structures,
and govern how data is saved in filesystem- and database-backed
storage. There's a pretty extensive API for manipulating both
individial documents and collections of docs.
You always hand-write the definitions -- that's the core exercise
of figuring out basic data structures and behaviors. Writing defs
feels like a kind of OO mating ritual: XML on one side, perl on the
other, and between the two of them a Class is born.
Most of our systems are built to assume completely tools-generated
docs (for various definitions of "tools"). You can write docs by hand,
and for your shopping cart system you might want to. For instance,
creating a new "item" in the product catalog could be the job for a
decent programmer who understands something about how the system is
used but doesn't have much knowledge of the
internals/architecture/subtleties.
Anyway, I threw together a bit of Comma stuff showing how you
might structure your widgets & gizmos example. Whether or not you
want to look more at Comma, it might provide some more food for
thought. There are a few example "item" and "customer" docs, followed
by defs for "item", "customer" and "cart".
<item>
<name>widget</name>
<base_cost>250</base_cost>
</product>
<item>
<name>gizmo</name>
<base_cost>125</base_cost>
<cost_hook>
<apply_order>100</apply_order>
<!-- buy three widgets, get a gizmo free -->
<cod><![CDATA[
sub {
my ( $cart, $item, $customer, $cost ) = @_;
if ( scalar(@{$cart->elements('widget')}) > 2 ) {
return 0;
} else {
die;
} ]]></cod>
</cost_hook>
<!-- buy 10 widgets, get $30-off per -->
<cost_hook>
<apply_order>110</apply_order>
<cod><![CDATA[
sub {
my ( $cart, $item, $customer, $cost ) = @_;
if ( $cart->number_of('gizmo') > 10 ) {
return $cost - 30;
} else {
die;
}
} ]]></cod>
</cost_hook>
</item>
<item>
<name>service_agreement_3yr</name>
<base_cost>150_000</base_cost>
<!-- bought widgets within the last thirty days,
first year is free -->
<cost_hook>
<apply_order>100</apply_order>
<cod><![CDATA[
sub {
my ( $cart, $item, $customer, $cost ) = @_;
my $widgets_order = $cart->customer()->last_ordered('widget');
if ( $widgets_order and
$widgets_order->date() > time()-60*60*24*30 ) {
return 100_000;
} else {
die;
}
} ]]></cod>
</cost_hook>
</item>
<customer>
<email>foo@bar.com</email>
<cost_hook>
<!-- 3 orders or more discount -->
<apply_order>400</apply_order>
<cod><![CDATA[
Foo::Util::get_Std_3_Orders_hook()
]]></cod></cost_hook>
<cost_hook>
<!-- Joe belongs to the 50%-off club -->
<apply_order>300</apply_order>
<cod><![CDATA[ sub {
sub {
my ( $cart, $item, $customer, $cost ) = @_;
return $cost / 2;
}
} ]]></cod>
</cost_hook>
</customer>
#----
<DocumentDefinition>
<name>item</name>
<element>widget</element>
<element><name>base_cost</name></element>
<nested_element>
<name>cost_hook</name>
<element>apply_order</element>
<element>code</element>
</nested_element>
<plural>'cost_hook'</plural>
<store>
<name>main</name>
<base>items</base>
<location>Read_only_file:'extension','.item'</location>
</store>
</DocumentDefinition>
<DocumentDefinition>
<name>customer</name>
<element><name>email</name></element>
<nested_element>
<name>cost_hook</name>
<defname>item:cost_hook</defname>
</nested_element>
<plural>'cost_hook'</plural>
<method>
<name>num_orders</name>
<cod><![CDATA[
sub {
my $self = shift;
return XML::Comma::Def->read('order')
->get_index->('completed')->count
( where_clause => 'doc_id="' . $self->doc_id() . '"' );
]]></cod>
</method>
<method>
<name>last_ordered</name>
<cod><![CDATA[
sub {
my ( $self, $item ) = @_;
return XML::Comma::Def->read('order')
->get_index->('orders')->single
( where_clause => 'doc_id="' . $self->doc_id() . '"',
collection_spec => "items:$item",
order_by => "timestamp DESC" );
]]></cod>
</method>
<store>
<name>main</name>
<base>customers</base>
<location>Sequential_dir</location>
<index_on_store>main</index_on_store>
</store>
<index>
<name>main</name>
<field>email</field>
</index>
</DocumentDefinition>
<DocumentDefinition>
<name>order</name>
<element><name>customer_id</name></element>
<element><name>timestamp</name></element>
<element><name>total_paid</name></element>
<nested_element>
<name>item</name>
<element>item_name</element>
<element>item_quantity</element>
<element>price_paid</element>
</nested_element>
<plural>'item'</plural>
<method>
<name>number_of</name>
<cod><![CDATA[
sub {
my ( $self, $item ) = @_;
my $total_quantity;
map {
$total_quantity += $_->item_quantity()
if $_->item_name() eq $item;
} $self->elements('item')
]]></cod>
</method>
<method>
<name>make_totals</name>
<cod><![[CDATA[
sub {
my $self = shift;
my $total = 0;
my $customer = XML::Comma::Doc
->read('customer|main|'.$self->customer_id()) ||
die "oops, couldn't get customer doc";
foreach my $i ( $self->elements('item') ) {
my $this_total = $i->base_cost();
foreach $hook
( sort { $a->apply_order() <=> $b->apply_order() }
( $customer->elements('cost_hook'),
$i->elements('cost_hook') ) ) {
my $cost;
eval { $this_total = $hook->code()->( $cart, $ ) };
return $cost unless $@;
}
my $extended = $this_total * $i->item_quantity();
$i->price_paid ( $extended );
$total += $extended;
}
$this->total_paid ( $total );
return $total;
}
]]></cod>
<store>
<name>carts</name>
<base>carts</base>
<location>Derived_file: 'derive_from','doc_id',
'extension','.order'</location>
</store>
<store>
<name>orders</name>
<base>orders</base>
<location>Sequential_file</location>
<index_on_store>orders</index_on_store>
</store>
<index>
<name>orders</name>
<field>customer_id</field>
<field>timestamp</field>
<field>total_paid</field>
<collection>
<name>items</name>
<cod><![CDATA[
sub { return map { $_->item_name() } $_[0]->item() };
]]></cod>
</collection>
</index>
</DocumentDefinition>
(Comma uses the code keyword pretty extensively, which
conflicts with the monks-tag of the same name. I shortened
Comma's to "cod".)
