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Introduction to Programming Ada

By Thomas Adam

Introduction

I'm quite old-fashioned when it comes to computers. I am one of these people whom prefers working at a command-line driven interface rather than a GUI. So it should not come as a shock to you that many of the programming languages I have experimented in are also based on textual input / output. Ada, along with Perl, Bash, Sed, Awk, C, etc is no such exception.

Over the years, there have been quite a few programming languages mentioned in the Linux Gazette. Ben Okopnik has done two very good tutorials on both Perl and Bash, and other people have contributed to describing other languages, such as: Smalltalk, C++, Python. Over the next couple of months, I shall be writing a series of articles to do with programming in Ada.

What is Ada?

Glad you asked :-) Originally Ada was a US governmental (DoD) developed programming language. The standard was originally known as Ada83, but this is now obsolete, as it was recently "overhauled" and re-born as Ada95. This is now the preferred standard and implementation of the Ada programming language.

In 1983, Ada was standardised by ANSI. Thus, it went through all the official motions and in that year, that first edition was released. Then four years later in 1987, ISO released an equivalent standard. At this time though, the idea of so called OOP (Object-Orientated Programming) was a concept that had not really been considered.

Ada however, was not designed by a committee. The original design was implemented by Jean Ichbiah, who won a language design competition. Then in 1995, Tucker Taft led a small group of developers and Ada95 was born. Unlike the previous version (Ada83), the implementation of Ada95 (or Ada9X, as it is sometimes known) underwent a public "benchmark" test; whereby testers of the language gave their feedback, and suggestions to make the syntactical and lexicographical layout more efficient.

The name Ada is attributed to a woman called Ada Loveless (1815-1852) who is considered to be the world's first programmer. Ada is used in all sorts of situations, and since it is a concurrent programming language, it is most commonly used in embedded systems. Ada has been used in some of the following:

Airplanes
Air Traffic Control systems
Financial Systems
Telecommunications Systems
Medical Devices

The list however is by no means exhaustive :-)

Ada Compilers

Unlike other scripting programming languages (Perl, Bash, Python, tcsh, etc), Ada like C is compiled rather than interpreted. This means that the person that is going to run the program does not need the interpreter installed to use it. Ada programs are therefore standalone from having any kind of Ada packages installed. [Unless you have used pragmas to interface with other languages, like C, in which case you might have libraries, but more on that later -- TA]

The Ada compiler that I recommend to you is called GNAT, which stands for: GNU NYU (New York University) Ada Translator. It is free (GNU license :-), and there is a wealth of information on it. It is based on the gcc compiler, which has had the Ada syntax bundled in with it.

It is available from the following website, which then has a link to the GNAT compiler:

www.gnuada.org/alt.html

A word of caution here. I recommend that you download a pre-compiled binary version of GNAT, and use the package alien if need be to convert to .DEB .RPM .TGZ, etc. The reason I say this, is because you will need an Ada version of gcc (often called gnatcc) to bootstrap the compiler. If this is the first time you are installing GNAT then the compilation from source code will not be possible.

That said, you can then go ahead and install the package once you have downloaded it. You'll find that with the RPM version of GNAT, that there should be one single RPM: "GNAT-3.13p-7.rpm" which will contain everything you need to start programming in this wonderful language :-).

Ada IDE's

Before we start our first program in Ada, I thought it would be good to make you aware of some of the IDE's (Integrated Development Environment). These are programs which help you to program in the specified language by offering features such as:

Syntax Highlightling
Compiler Support
Keyword Definitions
Pre-defined Templates

The two that I would recommend to you are:-

TIA (TIny Ada) -- a console based IDE written in Ada and is built around the use of GNAT

GRASP -- an X11 IDE which supports among other languages, Ada!

For all you EMACS fans, there is an extension called Glade which is installed as part of the main GNAT distribution. This is an EMACS extension which supports the GNAT compiler, syntax highlighting, etc. More information on that can be found at gnuada website

You do not have to use and IDE at all to be able to use GNAT. I actually don't bother, and instead use jed if I am at the console (although this does not yet support Ada syntax highlighting) and Nedit if I am in X11. This does support Ada syntax highlighting :-)

The Features of Ada

Ada95 has been greately enhanced over its predecesor Ada83. The biggest improvement has been object orientation, which many people will find useful. Some of the features that Ada has are:

Packages (Modules), related types, objects can all be defined
Packages and data types can be made generic
Data representation gives a means to support systems programming
Object-orientated programming is supported
Errors can be caught and delt with explicitly
Tasks (multiple paralell threads) can be created and used. This ability is inherent to Ada95 and is uncommon is most other langugaes
Interfaces to other languages (such as: C, Fortran, COLBOL) can be included in the language
Ada supports hard types of data, both for textual and mathematical Input/Output. As a result, Ada is well suited to writing and maintaining complex systems

In addition to the above, there are also:

Packages
Functions
Procedures
Task Units
Records
Arrays

And many more....

Hello World!

Now it's time to write our first ada program. In time-honoured tradition, we are going to start by writing a Hello World example. Open up a text editor, and type in the following:

  
  with text_io;
  use text_io;

  procedure hello_world is  

  begin

    put("Hello World!");

  end hello_world;

Easy, isn't it :-). Before we can run the program, we have to save it. But it has to be with the correct suffix. GNU/Linux doesn't require any suffix (file extension) as a rule, but it is essential when programming in Ada and using the GNAT compiler. A list of valid extensions are:

.ads - Ada package specification
.adb - Ada package body or program

When writing anything other than a package (which we won't be doing for some time yet -- I can assure you) :-) you should append a ".adb" extension to your filename. This is so that the compiler knows that the file it is compiling is a program and not a package specification!

So, save your file as hello_world.adb

Now we are ready to start to compile / build the program. This has to be done so that we can run it. You cannot run an Ada program until it has been compiled and built.

Change to the directory that you have just saved the file, and issue the command:

gnatmake hello_world.adb

This will compile -> link -> build your Ada code into a compiled program.

Now if you type in:

./hello_world

the response:

hello world!

is output to the screen, and the program exits.

You should also have noticed that as you issued the command, the following output was produced:

gnatgcc -c hello_world.adb
gnatbind -x hello_world.ali
gnatlink hello_world.ali

You could, if you wish, type each of the above commands in turn to both compile, bind and link your program (respectively). Luckily gnatmake provides a nice automation for this :-). If you now look in your directory, along with the main program, you'll find that GNAT has created other files too, namely:

hello_world.ali
hello_world.o

.ali files are GNAT link files that contain information about debugging and linking for the main program

.o files are object files which can be used in conjunction with the program-debugger: gdb.

In short, unless you plan to debug your program, you can delete these files.

Explanation: Hello World

In perl, you can issue a command such as: print("Hello"); and that can be the only line in your program (excluding the she-bang line), and it will run.

Ada however, has to be told exactly which packages it is to use before it can perform even the simplest of commands like echoing statements to the VDU. A package is a collection of functions and procedures that perform specific tasks. If you do not declare explicitly these at the start of the program, GNAT, when it comes to compile your program, will bomb out immediately.

Therefore, if we wish to read and write I/O (Input/Output) to a screen terminal, this has to be stated. All I/O functions are found within the package text_io, and the first two lines within our hello_world example are crucial....

 
  with text_io;
  use text_io;

The with statement in Ada indicates that we will be requiring the use of the named package, in this case text_io. If more than one package is required then this can be added, by separating each package name by a comma (,). When we have finished, we must append a semi-colon (;) to the end of the line, similiar to that of Perl. The with statement is a mandatory command that must ALWAYS be present at the start of your program in order for it to work.

The package text_io, as I have already stated allows I/O functions / procedures. This involves printing messages to the screen, allowing user input to be entered, etc. It is a package that is used in virtually every program you will ever write in Ada.

The use statement MUST be used only after the with statement has been made. It allows for unqualified references to be made to procedures and functions from other packages. Without the use of this statement, each procedure or function call must have the name of the package that it belongs to, followed by a period (full stop) preceeding it. For example, below is what the hello_world program would look like without the use statement.

with text_io;
  
  procedure hello_world is  

  begin

    text_io.put("Hello World!"); 

  end hello_world;

You can see how this has increased the amount of information that we have to type in, without the use of the use statement. When more than one package is used that might have the same procedure or function names, the compiler can usually tell to which package you are referring, based on the parameters passed to it.

The third line:

procedure hello_world is

declares that we are writing a new procedure with the name hello_world. The statement word is tells us that we are about to start the declarative section of the procedure, more on that later.

The keyword

begin

then tells us that we are going to start the executable part of the procedure -- i.e. where all the statements will appear and be executed, which in this case is:

put("Hello World!")

Which calls the procedure put from the package text_io to print the message Hello World! on the screen.

The last line:

end hello_world;

simply just ends the named procedure.

In short, the basic structure for an Ada program looks like the following:

with text_io;
use text_io;

procedure program_name is 

      [ declaritive part here ]

begin

      [ executable section here ]  

end program_name;

Also within the package text_io are commands such as:


put
put_line
get
get_line
new_line

Plus many others...

put does what we have already seen.
put_line does the same as put, except starts on a new line.
new_line is a command issued on its own, which starts a new line. If you use it, make sure that you put a semicolon at the end of it, like:

new_line;

In fact, that statement about the semicolon (;) goes for each command that you make in Ada.

Next month, we will be looking at:

text_io (inputting data via the get statement)
data types
simple arithmetic expressions
enumeration types

Exercises

Well, that is all for this month. I'm sorry if it seems like I'm not explaining enough things all in one go, but trying to explain anything more at this point, is I think overload. So, I am going to leave you with a few exercises for you to try.....

1. Print your name on the screen

2. Print your address on the screen, using only put and new_line

3. Repeat exercise 2, this time with put_line

If you submit them to me, I will print them in my next installment of this article!

As with all of my articles, if you have any questions, suggestions, rants or raves (hopefully not complaints :-) drop me a line!!

with text_io, ada.integer_text_io;
use text_io, ada.integer_text_io;

procedure happy_programming is

loop_number : integer :=0;

begin

  while loop_number /= 10 loop
    loop_number := loop_number + 1;
    put("Happy Programming in Ada");
    new_line;
  end loop;

end happy_programming;

Thomas Adam

My name is Thomas Adam. I am 18, and am currently studying for A-Levels (=university entrance exam). I live on a small farm, in the county of Dorset in England. I am a massive Linux enthusiast, and help with linux proxy issues while I am at school. I have been using Linux now for about six years. When not using Linux, I play the piano, and enjoy walking and cycling.