Fix overflow

System/Random.hs

@@ -137,16 +137,16 @@ class RandomGen g where
   next :: g -> (Int, g)
   next g = (minR + fromIntegral w, g') where
     (minR, maxR) = genRange g
-    range = fromIntegral $ maxR - minR
-    -- https://hackage.haskell.org/package/ghc-prim-0.5.3/docs/GHC-Prim.html#g:1
-    -- GHC always implements Int using the primitive type Int#, whose size
-    -- equals the MachDeps.h constant WORD_SIZE_IN_BITS. [...] Currently GHC
-    -- itself has only 32-bit and 64-bit variants [...].
+    range = minR `unsignedDiff` maxR
 #if WORD_SIZE_IN_BITS == 32
     (w, g') = genWord32R range g
 #elif WORD_SIZE_IN_BITS == 64
     (w, g') = genWord64R range g
 #else
+-- https://hackage.haskell.org/package/ghc-prim-0.5.3/docs/GHC-Prim.html#g:1
+-- GHC always implements Int using the primitive type Int#, whose size equals
+-- the MachDeps.h constant WORD_SIZE_IN_BITS. [...] Currently GHC itself has
+-- only 32-bit and 64-bit variants [...].
 # error unsupported WORD_SIZE_IN_BITS
 #endif
 
@@ -199,6 +199,19 @@ class RandomGen g where
   -- generators.
   split    :: g -> (g, g)
 
+-- | x `unsignedDiff` y is the difference between x and y as an unsigned number.
+unsignedDiff :: Integral a => Int -> Int -> a
+unsignedDiff x y
+  | y < x                                      = unsignedDiff y x
+  | x < 0 && y < 0                             = x' - y'
+  | x < 0 {- && y >= 0 (implied) -}            = x' + y'
+  | otherwise {- x >= 0 && y >= 0 (implied) -} = y' - x'
+  where
+    -- 'fromIntegral' conversion to unsigned type preserves magnitude but not
+    -- sign so x' and y' are implicitly the absolute values of x and y,
+    -- respectively
+    x' = fromIntegral x
+    y' = fromIntegral y
 
 class Monad m => MonadRandom g m where
   type Seed g :: *