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This section as adapted from a tutorial written for the Gimp 1 User Manual by Mike Terry.

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 5)

Typing this in and hitting Return 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 Script-Fu Console window. Now for a word of caution....

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+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 initial concepts.