Use a proper API for default parameters

Defaults.hs

@@ -0,0 +1,6 @@
+module Defaults where
+
+
+-- |Used to create a common interface for default settings of data types.
+class Def a where
+    def :: a

Diagram.hs

@@ -1,18 +1,28 @@
 module Diagram where
 
+import Defaults
 import Diagrams.Prelude
 import Diagrams.Backend.Cairo
 import Meshparser
 import Util
 
 
+instance Def DiagProp where
+    def = defaultProp
+
+
 -- |Holds the properties for a Diagram, like thickness of 2d points etc.
 data DiagProp = MkProp {
   -- |Get the thickness of the dot.
-  getThickness :: Double
+  t :: Double
 }
 
 
+-- |The default properties of the Diagram.
+defaultProp :: DiagProp
+defaultProp = MkProp 2
+
+
 -- |Create the Diagram from the VTable.
 diagFromVTable :: DiagProp -> VTable -> Diagram Cairo R2
 diagFromVTable prop vt
@@ -22,7 +32,7 @@ diagFromVTable prop vt
      `atop` vrule 500 # centerY # moveTo (p2(-250, 0))
      `atop` square 550 # lwG 0.00 # bg white
         where dot           = (circle $
-                               getThickness prop :: Diagram Cairo R2) # fc black
+                               t prop :: Diagram Cairo R2) # fc black
               mkPoint (x,y) = p2 (x,y)
 
 -- |Create the Diagram from a String.

Gtk.hs

@@ -2,6 +2,7 @@ module Gtk where
 
 import Control.Monad.IO.Class
 import Control.Monad
+import Defaults
 import Diagram
 import Diagrams.Prelude
 import Diagrams.Backend.Cairo
@@ -126,7 +127,7 @@ drawDiag' fp da scale' =
       scaleVal   <- adjustmentGetValue adjustment
       let (_, r) = renderDia Cairo
                      (CairoOptions "" (Width 600) SVG False)
-                     (diagFromString (MkProp scaleVal) mesh)
+                     (diagFromString (def{t = scaleVal}) mesh)
       renderWithDrawable dw r
       return True
     else return False
@@ -138,6 +139,6 @@ saveDiag' fp =
   if cmpExt "obj" fp
     then do
       mesh <- readFile fp
-      renderCairo "out.svg" (Width 600) (diagFromString (MkProp 2) mesh)
+      renderCairo "out.svg" (Width 600) (diagFromString def mesh)
       return True
     else return False