[plt-scheme] functional graphics

As I have been playing with graphics in Scheme I have been trying to
wrap my head around the concept of "functional graphics" -- a phrase
that originally seemed contradictory. Perhaps people here can help me
understand it better.

Usually in Scheme we try to emphasize the distinction between a value
and an effect. One aspect is that functions always return the same
value for same argument (that is, state of the system is theoretically
unchanged after the computation). A variation is that values are
separate from (possible) displayed representations: the value returned
for (+ 2 3) differs from the 5 on the screen.

Another aspect of this distinction is that, when we evaluate certain
kinds of expressions, there may be both a value and an effect, but we
can focus on "which is more important" to us for some particular case.
So, for (+2 3) -- if we have to choose, we tend to care more about the
fact that there is a returned value (and the fact that this can be an
argument to another function) than  about whether (how) that value is
presented onscreen. For typical imperative computer graphics, on the
other hand, we care more about the effect and less about whether there
are returned values (such as "done") after the command expression has
executed. (As far as I know, graphics commands in strongly imperative
languages don't even return values, so its not an option.)

Functional graphics seems to blur this last aspect of the distinction.
That is, we care about both the fact that compositions of image
expressions return values -- AND the fact that they have effects. So,
it is this compositional aspect of "functional graphics programming"
that seems to be the "functional" part.

Or is there some stronger interpretation of "functional graphics"? One
where there is even less emphasis on effects?

Oh, I'm thinking of examples like this: STACK as a procedure that
takes 2 images as arguments and a) displays image1 on top of image2,
and b) returns a value that can be used as an argument by another
invocation of STACK (or other image procedures).

(stack image1 image2) -> value and effect

(stack image1
         (stack image2 image3))

etc.

k