In this training course, we'll introduce you to the fundamentals of Scheme
necessary to use ScriptFu, 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
ScriptFu.
3.1.
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
function.
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
mentioned.
3.1.2.
Examples Of Prefix, Infix, And Postfix Notations
Here are some quick examples illustrating the differences between
prefix, 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 want it)
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 ScriptFu Console window, which allows us to work
interactively in Scheme. In a matter of moments, the ScriptFu
Console will appear:
3.1.4.
The ScriptFu Console Window
At the bottom of this window is an entryfield 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 problem:
(+ 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 ScriptFu Console window. Now for a word of
caution....
3.1.5.
Watch Out For Extra Parentheses
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 ScriptFu
Console until you're totally comfortable with these initial concepts.