Improve error handling

Checks the file extension before attempting to read the file.
Also allow empty argument list.

Gtk.hs

@@ -8,6 +8,7 @@ import Diagrams.Backend.Cairo.Internal
 import Graphics.UI.Gtk
 import Graphics.UI.Gtk.Windows.MessageDialog
 import System.Directory
+import Util
 
 
 -- |Handle the whole GTK gui.
@@ -126,12 +127,15 @@ showErrorDialog str = do
 
 -- |Draws a Diagram which is built from a given file to
 -- the gtk DrawingArea.
+-- Prints an error dialog if no valid mesh file is found.
 drawDiag' :: (WidgetClass widget, RangeClass scale)
           => FilePath
           -> widget
           -> scale
           -> IO ()
-drawDiag' fp da scale' = do
+drawDiag' fp da scale' =
+  case cmpExt "obj" fp of
+    True -> do
       mesh       <- readFile fp
       dw         <- widgetGetDrawWindow da
       adjustment <- rangeGetAdjustment scale'
@@ -140,10 +144,15 @@ drawDiag' fp da scale' = do
                      (CairoOptions "" (Width 600) SVG False)
                      (diagFromString (MkProp scaleVal) mesh)
       renderWithDrawable dw r
+    False -> showErrorDialog "No valid Mesh file!"
 
 
 -- |Saves a Diagram which is built from a given file as an SVG.
+-- Prints an error dialog if no valid mesh file is found.
 saveDiag' :: FilePath -> IO ()
-saveDiag' fp = do
+saveDiag' fp =
+  case cmpExt "obj" fp of
+    True -> do
       mesh <- readFile fp
       renderCairo "out.svg" (Width 600) (diagFromString (MkProp 2) mesh)
+    False -> showErrorDialog "No valid Mesh file!"

Main.hs

@@ -4,5 +4,7 @@ import System.Environment
 
 main :: IO ()
 main = do
-  [a] <- getArgs
-  startGUI a
+  a <- getArgs
+  case null a of
+    False -> startGUI (head a)
+    True  -> startGUI ""