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

[Edward Ing]

Hi,

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

Shapes.hs:40:40:
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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Re: [Haskell-cafe] Mis-understanding something in Haskell interpretation

[J. Garrett Morris]

On 10/3/06, Edward Ing [EMAIL PROTECTED] wrote:

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.


Yes - Haskell does not automatically promote numeric types.  In this
case, the following code compiles:

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

although I'm not sure it's exactly what you want.

/g
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Re: [Haskell-cafe] Mis-understanding something in Haskell interpretation

[Bulat Ziganshin]

Hello Edward,

Tuesday, October 3, 2006, 9:44:27 PM, you wrote:

 Couldn't match `Side' against `Int'
 In the first argument of `sin', namely `angle'

 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.

yes, Haskell don't make automatic conversions because together with
bi-directional type inferring it will make a headache.

 let  angle  = 1.0 * (360 / totalSides) *  currentSide  in

as Garret said, you should make conversions explicitly, hopefully the
'fromIntegral' function is enough in most cases


-- 
Best regards,
 Bulatmailto:[EMAIL PROTECTED]

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