Speed up the Eq instance for TArray

We don't actually have to check all the elements to see that
two `TArray`s are equal.

Control/Concurrent/STM/TArray.hs

@@ -41,7 +41,18 @@ import Control.Sequential.STM (STM)
 -- but it may be replaced by a more efficient implementation in the future
 -- (the interface will remain the same, however).
 --
-newtype TArray i e = TArray (Array i (TVar e)) deriving (Eq, Typeable)
+newtype TArray i e = TArray (Array i (TVar e)) deriving (Typeable)
+
+-- There are no provisions for moving/copying TVars between TArrays.
+-- Therefore, two TArrays are equal if and only if they are actually
+-- the same array in memory. We have no safe operations for checking that
+-- directly (though in practice we could use `unsafeCoerce#` with
+-- `sameMutableArray#`). So instead we take a quick look at the array
+-- sizes and then decide based on the first TVar of each.
+instance Eq (TArray i e) where
+  TArray t1 == TArray t2
+    = numElements t1 == numElements t2
+    && (numElements t1 == 0 || unsafeAt t1 0 == unsafeAt t2 0)
 
 instance MArray TArray e STM where
     getBounds (TArray a) = return (bounds a)