On Aug 24, 2015, at 2:32 PM, John Carmack <jo...@oculus.com> wrote:
> The idea that you functionally compose images like this:
>
> (place-image image-1 x y
> (place-image image-2 x y
> (place-image image-3 x y)))
>
> Which draws image1 on top of image2 on top of image 3, which is backwards
> from the "painters order" that would draw image 3, then image 2, then image 1.
>
> This imperative, side-effect-ing code is a little less clear to a beginner
> with the OOP and DC concepts, but It better represents what actually happens,
> and it is much easier to modify the code without worrying about the nesting.
>
> (send dc draw-bitmap imag3 x y)
> (send dc draw-bitmap imag2 x y)
> (send dc draw-bitmap imag1 x y)
This is a serious wart, but I didn't expect that when I read "backwards."
In HtDP/ISL, I use
[1] You can develop s0 ... s3 at the top-level, when you like them you esc-k,
ctrl-y place them into a local:
(define (render.v1 w)
(local ((define s0 (empty-scene 100 100))
(define s1 (place-image (circle 30 'solid 'yellow) 5 20 s0))
(define s2 (place-image (rectangle 10 20 'solid 'blue) 90 90 s1))
(define s3 (place-image (triangle 100 'solid 'red) 90 10 s2)))
s3))
[2] Or you can use let* as someone else pointed out
(define (render.v2 w)
(let* ((s (empty-scene 100 100))
(s (place-image (circle 30 'solid 'yellow) 5 20 s))
(s (place-image (rectangle 10 20 'solid 'blue) 90 90 s))
(s (place-image (triangle 100 'solid 'red) 90 10 s)))
s))
[3} But my favorite is this, when i try to explain kids abstraction and the
advantage of having values around instead of effects:
(define (render.v3 w)
(foldr (lambda (x s) (apply place-image- s x)) background pieces))
[4] We do need a for/image so that #lang racket programmers can do even better.
;; --- somewhere else:
(define background (empty-scene 100 100))
(define pieces
`((,(circle 30 'solid 'yellow)
,(rectangle 10 20 'solid 'blue)
,(triangle 100 'solid 'red))))
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