[Haskell-cafe] Mis-understanding something in Haskell interpretation

Edward Ing inge at ihitacow.com
Tue Oct 3 13:44:27 EDT 2006


I am new to Haskell and am learning Haskell on my own with "The Haskell
School of Expression". Unfortunately there is no teacher that comes
along with the book. I am having a problem with loading an excerise.

I get this message from ghci on a :l Shapes.hs

    Couldn't match `Side' against `Int'
      Expected type: Side
      Inferred type: Int
    In the first argument of `sin', namely `angle'
    In the second argument of `(*)', namely `(sin angle)'
Failed, modules loaded: none.

The source is below. Side is types as Float. My assumption was that
Haskell would know how to convert the Int to a float and all would be
well. I am I mistaken somewhere? The problem is with the last line.

Tips would be appreciated.

Source Shapes.hs:

module Shapes where

data Shape	= Rectangle Side Side
		| Ellipse Radius Radius
		| RtTriangle Side Side
		| Polygon [Vertex]
	deriving Show

type Radius 	= Float
type Side	= Float
type Vertex	= (Float, Float)
type Angle	= Float

rectangle :: Shape -> Shape
rectangle (Rectangle width height )= Polygon [(0, 0),(0, height),
(width, height), (width, 0)]

rtTriangle :: Shape -> Shape
rtTriangle (RtTriangle width height) = Polygon [(0,0),(0,height),
(width, height)]

regularPolygon :: Int -> Side -> Shape

regularPolygon totalSides sideLength = 
	let initial = (0.0,0.0) in
		Polygon (initial : vertices  initial 1 totalSides sideLength )    

vertices :: Vertex -> Int -> Int -> Side -> [Vertex]
vertices _ 0 _ _  = []
vertices lastVertex currentSide totalSides length = 
	let currentVertex = vertex lastVertex currentSide totalSides length in
	 	currentVertex: vertices currentVertex (totalSides - (currentSide +
1)) totalSides length 

vertex :: Vertex -> Int -> Int -> Side -> Vertex
vertex (a ,b) currentSide totalSides length  =
	let  angle  = 1.0 * (360 / totalSides) *  currentSide  in
		( a  + ( length * (sin  angle)), b + ( (*) (cos angle) length ) )

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