Hello,
One day, not long ago, slava fixed a bug in the golden-section demo. Like
modern day politicians that use a bill as a trojan horse to fast-track agenda
items, the bug fix was a payload to remove a variable and make everything use
the stack. :-) Looking back, it's kind of funny. Nice work senator Pestov. :-)
Now let me explain why the changes were bad from a design standpoint. I'd like
to follow an MVC model as much as possible in my code. golden-section.factor
has the following layout:
UTILITY WORDS
!!!!!!!!!!
MODEL
!!!!!!!!!!
SETUP
The MODEL portion is pure logic. The model portion is easily abstracted from
any drawing technology. I mistakenly claimed on the channel that the model
was free from openglisms. Actually, there are two calls to glColor4f. But no
matter what drawing technology the VIEW uses, you would have to set the color
in those places. So I could have an abstracted color setting word where
glColor4f is called.
This is the original model:
: phi ( -- phi ) 5 sqrt 1 + 2 / 1 - ;
: omega ( i -- omega ) phi * 2 * pi * ;
: x ( i -- x ) dup omega cos * 0.5 * ;
: y ( i -- y ) dup omega sin * 0.5 * ;
: center ( i -- point ) dup x swap y 2array ;
: radius ( i -- radius ) pi * 720 / sin 10 * ;
: color ( i -- color ) 360.0 / dup 0.25 1 4array ;
: rim ( i -- ) black first4 glColor4f dup center swap radius 1.5 * disk ;
: inner ( i -- ) dup color first4 glColor4f dup center swap radius disk ;
: dot ( i -- ) dup rim inner ;
: golden-section ( -- ) 720 [ dot ] each ;
Slava fixed the bug but changed the model:
: rim ( quadric i -- )
black first4 glColor4f dup radius 1.5 * swap center disk ;
: inner ( quadric i -- )
dup color first4 glColor4f dup radius swap center disk ;
: dot ( quadric i -- ) 2dup rim inner ;
: golden-section ( quadric -- ) 720 [ dot ] each-with ;
At least he was considerate in not indenting my code. ;-)
So now those four words accept a 'quadric' on the stack.
Quadric has no analogy in raster graphics libraries. Having it in the stack
effect of words that are part of the model makes the model incompatible with
possible backends such as Yuuki's web ui or pure xlib for examples.
I add a parameter to a function (or a word) whenever there's a piece of the
algorithm I'd like to change often. Quadric is not something I'd like to
change often. The OpenGL redbook advises that quadrics be reused when
possible. So I designed a 'disk' word for use in golden-section. The disk
word reuses a quadric that is stored in a 'quadric' variable. The design
matched the advice on how to use quadrics.
I really like how enthusiastic some of the Factor community is about doing
pure stack programming. I'm not kidding when I say this: I really really
really want folks to dig into the guts of Factory and the demos and rewrite
things in a way that only uses the stack. I think it's a good exercise. In
fact, I do this myself. For example, I rewrote cabal.factor using only the
stack and it came out pretty nice! In my experience, variable based code is
easier to develop and change. Once the design crystalizes, I usually try and
change things to only use a stack. I've rewriten parts of Factory this way.
Ed
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