TEST/Shape.hs at master · quantumman/TEST · GitHub

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-- file: Haskell/Graphic/Show.hs

module Shape ( Shape (Rectangle, Ellipse, RtTriangle, Polygon)
             , Radius
             , Side
             , Vertex
             , square
             , circle
             , distBetween
             , area ) 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)


square s = Rectangle s s

circle r = Ellipse r r

area :: Shape ->Float
area (Rectangle s1 s2)  = s1*s2
area (RtTriangle s1 s2) = s1*s2/2
area (Ellipse r1 r2)    = pi*r1*r2

area (Polygon (v1:vs))
    = foldl (\a (v2,v3) -> a + triArea v1 v2 v3) 0.0 list
    where list = zip (tail vs) vs

triArea :: Vertex -> Vertex -> Vertex -> Float
triArea v1 v2 v3 = let a = distBetween v1 v2
                       b = distBetween v2 v3
                       c = distBetween v3 v1
                       s = 0.5 * (a+b+c)
                   in sqrt(s*(s-a)*(s-b)*(s-c))

distBetween :: Vertex -> Vertex -> Float
distBetween (x1, y1) (x2, y2) = sqrt((x1-x2)^2+(y1-y2)^2)