# [Haskell-cafe] New New newbie question/help

Balu Raman braman09 at gmail.com
Wed Jun 27 11:37:13 EDT 2007

```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 <bf3 at telenet.be> 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 at haskell.org
> [mailto:haskell-cafe-bounces at haskell.org] On Behalf Of Balu Raman
> Sent: Wednesday, June 27, 2007 1:25 PM
> To: Haskell-Cafe at 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
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>
>
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