Building a Distributed Spreadsheet in Modula-3

Mr. Kominek introduces us to the Modula-3 language and shows us how it can be used for cross-platform programming.
Step 1: Basic Construction

We need some underlying data structure for our spreadsheet, so let's begin simply by typing:

TYPE
  Grid: REF ARRAY OF ARRAY OF INTEGER;

or

TYPE
  Grid: REF ARRAY OF ARRAY OF Money.T;
This defines a two dimensional grid of integers (in the first line), or, as a second option, of type Money.T. Integers are a built-in type. Money.T is a programmer-defined type; the “.T” suffix is a Modula-3 convention. (In a real spreadsheet, each column would have a distinct user-defined type. Let that detail pass for now.)

A new grid can be allocated on the heap during variable declarations, if you wish, or during program execution.

VAR
  myGrid : Grid := NEW (Grid, rows, cols);
BEGIN
  myGrid := NEW (Grid, 100, 20);
END.

The second assignment of myGrid will wipe out the first, but don't be alarmed—we do not have a memory leak. The Modula-3 garbage collector takes care of reclaiming lost memory. This is also true of object variables (no destructors necessary), including objects that allocate memory on remote machines.

To flesh out our spreadsheet object, we next attach some operator methods to the grid. A good place for this is in a separate “interface” file. Listing 1 contains an initial cut at spreadsheet.i3. Our object is now declared to be a Spreadsheet.T type.

The important property of an interface is that it contains no executable code whatsoever. That's reserved for “.m3” or module files. The interface does not say how something is computed, merely what it does. This is similar to .h files in C, but is more strict. Only the operations explicitly exposed in an interface—or “exported” to use the jargon—are available for outside use.

(The sharp reader may have noticed that the representation of Grid is exposed in spreadsheet.i3—a bad thing. Modula-3 does allow you to hide details of representation inside implementation files. That would take us into a discussion of opaque types, however, a more advanced topic.)

Step 2: User Interface Design

Modula-3 comes with a multi-platform windowing system called Trestle. Built upon Trestle is a user interface toolkit called VBTkit, and a UI builder, FormsVBT. You may call X directly if you wish (alternatively, the Win32 GDI), but in doing so you lose portability.

A description of your program's user interface is called a “Trestle Form”. A form is a textual description of names and values, organized using nested parentheses. Form elements consist of windows, frames, buttons and so on, as well as properties such as color. Listing 2 is a sample form for a popup calculator, as shown in Figure 1.

The important point is that a form is defined in its own file, outside any Modula-3 code. This separation of concerns proves valuable when the user interface designer is a different person from the primary coder. The form does not describe how to construct the interface, merely what it looks like. The FormsVBT library builds it at run time and hooks it into your code.

Figure 1. Appearance of Calculator.fv

Step 3: Building the Program

Suppose our spreadsheet is implemented, along with a suite of test functions. To build a program, we must inform the compiler what source files comprise our executable. This is done in a Modula-3 make file, or m3makefile. An example is shown in Listing 3.

To build your program, at the command-line prompt type:

m3build

The compiler will determine dependency relations for you, recompiling only what is necessary.

Step 4: Objects to Network Objects

Converting a regular object (restricted to a single address space) to a network object (visible over the Net) is not as difficult as you might imagine. You must attend to four details.

First, the network object library needs to be linked in. This is performed in the m3makefile (Listing 3).

Second, make the following two changes to the spreadsheet interface:

IMPORT Money;
IMPORT NetObj;  (* new statement *)
  TYPE
    T = NetObj.T OBJECT  (* modified line *)
      grid: Grid;
      name: TEXT;
    METHODS
      ...

Third, and this matters only at execution time, a network object daemon needs to be running in the background. The program is supplied as part of Modula-3. Start the daemon by typing:

netobjd &
In a client-server architecture, the spreadsheet object resides with the server, yet it is the client that issues method calls (to update a cell, for example). Clients need to find out about each other. This is the fourth detail.

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