On 27/06/07, Balu Raman
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)
Your problem lies in this section here. Let's look at the error message:
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
The problem comes from the calculations of 'a' and 'b'. The function sin doesn't return an Int value. It returns types within the type class Floating (annotated as below, for some unspecified 'a').
sin (pi/3) :: Floating a => a side :: Int
Since the type checker has one unknown type, a, and one known, Int, it tries to put the two together. Then it finds that Int is not an instance of the Floating class, so a /= Int. So it asks you to make one:
Probable fix: add an instance declaration for (Floating Int)
In this case, the advice is bad. There is no reasonable way of making a machine integer a member of the floating class. What you need to do instead is ensure that you're using a type that is a member of the Floating class - that is, convert from an Int before you start the calculation. The function fromIntegral should come in handy:
let n = 3 :: Int (fromIntegral n) * sin (pi/3) 2.598076211353316
Good luck! D.