Thursday, January 31, 2008

Scheme For Enlightenment Part Eight - Hello, World!

The book, Practical Common Lisp is said to be for people who want to do more than factorials. Hello, World! is the logical next step.

You've got a Linux machine. If your distribution doesn't have guile installed, perhaps this will help:

sudo apt-get install guile

Under Unix, scripts in script languages like the sh, csh, perl, awk, and guile can be executed by having a first line start with the name of the program that will run them. I'm not going to get into a discussion about what is a script, a program, an application, or whatever. Here, a scripting language is one where the source can be executed by some system from the command line, so the operating system needs to know what system will execute it. There is no other valuable distinction.

It turns out that there are many ways to get a script run by guile, but this is one of the easiest:

#!/usr/bin/guile -s
(display "Hello, World\n")

The pound bang and path to guile is reasonable enough. One must pass a command line option, -s to get guile to read the file and run it. The second line is a bit odd. In a departure from Scheme, Guile implements a multiple line comment syntax. It starts with pound bang, and ends with bang pound. Pound bang is not Scheme syntax. Scheme already has a comment to end of line - semicolon. So, Guile's authors decided on a multiple line comment concept. And it can get used for startup. The rest of the file is what you want to execute.

This script starts with a call to the function display, which writes it's argument to the standard output, which might be your terminal. Guile's display allows an embedded newline in the string, just as is done in C and other languages found in Unix environments. You may see this script written this way:

#!/usr/bin/guile -s
(display "Hello, World")

It does the same thing, but calls the newline function.

While the command line is important, there are also modules that allow GTK (graphical user interface) and CGI (web interface) applications written for Guile.

No comments: