From a1276fec996979baf4e0a9d45d9b60262ed5aed1 Mon Sep 17 00:00:00 2001
From: Leonhard Markert <curiousleo@users.noreply.github.com>
Date: Fri, 8 May 2020 15:40:18 +0200
Subject: [PATCH] StdGen: constructor accessible via Internal only (#123)

Fixes https://github.com/haskell/random/issues/59 by making 'StdGen' not
an instance of 'Read'.
---
 System/Random.hs          |  2 +-
 System/Random/Internal.hs | 10 ++++++----
 System/Random/Monad.hs    |  2 +-
 bench/Main.hs             |  8 ++++----
 4 files changed, 12 insertions(+), 10 deletions(-)

diff --git a/System/Random.hs b/System/Random.hs
index bff4145f7..120dd37da 100644
--- a/System/Random.hs
+++ b/System/Random.hs
@@ -293,7 +293,7 @@ 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 5ba916b8f..7ec513ad8 100644
--- a/System/Random/Internal.hs
+++ b/System/Random/Internal.hs
@@ -5,6 +5,7 @@
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE FunctionalDependencies #-}
 {-# LANGUAGE GHCForeignImportPrim #-}
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
 {-# LANGUAGE MagicHash #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE RankNTypes #-}
@@ -32,7 +33,7 @@ module System.Random.Internal
   , MonadRandom(..)
 
   -- ** Standard pseudo-random number generator
-  , StdGen
+  , StdGen(..)
   , mkStdGen
 
   -- * Monadic adapters for pure pseudo-random number generators
@@ -404,9 +405,10 @@ runStateGenST g action = runST $ runStateGenT g action
 
 
 -- | The standard pseudo-random number generator.
-type StdGen = SM.SMGen
+newtype StdGen = StdGen { unStdGen :: SM.SMGen }
+  deriving (RandomGen, Show)
 
-instance RandomGen StdGen where
+instance RandomGen SM.SMGen where
   next = SM.nextInt
   genWord32 = SM.nextWord32
   genWord64 = SM.nextWord64
@@ -420,7 +422,7 @@ 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 dc0110886..585023775 100644
--- a/System/Random/Monad.hs
+++ b/System/Random/Monad.hs
@@ -211,7 +211,7 @@ instance RandomGen r => RandomGenM STGenM r s (ST s) where
 --
 -- >>> import Data.Int (Int8)
 -- >>> runGenM (IOGen (mkStdGen 217)) (`uniformListM` 5) :: IO ([Int8], 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)
diff --git a/bench/Main.hs b/bench/Main.hs
index 0f1b8eaf4..0b6a6a537 100644
--- a/bench/Main.hs
+++ b/bench/Main.hs
@@ -22,10 +22,10 @@ main = do
   let !sz = 100000
   defaultMain
     [ bgroup "baseline"
-      [ let !stdGen = mkStdGen 1337 in bench "nextWord32" $ nf (genMany SM.nextWord32 stdGen) sz
-      , let !stdGen = mkStdGen 1337 in bench "nextWord64" $ nf (genMany SM.nextWord64 stdGen) sz
-      , let !stdGen = mkStdGen 1337 in bench "nextInt" $ nf (genMany SM.nextInt stdGen) sz
-      , let !stdGen = mkStdGen 1337 in bench "split" $ nf (genMany SM.splitSMGen stdGen) sz
+      [ let !smGen = SM.mkSMGen 1337 in bench "nextWord32" $ nf (genMany SM.nextWord32 smGen) sz
+      , let !smGen = SM.mkSMGen 1337 in bench "nextWord64" $ nf (genMany SM.nextWord64 smGen) sz
+      , let !smGen = SM.mkSMGen 1337 in bench "nextInt" $ nf (genMany SM.nextInt smGen) sz
+      , let !smGen = SM.mkSMGen 1337 in bench "split" $ nf (genMany SM.splitSMGen smGen) sz
       ]
     , bgroup "pure"
       [ bgroup "random"