From cd3b9d6217d48328703d14d91a883f2cd638adeb Mon Sep 17 00:00:00 2001
From: Leonhard Markert <curiousleo@users.noreply.github.com>
Date: Thu, 7 May 2020 14:53:25 +0200
Subject: [PATCH] StdGen: constructor accessible via Internal only

Fixes https://github.com/haskell/random/issues/59 by making 'StdGen' not
an instance of 'Read'.
---
 System/Random.hs          |  2 +-
 System/Random/Internal.hs | 17 ++++++++++-------
 System/Random/Monad.hs    |  2 +-
 3 files changed, 12 insertions(+), 9 deletions(-)

diff --git a/System/Random.hs b/System/Random.hs
index 3741ce0d3..8a93e5468 100644
--- a/System/Random.hs
+++ b/System/Random.hs
@@ -250,7 +250,7 @@ getStdGen :: MonadIO m => m StdGen
 getStdGen = liftIO $ readIORef theStdGen
 
 theStdGen :: IORef StdGen
-theStdGen = unsafePerformIO $ SM.initSMGen >>= newIORef
+theStdGen = unsafePerformIO $ SM.initSMGen >>= newIORef . StdGen
 {-# NOINLINE theStdGen #-}
 
 -- |Applies 'split' to the current global pseudo-random generator,
diff --git a/System/Random/Internal.hs b/System/Random/Internal.hs
index 76e101660..42682d395 100644
--- a/System/Random/Internal.hs
+++ b/System/Random/Internal.hs
@@ -32,7 +32,7 @@ module System.Random.Internal
   , Frozen(..)
 
   -- ** Standard pseudo-random number generator
-  , StdGen
+  , StdGen(..)
   , mkStdGen
 
   -- * Monadic adapters for pure pseudo-random number generators
@@ -402,13 +402,16 @@ runPureGenST g action = runST $ runGenStateT g $ action
 
 
 -- | The standard pseudo-random number generator.
-type StdGen = SM.SMGen
+newtype StdGen = StdGen { unStdGen :: SM.SMGen }
+  deriving (Show)
 
 instance RandomGen StdGen where
-  next = SM.nextInt
-  genWord32 = SM.nextWord32
-  genWord64 = SM.nextWord64
-  split = SM.splitSMGen
+  next = second StdGen . SM.nextInt . unStdGen
+  genWord32 = second StdGen . SM.nextWord32 . unStdGen
+  genWord64 = second StdGen . SM.nextWord64 . unStdGen
+  split g = (StdGen g1, StdGen g2)
+    where
+      (g1, g2) = SM.splitSMGen $ unStdGen g
 
 instance RandomGen SM32.SMGen where
   next = SM32.nextInt
@@ -418,7 +421,7 @@ instance RandomGen SM32.SMGen where
 
 -- | Constructs a 'StdGen' deterministically.
 mkStdGen :: Int -> StdGen
-mkStdGen s = SM.mkSMGen $ fromIntegral s
+mkStdGen = StdGen . SM.mkSMGen . fromIntegral
 
 -- | The class of types for which a uniformly distributed value can be drawn
 -- from all possible values of the type.
diff --git a/System/Random/Monad.hs b/System/Random/Monad.hs
index 309e8bfad..f5250e346 100644
--- a/System/Random/Monad.hs
+++ b/System/Random/Monad.hs
@@ -216,7 +216,7 @@ instance RandomGen r => RandomGenM STGen r s (ST s) where
 --
 -- >>> import Data.Int (Int8)
 -- >>> runGenM (IOGen (mkStdGen 217)) (`uniformListM` 5) :: IO ([Int8], Frozen (IOGen StdGen))
--- ([-74,37,-50,-2,3],IOGen {unIOGen = SMGen 4273268533320920145 15251669095119325999})
+-- ([-74,37,-50,-2,3],IOGen {unIOGen = StdGen {unStdGen = SMGen 4273268533320920145 15251669095119325999}})
 --
 -- @since 1.2
 runGenM :: MonadRandom g s m => Frozen g -> (g s -> m a) -> m (a, Frozen g)