Finish up StatefulGen implementation

src/Test/QuickCheck.hs

@@ -128,6 +128,9 @@ module Test.QuickCheck
 
     -- * The 'Gen' monad: combinators for building random generators
   , Gen
+#ifndef OLD_RANDOM
+  , QC(..)
+#endif
     -- ** Generator combinators
   , choose
   , chooseInt

src/Test/QuickCheck/Gen.hs

@@ -1,4 +1,5 @@
 {-# LANGUAGE CPP #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
 #ifndef NO_ST_MONAD
 {-# LANGUAGE Rank2Types #-}
 #endif
@@ -53,6 +54,9 @@ import GHC.Stack
 #define WITHCALLSTACK(ty) ty
 #endif
 
+#ifndef OLD_RANDOM
+import System.Random.Stateful
+#endif
 --------------------------------------------------------------------------
 -- ** Generator type
 
@@ -102,6 +106,34 @@ instance MonadFix Gen where
       let a = unGen (f a) r n
       in a
 
+#ifndef OLD_RANDOM
+-- | A monadic adapter that can be passed to stateful generators to make them 
+-- use @Gen@ as the generator monad.
+--
+-- The stateful generator interface can be used as follows
+-- > -- 1. Define a stateful generator
+-- > myGen :: StatefulGen g m => g -> m (Int, String)
+-- > myGen g = do
+-- >   l <- uniformRM (0, 5) g
+-- >   s <- replicateM l (uniformM g)
+-- >   pure (l, s)
+-- >
+-- > -- 2. Pass QC to the stateful generator to turn it into a QuickCheck generator
+-- > myQCGen :: Gen (Int, String)
+-- > myQCGen = myGen QC
+data QC = QC
+
+instance StatefulGen QC Gen where
+  uniformWord32 QC = MkGen (\r _n -> runStateGen_ r uniformWord32)
+  uniformWord64 QC = MkGen (\r _n -> runStateGen_ r uniformWord64)
+#if MIN_VERSION_random(1,3,0)
+  uniformByteArrayM pinned sz QC = 
+    MkGen (\r _n -> runStateGen_ r (uniformByteArrayM pinned sz))
+#else
+  uniformShortByteString k QC =
+    MkGen (\r _n -> runStateGen_ r (uniformShortByteString k))
+#endif
+#endif
 --------------------------------------------------------------------------
 -- ** Primitive generator combinators