Water 5-Advanced Abstractions-Meta-field

Contract
Parameter key	Default value	Type
container	opt
Parameter kind	Default value	Type
Other unkeyed arguments	opt
Other keyed arguments	opt

Water Contract
<class field container=opt _other_keyed=opt=wob="_add_to_environment" _other_unkeyed=opt=wob=ekind.code="_add_to_environment" />

Background

Objects are made up of fields. Each field has a key and a value. The key and the value may be any object though usually the key is a string or an integer.

Meta-fields

Water has special machinery to attach "meta data" to fields by creating "meta fields". These meta fields are meant to hold data about a field, which is not necessarily about the VALUE of the field but has more to do about what kind of information may be in the field. the Water system has meta fields for "type" and "doc" to help users and system software know about potential values for fields. Developers may add their own meta fields using names appropriate to the domain of their application. Meta fields must be named by a string.

Syntax

Meta fields are actually just regular fields that have a key that is a string composed of the key of the field, the string "_f_" and the meta key itself. Example: weight_f_type You can create meta fields in a number of ways. The most common is when defining a method's parameter that has a type:

<class vehicle/>
<method vehicle.drive speed=65=integer> </method>

Makes a meta field in vehicle of speed_f_type whose value is integer.

vehicle.drive.speed_f_type

integer

You can also directly set a meta field via:

thing.<set foo_f_bar = 23/>
thing.foo_f_bar

You can avoid knowing the details of the metafield infix "_f_" by using the method field_key to make the key:

<field_key "foo" "bar"/>

"foo_f_bar"

thing.<set <field_key "foo" "bar"/> = 23/>
thing.<get <field_key "foo" "bar"/>/>

Restrictions

The meta_key itself, i.e. "type" must be a string and cannot start with underscore. You also can't have a meta_key of "key" , "value" , or "container" .

Field Objects

Most of the time the above knowledge is all you'll need to know about meta fields. But for more advanced work, you may find it convenient to think about a field as an actual object with keys of the meta-keys of the field. In order to make a field object, use the field class to make a field instance. Each field object must have a key and may have either a container or some other meta-key fields. Field Objects with a Container

thing.<set weight=48 weight_f_type=integer/>
thing.weight

thing.weight_f_type

integer

<set my_field = thing.<field "weight"/>/>
my_field.<get "type"/>

integer

my_field.<get "value"/>

48

(Note: "value" is the default meta_key. It stands for the regular field value.)

my_field.<set doc="how heavy it is"/>
my_field.<get "doc"/>

"how heavy it is"

thing.weight_f_doc

"how heavy it is"

When a field object has a container, it becomes a proxy for the field within its containing object. A field object with a container specifies a location as an object and the key within the object. This can be used to get, set and test if the field exists within the object. Implementation Field objects have special definitions for the methods: has , get , and set to achieve their behavior.