Oberon Modules and Procedures

By R. S. Doiel, 2020-04-12

This is the second post in the Mostly Oberon series. Mostly Oberon documents my exploration of the Oberon Language, Oberon System and the various rabbit wholes I inevitably fell into.

Modules

The module is a primary code unit of Oberon language. Modules allow you to focus on functional units of code and can be readily composed into larger solutions. A module's name should match the filename you are saving it under. A module starts with declaring it's name and ends the declaration with a semicolon the statement separator in Oberon. Our simple "Hello World" example shows the basic code shape.

          MODULE HelloWorld;
            IMPORT Out;
          BEGIN
            Out.String("Hello World!"); Out.Ln;
          END HelloWorld.
      
      

Modules end with a END followed by the module's name and a period. Any text following the END statement is ignored by the compiler. This turns out to be very useful as a place to write up ideas about the code you're working on. You can also write any additional special instructions there (e.g. document usage). You can even use it as a scratch pad knowing that the compiler will ignore it.

Here's an example

          MODULE HelloWorld;
            IMPORT Out;
          BEGIN
            Out.String("Hello World!"); Out.Ln;
          END HelloWorld.
      
          This program isn't very useful. It has no interactive ability.
          It'd be nice if it could be more specific about who it was saying
          hello to.
      
      

For a module to be really useful you want to have the capability of including both private and public code. Public code allows us to reuse our code in other modules while the private code keeps internal things inside the module safe from colliding with other modules private code. This technique is classically known as "information hiding" and in computer sciences texts as "scope". Lets create a a more composable module called SayingHi.Mod. In addition to display "Hello World!" we want a public method (procedure in Oberon terminology) that can ask for a name and print out a salutation. We will use the SayingHi.Mod module along with a newer version of HelloWorld.Mod named HelloWorld2.Mod.

Procedures

How do we write methods in Oberon? Methods are declared using the keyword PROCEDURE followed by their name, a declaration of any parameters and if the procedure returns a value (i.e. is a function) it also includes that declaration. Next we declare any internal variables needed by the procedure. This is followed by the procedure's body. The body of the procedure is defined by a BEGIN and END statement structure. The body contains the steps the procedure needs to execute.

We'll create a procedure called "HelloWorld" in our new module. Since we will use this procedure from our new HelloWorld2.Mod our new "HelloWorld" procedure needs to be public. A public procedure in SayingHi.Mod is available for use in our new HelloWorld2.Mod (or by another module). Marking a procedure public in Oberon is a little different than in other languages. A Module's procedure is public if its name ends with an asterisk. Below is a sketch of our module SayingHi.Mod so far.

NOTE: This technique is also used to mark variables, records and constants as public and available to other modules. Public variables are "read only" in other modules.

          MODULE SayingHi;
            IMPORT Out;
          
            PROCEDURE HelloWorld*;
            BEGIN
              Out.String("Hello World!"); Out.Ln;
            END HelloWorld;
          END SayingHi.
      
      

This modules looks allot like HelloWorld.Mod with a couple key differences. Rather than relying on the module's begin and end statements we declare a procedure with its own begin and end statements. Notice the procedures end statement includes the procedure name and is terminated by semicolon rather than a period. Like HelloWorld.Mod we import the Out module to display our greeting.

Putting it all together

Let's create a new "Hello World" module called HelloWorld2.Mod and use our SayingHi module instead of directly importing Out.

          MODULE HelloWorld2;
            IMPORT SayingHi;
          BEGIN
            SayingHi.HelloWorld;
          END HelloWorld2.
      
      

We can compile our module with OBNC using the command

          obnc HelloWorld2.Mod
      
      

We can run our new "Hello World" with the command

          ./HelloWorld2
      
      

At this point we have a way of saying "Hello World!" whenever we need in our Oberon programs. But just printing "Hello World!" to the screen isn't very interactive. It'd be nice if we could have the computer ask our name and then respond with a greeting.

We'll modify our SayingHi to include a new procedure called "Greetings" and that procedure needs to ask us our name and then display an appropriate greeting. "Greetings" will be a public procedure marked by an asterisk like "HelloWorld".

"Greetings" has three tasks

1. Ask politely for our name
2. Get the name typed in with our keyboard
3. Assemble and display a polite greeting

To keep our "Greeting" procedure short we'll split this up into some private procedures. These will not be available outside SayingHi.Mod. Here's a sketch of our improved module.

          MODULE SayingHi;
            IMPORT In, Out;
          
            PROCEDURE HelloWorld*;
            BEGIN
              Out.String("Hello World!"); Out.Ln;
            END HelloWorld;
          
            PROCEDURE AskOurName;
            BEGIN
              Out.String("Excuse me, may I ask your name? ");
            END AskOurName;
          
            PROCEDURE GetName(VAR ourName : ARRAY OF CHAR);
            BEGIN
              In.Line(ourName);
            END GetName;
          
            PROCEDURE AssembleGreeting(ourName : ARRAY OF CHAR);
            BEGIN
              Out.String("Hello ");Out.String(ourName);
              Out.String (", very nice to meeting you."); Out.Ln;
            END AssembleGreeting;
          
            PROCEDURE Greetings*;
              VAR ourName : ARRAY 256 OF CHAR;
            BEGIN
              AskOurName;
              GetName(ourName);
              AssembleGreeting(ourName);
            END Greetings;
          END SayingHi.
      
      

Now let's add our Greetings procedure to HelloWorld2.Mod.

          MODULE HelloWorld2;
            IMPORT SayingHi;
          BEGIN
            SayingHi.HelloWorld;
            SayingHi.Greetings;
          END HelloWorld2.
      
      

We compile and run it the same way as before

          obnc HelloWorld2
          ./HelloWorld2
      
      

When you run HelloWorld2 you should now see something like (I've answered "Robert" and pressed return after the second line.

         Hello World!
         Excuse me, may I ask your name? Robert
         Hello Robert, very nice to meeting you.
      
      

Reading our code

While our revised modules are still short they actually exercise a number of language features. Let's walk through the code block by block and see what is going.

HelloWorld2.Mod is responsible for the general management of our program namely say "Hello World!" and also for initiating and responding with a more personal greeting. It does this by first importing our SayingHi.Mod module.

          IMPORT SayingHi;
      
      

HelloWorld2.Mod doesn't have any of its own procedures and like our original HelloWorld.Mod relies on the module's initialization block to run our two public procedures from SayingHi. It calls first SayingHi.HelloWorld; then SayingHi.Greetings' before existing. Other than using the SayingHi module it is similar in spirit to our first HelloWorld.Mod.

Our second module SayingHi.Mod does the heavy lifting. It contains both public and private procedures. If you tried to use GetName from SayingHi in HelloWorld2.Mod you would get a compiler error. As far as HelloWorld2.Mod is concerned GetName does not exist. This is called information hiding and is an important capability provided by Oberon's Modules system.

explore SayingHi more deeply

In SayingHi.Mod we introduce two important concepts.

1. Public and Private procedures
2. variables to hold user input

SayingHi.Mod imports two module, In which is for getting text input from the keyboard, and Out which is used for displaying text to standard output.

          IMPORT In, Out;
      
      

In and Out are to modules you will commonly use to either receive input (In) from the keyboard or display output (Out) to the terminal or shell. They provide simple methods for working with variables and constants and built-in Oberon data types. This is a very useful as it lets us focus our procedures on operating on data rather than the low level steps needed to interact with the operating system and hardware.

NOTE: basic types, Oberon has a number of basic types, BYTE holds a byte as a series of bit, CHAR holds a single ASCII character, INTEGER holds a signed integer value, REAL holds a floating point number and BOOLEAN holds a True/False value.

The first procedure is HelloWorld and it's pretty straight forward. It displays a "Hello World!" message in our terminal. It uses Out.Out.String to display the "Hello World!" and Out.Ln to force a new line. Out.String is responsible for displaying values that are of typeARRAY OF CHAR. This includes text we provided in double quotes.

          PROCEDURE HelloWorld*;
          BEGIN
            Out.String("Hello World!"); Out.Ln;
          END HelloWorld;
      
      

The notable thing about HelloWorld* is its annotation *. This asterisk indicates to the compiler that this is a public procedure and should be made available to other modules. Procedures, variables, constants, records (data structures) can be made public with this simple annotation. If we left off the * then we would not be able to use HelloWorld procedure from other module.

Our second procedure is AskOurName. It's private because it lacks the *. It is invisible to HelloWorld2.Mod. It is visible withinSayingHi module and we'll use it later in Greetings*. Before a procedure, variable, constant or record can be used it must be declared. That is why we most define AskOurName before we defineGreetings*. AskOurName is in other respects very similar to HelloWorld*.

          PROCEDURE AskOurName;
          BEGIN
            Out.String("Excuse me, may I ask your name? ");
          END AskOurName;
      
      

Our third procedure GetName is a little more interesting. It demonstrates several features of the Oberon language. Most obvious is that it is the first procedure which contains a parameter list.

          PROCEDURE GetName(VAR ourName: ARRAY OF CHAR);
      
      

There is allot packed in this single statement in addition to putting a name to our procedure. Specifically it uses a VAR in the parameter. Oberon provides two kinds of parameters in declaring procedures. The two are VAR and static. A VAR parameter means that the procedure is allowed to up date the value in the memory location indicated by the name. A static variable (a parameter without the VAR prefix passes in a read only value. This allows us to distinguish between those procedures and variables where that can be modified by the procedure and those which will be left the same. Inside of GetName we call the In module using the Line. This retrieves a line of text (a sequence of keyboard strokes ended with the return key).

          In.Line(ourName);
      
      

Because ourName was a variable parameter in GetName it can be modified by In.Line.

Our next procedure AssembleGreeting is private likeAskOurName and GetName. Like HelloWorld* and AskOurName it makes use of the Out module to display content. Unlike HelloWorld* it has a parameter but this time a static one. Notice the missing VAR. This indicates thatAssembleGreeting doesn't modify, cannot modify ourName.

          PROCEDURE AssembleGreeting(ourName : ARRAY OF CHAR);
          BEGIN
            Out.String("Hello ");Out.String(ourName);
            Out.String (", very nice to meeting you."); Out.Ln;
          END AssembleGreeting;
      
      

The use of Out.String is more elaborate then before. Notice how we use trailing spaces to make the output more readable.

Our final procedure is public, Greetings*. It does not have any parameters. Importantly it does include a variable for use inside the procedure called ourName. The VAR line declares ourName as an ARRAY 256 OF CHAR. This declaration tells the compiler to allocate memory for storing ourName while Greetings* is being executed. The declaration tells us three things. First the storage is continuous block of memory, that is what ARRAY means. The second is the size of this memory block is 256 CHAR long and the that we will be storing CHAR values in it.

The memory for ourName will be populated when we pass the variable to GetName based on what we type at the keyboard. If we type more than 256 ASCII characters they will be ignored. After GetName records the typed character we use the memory associated with the ourName variable we read that memory to display what we typed in the procedure named AssembleGreeting.

Going a little deeper

Oberon is a typed language meaning that variables are declared, allocated and checked during compile time for specific characteristics. The one variable we created ourName in the Greetings procedure reserves the space for 256 ASCII characters. In Oberon we call a single ASCII character a CHAR. Since it would be useful to work with more than one CHAR in relationship to others Oberon also supports a variable type called ARRAY. An ARRAY is represented as a block of memory that is allocated by the Oberon run time. Because it is allocated ahead of time we need to know its size (i.e. how many CHAR are we storing). In our case we have declared ARRAY 256 OF CHAR. That means we can hold names up to 256 ASCII characters.

Greetings* does three things and the second thing, GetName receives the characters typed at the keyboard. GetName has a parameter list. In this case the only one parameter is declaredVAR ourName : ARRAY OF CHAR. Notice the similarity and difference between the VAR statement in Greetings versions the parameter list. Our GetName can accept any length ofARRAY OF CHAR and it only can accept an ARRAY OF CHAR. If you try to pass another type of variable to GetName the compiler will stop with an error message.

Why is this important?

We've minimized the memory we've used in our program. Memory is typically allocated on the stack (a block of memory made available by the operating system to the program). We've told the operating system we need 256 CHAR worth of consecutive memory locations when we allocated room the variable ourName in Greetings. When we invoke GetName Oberon knows to use that same memory location for the value of ourName defined in the parameter. In turn when In.String(ourName); is called the module In knows to store the name typed on the keyboard in that location of memory. When Out.String(outName); is called the compiler knows to use the same location of memory to send the contents to the display. When we finally finish the Greetings* procedure the memory is released back to the operating system for re-use by this or other programs.

What we've explored

1. Using a module to break down a simple problem
2. Using a module's ability to have public and private procedures
3. Touched on how memory is used in a simple interactive program

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