Fix wrong NFData instance.

nh2 · nh2 · commit 31d51243bd7f · 2018-12-18T22:30:59.000+01:00
The code

    rnf _ = ()

is *always* wrong!

Any request to evaluate to normal form must at least evaluate to
weak head normal form.

Even worse in this case, the `ForeignPtr` inside the type is often created
by `unsafePerformIO`, and you'd _really_ expect that `rnf` enforces that.
diff --git a/opencv/src/OpenCV/Internal/Core/Types/Mat.hs b/opencv/src/OpenCV/Internal/Core/Types/Mat.hs
@@ -203,7 +203,7 @@ instance FreezeThaw (Mat shape channels depth) where
     unsafeThaw = pure . Mut
 
 instance NFData (Mat shape channels depth) where
-    rnf _ = ()
+    rnf (Mat !_fptr) = ()
 
 {- | Tests whether a 'Mat' is deserving of its type level attributes
 
