Chapter 2 - Getting started in Modula-2
Our first Modula-2 program
We are ready to look at our first instructional program in Modula-2. Open the program file PUPPYDOG.MOD and display it on your screen. It is an example of the minimum Modula-2 program. There is nothing that can be left out of this program and still have a compileable, executable program.
The first word in the program, "MODULE", is the name that identifies a module, and it must be written as given here, in all capital letters. During the entire first part of this tutorial, we will use only this type of a module. There are other types but we will not look at any of them until we get to part III of this tutorial. Modula-2 requires us to name our module so we give it a name, "PuppyDog". We could have used any name that qualifies as an identifier but we have chosen a name that has nothing to do with computers as an illustration that any name could be used. In a practical program, you would probably use a name that was descriptive of the program in some way.
What is an identifier?
An identifier is a combination of letters and numbers that Modula-2 uses to identify a variable, program name, procedure name, and several other quantities. In Modula-2, an identifier is composed of any number of characters. The characters may be any mix of alphabetic and numeric characters, but the first character must be an alphabetic character. The case of the alphabetic character is significant such that "IdentNumber1", "IDENTNUMBER1", and "IdEnTnUmBeR1" are all different identifiers. No spaces or any other special characters are allowed.
Back to the program under consideration
The "header" line is terminated with a semicolon according to the formal definition of Modula-2. A semicolon is a statement separator and many will be used in large programs. Following the semicolon, we come to the program itself. The program statements are enclosed between the two words "BEGIN" and "END". In this case there are no statements, but if there were some, they would be placed between the two indicated words. Finally, the module name is repeated after the "END" and it is followed by a period. The module name is repeated in order to make the program easier to understand by clearly marking its limits. In this case it really doesn't add to the clarity of the program, but in a large program it can be of significant help. The period marks the end of the listing and can be thought of as the period that marks the end of a sentence.
The three words, MODULE, BEGIN, and END, are special words in Modula-2. They are "reserved words" because they are used for a specific purpose and cannot be used for any other purpose. They are not available for your use in any way except for the defined purpose. The reserved words in Modula-2 are always capitalized or the compiler will not consider them as reserved words. Remember that alphabetic characters must have the correct case in Modula-2. Some other languages, most notably Pascal, allow you to use either case anywhere and it converts them internally so that they are the same. It would be permissible for you to use words such as "Begin" or "End" as variables in a Modula-2 program, but it would be very poor programming practice and should be avoided. We will come across many other reserved words in these lessons. There are 40 reserved words in Modula-2.
You should have learned how to use your compiler by now so you can compile and run this program. It will do nothing, but that is significant in itself, because it should at least return to the operating system after it finishes doing nothing. That may sound a little silly, but it does take a considerable amount of effort to load, transfer control to the program, and set up linkage back to your Operating System.
It should be noted at this time that the Modula-2 compiler doesn't care about extra blanks or linefeeds and the careful programmer will insert extra blanks and linefeeds as desired in order to make the program easier to read. As you continue to program in Modula-2, you will no doubt develop a style of your own and hopefully your programs can be read easily by other programmers.
A program that does something
Load and display the program named WRITESM.MOD for an example of a Modula-2 program that does something. First you should notice that the elements of the first program are still here as they will be in every Modula-2 program. The same three reserved words are used here as before, but now there are some added statements.
The line near the beginning (line 3) that begins with the reserved word "FROM" is a special line that must be used in any program that accesses external procedures. We will not try to define this line at this time. We will only say that every external call in Modula-2 requires a definition of where to find the procedure. The module named "InOut" is a collection of input and output routines that are available for our use and this line in the program tells the system to look in the "InOut" collection for the procedures named "WriteLn" and WriteString". When the program needs these particular functions to do what we ask it to do, it knows where to find them. We will cover the IMPORT list in detail later in this tutorial. Until then, simply use the example programs as a guide when you wish to write a practice program.
Our first program statements
Between the BEGIN and END statements, which we defined previously as the place where the actual program is placed, we have a series of "WriteString" and WriteLn" statements. These statements are almost self explanatory, but we will say a few words about them anyway. Each line is a call to a "procedure" which is a very important feature of Modula-2. A "procedure" is an external servant that does a certain job for us in a well defined way. In the case of the "WriteString", it looks at the string of characters supplied to it and displays the string of characters on the monitor at the current cursor position. In the case of the "WriteLn" procedure, it serves us by moving the cursor down one line on the monitor and moving it to the left side of the screen.
The parentheses are required for the WriteString because it has data following it. The data within the parentheses is data supplied to our slave or helper. It gets the string of characters between the quotation marks or the apostrophes and displays the string on the monitor. You have a choice of delimiters so that you can output the delimiters themselves. If you desire to output a quotation mark to the monitor, use apostrophes for delimiters, and if you wish to output apostrophes, use quotation marks. If you wish to output both, break the line up and output it piecemeal as in the last example line.
This program should be very clear to you by now. First we tell the system where to get the procedures, then we list the procedures in the order required to produce the desired results. It should be apparent that the lines of the program between the reserved words BEGIN and END are simply executed in order. Compile and run the program and observe the output on your monitor. It should be mentioned at this point that it is possible to redirect the output to the printer or to a disk file but we will not be doing that for quite some time. We will stay with the basic syntax of Modula-2 for now.
No program is complete without a few comments embedded in the program as notes to the programmer describing the reasons for doing some particular thing. The notes are particularly helpful to another programmer who needs to modify the program some day. It is not necessary for the computer to understand the notes and in fact, you don't want the computer to try to understand the notes, so you tell the compiler to ignore the notes completely. How to do this is the object of our next program named MODCOMS.MOD which you should load and display on your monitor.
In Modula-2, comments are enclosed in pairs of double characters. The comment is started with the "(*", and ended with the "*)". The program on your monitor has several examples of comments in it. If the comments were completely removed, the program would be very similar to the last one but a lot shorter. Notice that comments can go nearly anywhere in a program, even before the header statement or after the ending period. Comments can be used to remove a section of program from consideration by the compiler so that a particularly troublesome section of code can be "commented out" until you solve some of the other problems in program debugging. It is important to remember that comments can be "nested" in Modula-2 so that a section of code can be "commented out" even if it contains other comments.
This particular program is not meant to be an example of good commenting. It is really a sloppy looking program that would need some work to put it into a good style, but it does illustrate where it is possible to put comments.
Good programming style
Load and display the program named GOODFORM.MOD for an example of a well formatted program. Since Modula-2 allows you to use extra spaces and blank lines freely, you should use them in any way you can to make your programs easy to understand, and therefore easy to debug and modify. Special care has been given to style in this program and it payed off in a very easy to understand program. Even with your very limited knowledge of Modula-2 programming you can very quickly decipher what it does. It is so well formatted that comments are not needed and they would probably detract from its readability. No further comment is needed or will be given. Compile and run this program to see if it does what you think it will do.
Really bad formatting
Load and display UGLYFORM.MOD for an excellent example of bad formatting. If you can see at a glance what this program does you deserve the Nobel Prize for understanding software if such a thing exists. The syntax for this program follows all of the rules of Modula-2 programming except for good style. Without saying anything else about this mess, I would suggest that you try to compile and run it. You may be surprised to find that it does compile and run, and in fact it is identical to the last program. Keep in mind that you can add extra blanks and linefeeds anyplace you desire in a program to improve its readability.
Hopefully, the last two programs will be an indication to you that good programming style is important and can be a tremendous aid in understanding what a program is supposed to do. You will develop your own programming style as time goes by. It is good for you to spend some effort in making your program look good, but don't get too excited about it yet. Initially, you should expend your effort in learning how to program in Modula-2 with reasonable style and strive to improve your style as you go along. It would be good for now if you simply tried to copy the style given in these lessons.