In this training course, we'll introduce you to the fundamentals
of Scheme necessary to use Script-Fu, and then build a handy
script that you can add to your toolbox of scripts. The script
prompts the user for some text, then creates a new image sized
perfectly to the text. We will then enhance the script to allow
for a buffer of space around the text. We will conclude with a few
suggestions for ways to ramp up your knowledge of Script-Fu.
Getting Acquainted With Scheme
3.1.1. Let's Start Scheme'ing
The first thing to learn is that:
Every statement in Scheme is surrounded by parentheses ().
The second thing you need to know is that:
The function name/operator is always the first item in the
parentheses, and the rest of the items are parameters to the
However, not everything enclosed in parentheses is a function --
they can also be items in a list -- but we'll get to that
later. This notation is referred to as prefix notation, because
the function prefixes everything else. If you're familiar with
postfix notation, or own a calculator that uses Reverse Polish
Notation (such as most HP calculators), you should have no
problem adapting to formulating expressions in Scheme.
The third thing to understand is that:
Mathematical operators are also considered functions, and thus
are listed first when writing mathematical expressions.
This follows logically from the prefix notation that we just
3.1.2. Examples Of Prefix, Infix, And Postfix Notations
Here are some quick examples illustrating the differences
infix, and postfix
notations. We'll add a 1 and 3 together:
Prefix notation: + 1 3 (the way Scheme will want it)
Infix notation: 1 + 3 (the way we "normally" write it)
Postfix notation: 1 3 + (the way many HP calculators will
3.1.3. Practicing In Scheme
Now, let's practice what we have just learned. Start up Gimp,
if you have not already done so, and choose
-> -> . This will start
up the Script-Fu Console window, which allows us to work
interactively in Scheme. In a matter of moments, the Script-Fu
Console will appear:
3.1.4. The Script-Fu Console Window
At the bottom of this window is an entry-field entitled
Current Command. Here, we can test out
simple Scheme commands interactively. Let's start out easy,
and add some numbers:
(+ 3 5)
Typing this in and hitting Enter yields the
expected answer of 8 in the center window.
Now, what if we wanted to add more than one number? The "+"
function can take two or more arguments, so this is not a
(+ 3 5 6)
This also yields the expected answer of 14.
So far, so good -- we type in a Scheme statement and it's
executed immediately in the Script-Fu Console window. Now for
a word of caution....
3.1.5. Watch Out For Extra Parens
If you're like me, you're used to being able to use extra
parentheses whenever you want to -- like when you're typing a
complex mathematical equation and you want to separate the
parts by parentheses to make it clearer when you read it. In
Scheme, you have to be careful and not insert these extra
parentheses incorrectly. For example, say we wanted to add 3
to the result of adding 5 and 6 together:
3 + (5 + 6) + 7= ?
Knowing that the + operator can take a list of numbers to add,
you might be tempted to convert the above to the following:
(+ 3 (5 6) 7)
However, this is incorrect -- remember, every statement in
Scheme starts and ends with parens, so the Scheme interpreter
will think that you're trying to call a function named "5" in
the second group of parens, rather than summing those numbers
before adding them to 3.
The correct way to write the above statement would be:
(+ 3 (+ 5 6) 7)
3.1.6. Make Sure You Have The Proper Spacing, Too
If you are familiar with other programming languages, like
C/C++, Perl or Java, you know that you don't need white space
around mathematical operators to properly form an expression:
3+5, 3 +5, 3+ 5
These are all accepted by C/C++, Perl and Java
compilers. However, the same is not true for Scheme. You must
have a space after a mathematical operator (or any other
function name or operator) in Scheme for it to be correctly
interpreted by the Scheme interpreter.
Practice a bit with simple mathematical equations in the
Script-Fu Console until you're totally comfortable with these