First Data Type Class Model

The good news is that I've sorted out some terminology since the last post (as promised!). The bad news is that I've clearly got some more studying to do before nailing this data type thing down the way I want. I suppose, though, it's good news that I've figured that much out. After my conversation with Andrew, I'm concerned that this subsystem could turn into a serious project of its own. So we will probably limp along with a miminal data typing system whilst the rest of the action language gets sorted out.

For now, I propose the following model be inserted into the datatype subsystem of the * UML metamodel.


If you don't have an image resizer plugin (firefox) the diagrams might be hard to read. I'm new to the blog - writely technology and haven't figured out the easiest way to insert readable model fragments. I will post the full pdf and BridgePoint files on the sourceforge site, though. Probably in a separate file package.

Here I just want to excerpt and explain what I have so far. This is only a model of data type structure, not data type operations. The model is best explained in two pieces: core and user data types. Then I will construct the Position data type to show how the model works.

Core Data Types

First let's examine the left side - the core data types.

Nothing mysterious here. I have superclassed the Real and Integer Numeric types because they each may specify units.

Note that the max/min values on Integer and Real are kept in the subclasses since the types are not the same. The real type also specifies a precision as a positive integer representing decimal places to the right.

The Enumerated data type consists of a set of Symbols, each of which has a Value.

Finally, String has a positive integer maximum length. Now let's flip over to the right side.

User Data Types

A Vector is simply a collection of one or more values (as determined by Vector.Scale) all specified by the same Datatype. Since the Datatype can be either User or Core we can set up a hierarchical structure. Since Vector.Scale can be 1 or more we can build an N-dimensional matrix at any level of the hierarchy. Let's review the examples mentioned in my earlier post.

First we'll work from the bottom up to define the Position data type. Since minutes and seconds are the same in both latitude and longitude, let's define those.

Minutes

Our specification for Minutes is:
Name: Minutes
Core Type: Integer
Min_value: 0
Max_value: 59
Units: minutes

So this gives us an instance each of: Integer, Numeric, Core Data Type and Data Type

Seconds

Using a more compact syntax, we specify:
Seconds: real[0..60] prec 2 units seconds

This gives us an instance each of: Real, Numeric, Core Data Type and Data Type

Degrees Latitude

Lat_Degrees: int[-90..90] units degrees

Degrees Longitude

Long_Degrees: int[-180..180] units degrees

Unlike thte earlier post I am using negative degrees to avoid defining an enumerated type for [N | S] and [W | E].
No particular reason - just lazy. Also the max Minutes is 59 while max Seconds is 60 (59.999...). Seems like it is better to say "60". Otherwise you end up redundantly specifying the precision with a sequence of trailing nine's.

Latitude and Longitude

Now we start creating User Data Types as we work our way up the hierarchy.

Name: Latitude
Vector: name Degrees, scale 1, value_datatype Lat_Degrees
Vector: name Minutes, scale 1, value_datatype Minutes
Vector: name Seconds, scale 1, value_datatype Seconds

Name: Longitude
Vector: name Degrees, scale 1, value_datatype Long_Degrees
Vector: name Minutes, scale 1, value_datatype Minutes
Vector: name Seconds, scale 1, value_datatype Seconds

Position

Name: Position

Vector: name Latitude, scale 1, value_datatype Latitude

Vector: name Longitude, scale 1, value_datatype Longitude

Matrix example

Now if, for some reason, we wanted a 5x10 matrix of Position data we could do this:

Name: Position_Column

Vector: name Position, scale 10, value_datatype Position

Name: Position_Matrix

Vector: name Position_Rows, scale 5, value_datatype Position_Column

Moving on

Okay, that's it for now. I might shelve the data type subsystem for a few days while I get some other tasks under control. Namely: Publish up a project roadmap, resume work on the model build script parser, sketch out key parts of the action language subsystem.

Stay tuned!

- Leon