2.3 Introduction to Programming

2.3.1 Interpreters

With an interpreter, the language comes as an environment, where you type in commands at a prompt and the environment executes them for you. For more complicated programs, you can type the commands into a file and get the interpreter to load the file and execute the commands in it. If anything goes wrong, many interpreters will drop you into a debugger to help you track down the problem.

The advantage of this is that you can see the results of your commands immediately, and mistakes can be corrected readily. The biggest disadvantage comes when you want to share your programs with someone. They must have the same interpreter, or you must have some way of giving it to them, and they need to understand how to use it. Also users may not appreciate being thrown into a debugger if they press the wrong key! From a performance point of view, interpreters can use up a lot of memory, and generally do not generate code as efficiently as compilers.

In my opinion, interpreted languages are the best way to start if you have not done any programming before. This kind of environment is typically found with languages like Lisp, Smalltalk, Perl and Basic. It could also be argued that the Unix shell (sh, csh) is itself an interpreter, and many people do in fact write shell ``scripts'' to help with various ``housekeeping'' tasks on their machine. Indeed, part of the original Unix philosophy was to provide lots of small utility programs that could be linked together in shell scripts to perform useful tasks.

2.3.2 Interpreters available with FreeBSD

Here is a list of interpreters that are available as FreeBSD packages, with a brief discussion of some of the more popular interpreted languages.

To get one of these packages, all you need to do is to click on the hotlink for the package, to download the package and then install the package by running:

    # pkg_add package name

as root. Obviously, you will need to have a fully functional FreeBSD 2.1.0 or later system for the package to work!

2.3.3 Compilers

Compilers are rather different. First of all, you write your code in a file (or files) using an editor. You then run the compiler and see if it accepts your program. If it did not compile, grit your teeth and go back to the editor; if it did compile and gave you a program, you can run it either at a shell command prompt or in a debugger to see if it works properly. [1]

Obviously, this is not quite as direct as using an interpreter. However it allows you to do a lot of things which are very difficult or even impossible with an interpreter, such as writing code which interacts closely with the operating system--or even writing your own operating system! It is also useful if you need to write very efficient code, as the compiler can take its time and optimise the code, which would not be acceptable in an interpreter. Moreover, distributing a program written for a compiler is usually more straightforward than one written for an interpreter--you can just give them a copy of the executable, assuming they have the same operating system as you.

Compiled languages include Pascal, C and C++. C and C++ are rather unforgiving languages, and best suited to more experienced programmers; Pascal, on the other hand, was designed as an educational language, and is quite a good language to start with. FreeBSD does not include Pascal support in the base system, but the GNU Pascal Compiler (gpc) is available in the ports collection as lang/gpc.

As the edit-compile-run-debug cycle is rather tedious when using separate programs, many commercial compiler makers have produced Integrated Development Environments (IDEs for short). FreeBSD does not include an IDE in the base system, but devel/kdevelop is available in the ports tree and many use Emacs for this purpose. Using Emacs as an IDE is discussed in Section 2.7.