index buffer failing

src/Main.hs

@@ -67,17 +67,21 @@ main = do
   bo <- GL.genObjectName
   GL.bindVertexArrayObject $= Just bo
 
-  -- create and bind buffer for a triangle
+      -- define vertices (positions of the triangle corners) as List of Floats
+  let vertexPositions =
+        [ (-0.5), (-0.5) -- 0
+        , 0.5   , (-0.5) -- 1
+        , 0.5   , 0.5    -- 2
+        , (-0.5), 0.5    -- 3
+        ] :: [Float]
+
+      -- create draw order indices
+      indices = [0, 1, 2, 2, 3, 0] :: [Word]
+
+  -- create and bind buffer for vertices
   buf <- GL.genObjectName
   GL.bindBuffer GL.ArrayBuffer $= Just buf
 
-  -- define vertices (positions of the triangle corners) as List of Floats
-  let vertexPositions =
-        [ (-0.5), (-0.5)
-        , 0     , 0.5
-        , 0.5   , (-0.5)
-        ] :: [Float]
-
   -- put vertices into the buffer
     -- turn the list into a pointer
   withArray vertexPositions $ \ptr ->
@@ -106,6 +110,23 @@ main = do
     )
   GL.vertexAttribArray (GL.AttribLocation 0) $= GL.Enabled
 
+  -- create and bind buffer index buffer
+  ibo <- GL.genObjectName
+  GL.bindBuffer GL.ElementArrayBuffer $= Just ibo
+
+  -- put indices into index buffer
+    -- turn the list into a pointer
+  withArray indices $ \ptr -> do
+    -- Feed the data to the buffer
+    GL.bufferData GL.ElementArrayBuffer $=
+      -- how much bytes of memory we are going to write (as an Int32)
+      ( fromIntegral $ length indices * sizeOf (undefined :: Word)
+      -- The pointer to the data
+      , ptr
+      -- The data's usage
+      , GL.StaticDraw
+      )
+
   -- -- SHADERS
 
   -- read in shaders from source file
@@ -114,6 +135,8 @@ main = do
 
   sp <- createShaderProgram vertSrc fragSrc
   GL.currentProgram $= Just sp
+  err <- get GL.errors
+  print $ "pre-loop errors: " <> show err
 
   -- -- EVENTING AND DRAWING
 
@@ -138,7 +161,10 @@ main = do
       GL.clear [GL.ColorBuffer]
       
       -- tha actual drawing happens here
-      GL.drawArrays GL.Triangles 0 3
+      void $ get GL.errors
+      GL.drawElements GL.Triangles 6 GL.UnsignedInt nullPtr
+      err <- get GL.errors
+      when (not $ null err) (print $ "loop errors: " <> show err)
 
       -- make GL finish things up
       -- GL.flush