On 27/06/07, Paul Hudak wrote:

I think that an easier solution is to just change the type of equilateralTri to:

equilateralTri :: Window -> Float -> Float -> Float -> IO()

But I am on the road and wasn't able to actually run the code.

I don't have my copy of SOE to hand, so I didn't want to say that in case it subverted the point of the exercise. But of course I will defer to authority on this matter ;-) D.

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 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: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Balu Raman Sent: Wednesday, June 27, 2007 1:25 PM To: Haskell-Cafe@haskell.org 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 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

Newbie helping newbie, cool J And indeed, this is an amazing mailing list! Personally, I prefer to read fromIntegral :: (Num b, Integral a) => a -> b like IF (b is a Num) AND (a is an Integral) THEN (fromIntegral is defined and is a function from a to b) This way it resembles the mathematical symbol for implication (=>) PS: Haskells "generic number system" can be very confusing for the beginner, but it becomes very cool when you start working with type classes. You will see that in the later chapters of the great SOE book (animation and reactive behaviors). The reactive behavior chapter is really hard, but don't give up. In my case I got a real revelation, finally understanding the real power of streams and lazy evaluation; it really changes the way you look at the "world". As a videogames developer, I still have a lot of unanswered questions though (for example, how to efficiently handle events between behaviors, like collision, but I hope to find that in Yampa or newer work) From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Balu Raman Sent: Wednesday, June 27, 2007 5:37 PM To: Haskell-Cafe@haskell.org Subject: Re: [Haskell-cafe] New New newbie question/help 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 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: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Balu Raman Sent: Wednesday, June 27, 2007 1:25 PM To: Haskell-Cafe@haskell.org 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 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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 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 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: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Balu Raman Sent: Wednesday, June 27, 2007 1:25 PM To: Haskell-Cafe@haskell.org 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