ADVANCED COMPOSITING & RETOUCHING - Chapter 6, Exercise 2
Understanding Prefix Notation in (gimp-image-flatten image)
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QUESTION:
Explain what (gimp-image-flatten image) means in terms of ordinary
function-call syntax, and why this notation style is called "prefix
notation."

SOLUTION / EXPLANATION:

In many mainstream languages, calling a function on an object is usually
written with the function name coming AFTER (or attached to) the object it
operates on - for example, something like image.flatten() in a
dot-notation, object-oriented style, where "image" comes first and the
operation "flatten" follows it.

Scheme (and Script-Fu, since it's built on Scheme) does the opposite: the
function name comes FIRST, immediately inside the opening parenthesis,
followed by whatever arguments it takes. So:

    (gimp-image-flatten image)

reads as: "call the function named gimp-image-flatten, passing image as its
single argument." The function name (gimp-image-flatten) is written before
its argument (image), both wrapped together in one set of parentheses. This
is exactly equivalent in meaning to the dot-notation image.flatten() from
other languages - it flattens the given image - but the ORDER the function
name and its argument appear in is reversed, and everything is wrapped in
parentheses rather than using a dot or a separate pair of call-parentheses
after the function name.

This is called "prefix notation" (also known as Polish notation) precisely
because the operator or function name is always placed BEFORE (as a
"prefix" to) its arguments, rather than after them or between them. This is
consistent throughout all of Scheme, not just for GIMP-specific functions -
even ordinary arithmetic is written this way, e.g. (+ 2 3) means "add 2 and
3," with the + operator itself written first, before the numbers it operates
on.

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WHY THIS WORKS AS AN ANSWER:
It translates the specific example into an equivalent, more familiar
dot-notation phrasing so the meaning is clear, then explains precisely why
the term "prefix notation" applies (the function name always precedes its
arguments), and generalizes the pattern beyond just this one GIMP-specific
example to show it's a consistent language-wide rule, not a one-off quirk.