From f3e0e3f5487f80d39d998f4af6222909f58f5f41 Mon Sep 17 00:00:00 2001
From: Alexey Kuleshevich <alexey@kuleshevi.ch>
Date: Sun, 24 Jan 2021 17:52:07 +0300
Subject: [PATCH] Improve `RandomGenM` interface:

* Add a new class method `newRandomGenM`, which also allowed addition
  of `splitRandomGenM`
* Add `getGenM` and getGenM`
---
 CHANGELOG.md                  |  9 +++++++
 src/System/Random/Stateful.hs | 44 ++++++++++++++++++++++++++++++++++-
 2 files changed, 52 insertions(+), 1 deletion(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 16facc7aa..d1070c6b0 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,12 @@
+# 1.3.0
+
+* Improve `RandomGenM` interface:
+
+   * Add a new class method `newRandomGenM`, which also allowed addition
+     of `splitRandomGenM`
+   * Add `getGenM` and getGenM`
+
+
 # 1.2.0
 
 1. Breaking change which mostly maintains backwards compatibility, see
diff --git a/src/System/Random/Stateful.hs b/src/System/Random/Stateful.hs
index 6ab1cde0d..3ff7b9b78 100644
--- a/src/System/Random/Stateful.hs
+++ b/src/System/Random/Stateful.hs
@@ -28,13 +28,19 @@ module System.Random.Stateful
 
   -- * Mutable pseudo-random number generator interfaces
   -- $interfaces
+  -- ** StatefulGen
   , StatefulGen(..)
+  -- ** FrozenGen
   , FrozenGen(..)
-  , RandomGenM(..)
   , withMutableGen
   , withMutableGen_
+  -- ** RandomGenM
+  , RandomGenM(..)
+  , splitRandomGenM
   , randomM
   , randomRM
+  , getGenM
+  , putGenM
   , splitGenM
 
   -- * Monadic adapters for pure pseudo-random number generators #monadicadapters#
@@ -203,8 +209,23 @@ import System.Random.Internal
 --
 -- @since 1.2.0
 class (RandomGen r, StatefulGen g m) => RandomGenM g r m | g -> r where
+
+  -- | Construct a mutable pseudo-random number generator out of the pure one
+  --
+  -- @since 1.3.0
+  newRandomGenM :: r -> m g
+
+  -- | Apply a pure function to the undelying pure pseudo-random number generator
+  --
+  -- @since 1.2.0
   applyRandomGenM :: (r -> (a, r)) -> g -> m a
 
+-- | Splits mutable pseudo-random number generator into two and retuns the new one
+--
+-- @since 1.3.0
+splitRandomGenM :: RandomGenM g r m => g -> m g
+splitRandomGenM = applyRandomGenM split >=> newRandomGenM
+
 -- | Splits a pseudo-random number generator into two. Overwrites the mutable
 -- wrapper with one of the resulting generators and returns the other.
 --
@@ -212,16 +233,37 @@ class (RandomGen r, StatefulGen g m) => RandomGenM g r m | g -> r where
 splitGenM :: RandomGenM g r m => g -> m r
 splitGenM = applyRandomGenM split
 
+-- | Get the underlying pure pseudo-random number generator from the mutable one.
+--
+-- @since 1.3.0
+getGenM :: RandomGenM g r m => g -> m r
+getGenM = applyRandomGenM (\g -> (g, g))
+
+-- | Get the underlying pure pseudo-random number generator from the mutable one.
+--
+-- @since 1.3.0
+putGenM :: RandomGenM g r m => g -> r -> m ()
+putGenM gen rng = applyRandomGenM (\_ -> ((), rng)) gen
+
+
 instance (RandomGen r, MonadIO m) => RandomGenM (IOGenM r) r m where
+  newRandomGenM = newIOGenM
+
   applyRandomGenM = applyIOGen
 
 instance (RandomGen r, MonadIO m) => RandomGenM (AtomicGenM r) r m where
+  newRandomGenM = newAtomicGenM
+
   applyRandomGenM = applyAtomicGen
 
 instance (RandomGen r, MonadState r m) => RandomGenM (StateGenM r) r m where
+  newRandomGenM g = StateGenM <$ put g
+
   applyRandomGenM f _ = state f
 
 instance RandomGen r => RandomGenM (STGenM r s) r (ST s) where
+  newRandomGenM = newSTGenM
+
   applyRandomGenM = applySTGen