ok zoomer

app/Main.hs

@@ -31,11 +31,12 @@ toPicture figure@(Compound figures) = G.pictures $ map toPicture' figures ++ [ce
                 mirror XY = G.scale (-1) (-1)
 
                 color Positive = G.color G.blue
-                color Negative = G.color G.red
+                color Negative = G.color G.black
 
         centroid' = let dx = getX $ centroid figure
                         dy = getY $ centroid figure
-                    in G.translate dx dy . G.color G.red $ G.circleSolid 5
+                    in G.pictures [ G.translate dx dy . G.color G.red . G.scale 10 10 $ cross
+                                   , G.scale 1000 1000 . G.color G.white $ cross ]
 
           where getX (Location x _) = realToFrac x
                 getY (Location _ y) = realToFrac y
@@ -55,3 +56,7 @@ rectangle w h = G.polygon [(0, 0), (w, 0), (w, h), (0, h)]
 
 semicircle :: Float -> G.Picture
 semicircle r = G.arcSolid 0 180 r
+
+cross :: G.Picture
+cross = G.pictures $ [ G.line [ (-1, 0), (1, 0) ]
+                     , G.line [ (0, -1), (0, 1) ] ]

src/Lib.hs

@@ -70,7 +70,13 @@ instance Centroid Figure where
           yb' NS = yb
           yb' S = xb
 
-centroid :: Compound -> Location
-centroid (Compound figures) = Location (dist xb) (dist yb)
-  where dist f = (sum $ map (\figure -> area figure * f figure) figures)
-                 / (sum $ map area figures)
+dist f (Compound figures) = (sum $ map (\figure -> area figure * f figure) figures)
+                            / (sum $ map area figures)
+
+instance Centroid Compound where
+  area (Compound figures) = sum $ map area figures
+  xb = dist xb
+  yb = dist yb
+
+centroid :: Centroid a => a -> Location
+centroid figure = Location (xb figure) (yb figure)