Thanks Paul.
Yes, I was missing a node in the polygon list.
I did change to equilateralTri :: Float -> Float -> Float -> IO() for
the scalings that you mentioned. Yes, i was doing the snowflake
problem.
thanks,
balu raman
On 6/27/07, Paul Hudak <[EMAIL PROTECTED]> wrote:
Hi Balu. It looks like you've gotten some excellent advice from
others, but permit me to add a further comment regarding the broader
context, now that I've had a chance to look a little closer.
It looks like you're trying to solve the "fractal snowflake" exercise. One
of the challenges in programming with numbers is deciding what
representation to use. Ints are great because they are efficient, but if
you need to use trigonometric functions such as sine, etc. then you need
Floats or Doubles. The problem here is that you need both -- you need Ints
because polygon is defined in terms of pixels, which are represented as
Ints, and you need Floats because you need to compute the coordinates of an
equilateral triangle, which (interestingly) can't be represented using
integer coordinates. But also, in the case of the snowflake fractal, you
will need to scale the size as you recurse. The reason that the latter is
important is that it implies that the arguments to equilateralTri should
perhaps be floats -- otherwise you will once again run into numeric
conversion problems as you try to scale the arguments (unless you always
start with a pixel size that is a multiple of six).
So -- I would still suggest using Window -> Float -> Float -> Float -> IO()
as the type for equilateralTri. It's only when you make the call to polygon
that you need Ints. And there you can just use "round" to convert the
Floats to Ints.
As an aside, looking at your code a bit closer, I see this:
(polygon [(x,y),(a,b),(x,y)]))
where
b = y + side * sin(pi/3)
a = x + side * cos(pi/3)
Something is not right here -- you repeat (x,y) as a vertex. Probably the
third vertex should be (x+side,y). Also, note that sin (pi/3) and cos
(pi/3) are constants (namely 0.866... and 0.5, resp.).
I hope this helps,
-Paul
Balu Raman wrote:
I am for ever obliged to this haskell community. Who would have thought that
Prof.Hudak would reply instantly, from on-the-road. I am reading his SOE.
Thanks so much.
I went with peterv's response after trying so many things.
I tried to change to : equilateralTri Window -> Float -> Float -> Float ->
IO()
which bombed because polygon wants list of integer-pairs.
I read the definitions of fromIntegral and round and they are defined as :
fromIntegral :: (Num b, Integral a) => a -> b
round :: (RealFrac a, Integral b) => a->b
Is it proper/ok to defines them as :
fromIntegral :: (a::Integral) -> (b::Num)
and
round :: (a::RealFrac) -> (b::Integral) ?
Is RealFrac is-a Num ?
Does the order matters in (Num b,Integral a) => a -> b or
(Integral a,Num b) => a -> b
With your encouragements, I'll keep pluuging. Thanks.
- br
On 6/27/07, peterv <[EMAIL PROTECTED]> wrote:
I'm also a haskell newbie, but I'll try to help; the experts here will
correct me if I'm wrong.
The compiler cannot in all cases infer the type of a number. pi can be a
Float, a Double, or even a complex number.
Furthermore unlike in C/C++ you cannot just mix integer and floating
operations.
For example, the following works for me:
f :: Int -> Int
f side = round ( (fromIntegral side) * sin ( (pi::Float) / 3 ) )
or easier
f side = round ( (fromIntegral side) * sin (pi / 3.0) )
I'm sure the experts here will have a better solution.
Peter
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Balu Raman
Sent: Wednesday, June 27, 2007 1:25 PM
To: [email protected]
Subject: [Haskell-cafe] New New newbie question/help
Hi,
Hope someone can help me, just starting out with SOE.My code :
module Main where
import Graphics.SOE.Gtk
spaceClose :: WIndow -> IO()
spaceClose w = do k <- getKey w
if k == ' ' then closeWindow w
else spaceClose w
equilateralTri :: Window -> Int -> Int -> Int -> IO()
equilateralTri w x y side
= drawInWindow w (withColor Red
(polygon
[(x,y),(a,b),(x,y)]))
where
b = y + side * sin(pi/3)
a = x + side * cos(pi/3)
main =
runGraphics(
do w <- openWindow "Equilateral
Triangle" (400,400)
equilateralTri w 50 300 200
spaceClose w
)
all of the above in file triangle.hs
when I do a :l triangle.h in ghci, I get the following error
triangle.hs:17:36:
No instance for (Floating Int)
arising from use of 'pi' at triangle.hs:17:36-37
Probable fix: add an instance declaration for (Floating Int)
In the first argument of '(/)', namely 'pi'
In the first argument of 'cos', namely '(pi / 3)'
In the second argument of '(*)', namely 'cos (pi/3)'
Failed, modules loaded: none
Can someone help me what's going on to a brand new newbie. All I can
figure out is that some type mismatch between float and int . I tried
various
combinations of lets and wheres and I still get the same complaints.
I am just linearly studying SOE
Thanks,
- br
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