Move thawGen fromFrozenGen into new ThawedGen type class:

* Also add some laws for `FrozenGen` and `ThawedGen`

This split of `FrozenGen` type class into two is needed because some
mutable generators can't be cloned, becase the mutable state is
stored in the monad they are running in, rather than in the mutable
generator itself.

Author: lehins

CHANGELOG.md

@@ -1,5 +1,7 @@
 # 1.3.0
 
+* Move `thawGen` from `FreezeGen` into the new `ThawGen` type class. Fixes an issue with
+  an unlawful instance of `StateGen` for `FreezeGen`.
 * Add `modifyGen` and `overwriteGen` to the `FrozenGen` type class
 * Add `splitGen` and `splitMutableGen`
 * Switch `randomM` and `randomRM` to use `FrozenGen` instead of `RandomGenM`

src/System/Random/Internal.hs

@@ -30,6 +30,7 @@ module System.Random.Internal
     RandomGen(..)
   , StatefulGen(..)
   , FrozenGen(..)
+  , ThawedGen(..)
   , splitGen
   , splitMutableGen
 
@@ -286,40 +287,85 @@ class Monad m => StatefulGen g m where
   {-# INLINE uniformShortByteString #-}
 
 
-
--- | This class is designed for stateful pseudo-random number generators that
--- can be saved as and restored from an immutable data type.
+-- | This class is designed for mutable pseudo-random number generators that have a frozen
+-- imutable counterpart that can be manipulated in pure code.
+--
+-- It also works great with frozen generators that are based on pure generators that have
+-- a `RandomGen` instance.
+--
+-- Here are a few of laws that are important for this interface:
+--
+-- * Roundtrip and complete destruction on overwrite:
+--
+-- @
+-- (overwriteGen mg fg >> freezeGen mg) = pure fg
+-- @
+--
+-- * Modification of mutable generator:
+--
+-- @
+-- overwriteGen mg fg = modifyGen mg (const ((), fg)
+-- @
 --
--- It also works great on working with mutable generators that are based on a pure
--- generator that has a `RandomGen` instance.
+-- * Freeing of mutable generator:
+--
+-- @
+-- freezeGen mg = modifyGen mg (\fg -> (fg, fg))
+-- @
 --
 -- @since 1.2.0
 class StatefulGen (MutableGen f m) m => FrozenGen f m where
+  {-# MINIMAL (modifyGen|(freezeGen,overwriteGen)) #-}
   -- | Represents the state of the pseudo-random number generator for use with
   -- 'thawGen' and 'freezeGen'.
   --
   -- @since 1.2.0
   type MutableGen f m = (g :: Type) | g -> f
+
   -- | Saves the state of the pseudo-random number generator as a frozen seed.
   --
   -- @since 1.2.0
   freezeGen :: MutableGen f m -> m f
-  -- | Restores the pseudo-random number generator from its frozen seed.
-  --
-  -- @since 1.2.0
-  thawGen :: f -> m (MutableGen f m)
+  freezeGen mg = modifyGen mg (\fg -> (fg, fg))
+  {-# INLINE freezeGen #-}
 
   -- | Apply a pure function to the frozen pseudo-random number generator.
   --
   -- @since 1.3.0
   modifyGen :: MutableGen f m -> (f -> (a, f)) -> m a
+  modifyGen mg f = do
+    fg <- freezeGen mg
+    case f fg of
+      (a, !fg') -> a <$ overwriteGen mg fg'
+  {-# INLINE modifyGen #-}
 
   -- | Overwrite contents of the mutable pseudo-random number generator with the
   -- supplied frozen one
   --
   -- @since 1.3.0
   overwriteGen :: MutableGen f m -> f -> m ()
   overwriteGen mg fg = modifyGen mg (const ((), fg))
+  {-# INLINE overwriteGen #-}
+
+-- | Functionality for thawing frozen generators was split into a separate type class,
+-- becase not all mutable generators support functionality of creating new mutable
+-- generators, which is what thawing is in its essence. For this reason `StateGen` does
+-- not have an instance for this type class, but it has one for `FrozenGen`.
+--
+-- Here is an important law that relates this type class to `FrozenGen`
+--
+-- * Roundtrip and independence of mutable generators:
+--
+-- @
+-- (mapM thawGen fgs >>= mapM freezeGen) = pure fgs
+-- @
+--
+-- @since 1.3.0
+class FrozenGen f m => ThawedGen f m where
+  -- | Create a new mutable pseudo-random number generator from its frozen state.
+  --
+  -- @since 1.2.0
+  thawGen :: f -> m (MutableGen f m)
 
 -- | Splits a pseudo-random number generator into two. Overwrites the mutable
 -- wrapper with one of the resulting generators and returns the other.
@@ -328,11 +374,11 @@ class StatefulGen (MutableGen f m) m => FrozenGen f m where
 splitGen :: (RandomGen f, FrozenGen f m) => MutableGen f m -> m f
 splitGen = flip modifyGen split
 
--- | Splits a pseudo-random number generator into two. Overwrites the mutable
--- wrapper with one of the resulting generators and returns the other.
+-- | Splits a pseudo-random number generator into two. Overwrites the mutable wrapper with
+-- one of the resulting generators and returns the other as a new mutable generator.
 --
 -- @since 1.3.0
-splitMutableGen :: (RandomGen f, FrozenGen f m) => MutableGen f m -> m (MutableGen f m)
+splitMutableGen :: (RandomGen f, ThawedGen f m) => MutableGen f m -> m (MutableGen f m)
 splitMutableGen = splitGen >=> thawGen
 
 
@@ -481,7 +527,6 @@ instance (RandomGen g, MonadState g m) => StatefulGen (StateGenM g) m where
 instance (RandomGen g, MonadState g m) => FrozenGen (StateGen g) m where
   type MutableGen (StateGen g) m = StateGenM g
   freezeGen _ = fmap StateGen get
-  thawGen (StateGen g) = StateGenM <$ put g
   modifyGen _ f = state (coerce f)
   {-# INLINE modifyGen #-}
   overwriteGen _ f = put (coerce f)

src/System/Random/Stateful.hs

@@ -30,6 +30,7 @@ module System.Random.Stateful
   -- $interfaces
   , StatefulGen(..)
   , FrozenGen(..)
+  , ThawedGen(..)
   , withMutableGen
   , withMutableGen_
   , randomM
@@ -257,7 +258,7 @@ instance RandomGen r => RandomGenM (TGenM r) r STM where
 -- ([-74,37,-50,-2,3],IOGen {unIOGen = StdGen {unStdGen = SMGen 4273268533320920145 15251669095119325999}})
 --
 -- @since 1.2.0
-withMutableGen :: FrozenGen f m => f -> (MutableGen f m -> m a) -> m (a, f)
+withMutableGen :: ThawedGen f m => f -> (MutableGen f m -> m a) -> m (a, f)
 withMutableGen fg action = do
   g <- thawGen fg
   res <- action g
@@ -274,7 +275,7 @@ withMutableGen fg action = do
 -- 4
 --
 -- @since 1.2.0
-withMutableGen_ :: FrozenGen f m => f -> (MutableGen f m -> m a) -> m a
+withMutableGen_ :: ThawedGen f m => f -> (MutableGen f m -> m a) -> m a
 withMutableGen_ fg action = thawGen fg >>= action
 
 
@@ -311,6 +312,7 @@ uniformListM n gen = replicateM n (uniformM gen)
 -- @since 1.2.0
 randomM :: forall a g m. (Random a, RandomGen g, FrozenGen g m) => MutableGen g m -> m a
 randomM = flip modifyGen random
+{-# INLINE randomM #-}
 
 -- | Generates a pseudo-random value using monadic interface and `Random` instance.
 --
@@ -331,6 +333,7 @@ randomM = flip modifyGen random
 -- @since 1.2.0
 randomRM :: forall a g m. (Random a, RandomGen g, FrozenGen g m) => (a, a) -> MutableGen g m -> m a
 randomRM r = flip modifyGen (randomR r)
+{-# INLINE randomRM #-}
 
 -- | Wraps an 'IORef' that holds a pure pseudo-random number generator. All
 -- operations are performed atomically.
@@ -386,13 +389,15 @@ instance (RandomGen g, MonadIO m) => StatefulGen (AtomicGenM g) m where
 instance (RandomGen g, MonadIO m) => FrozenGen (AtomicGen g) m where
   type MutableGen (AtomicGen g) m = AtomicGenM g
   freezeGen = fmap AtomicGen . liftIO . readIORef . unAtomicGenM
-  thawGen (AtomicGen g) = newAtomicGenM g
   modifyGen (AtomicGenM ioRef) f =
     liftIO $ atomicModifyIORef' ioRef $ \g ->
       case f (AtomicGen g) of
         (a, AtomicGen g') -> (g', a)
   {-# INLINE modifyGen #-}
 
+instance (RandomGen g, MonadIO m) => ThawedGen (AtomicGen g) m where
+  thawGen (AtomicGen g) = newAtomicGenM g
+
 -- | Atomically applies a pure operation to the wrapped pseudo-random number
 -- generator.
 --
@@ -466,7 +471,6 @@ instance (RandomGen g, MonadIO m) => StatefulGen (IOGenM g) m where
 instance (RandomGen g, MonadIO m) => FrozenGen (IOGen g) m where
   type MutableGen (IOGen g) m = IOGenM g
   freezeGen = fmap IOGen . liftIO . readIORef . unIOGenM
-  thawGen (IOGen g) = newIOGenM g
   modifyGen (IOGenM ref) f = liftIO $ do
     g <- readIORef ref
     let (a, IOGen g') = f (IOGen g)
@@ -476,6 +480,9 @@ instance (RandomGen g, MonadIO m) => FrozenGen (IOGen g) m where
   overwriteGen (IOGenM ref) = liftIO . writeIORef ref . unIOGen
   {-# INLINE overwriteGen #-}
 
+instance (RandomGen g, MonadIO m) => ThawedGen (IOGen g) m where
+  thawGen (IOGen g) = newIOGenM g
+
 -- | Applies a pure operation to the wrapped pseudo-random number generator.
 --
 -- ====__Examples__
@@ -533,7 +540,6 @@ instance RandomGen g => StatefulGen (STGenM g s) (ST s) where
 instance RandomGen g => FrozenGen (STGen g) (ST s) where
   type MutableGen (STGen g) (ST s) = STGenM g s
   freezeGen = fmap STGen . readSTRef . unSTGenM
-  thawGen (STGen g) = newSTGenM g
   modifyGen (STGenM ref) f = do
     g <- readSTRef ref
     let (a, STGen g') = f (STGen g)
@@ -543,6 +549,9 @@ instance RandomGen g => FrozenGen (STGen g) (ST s) where
   overwriteGen (STGenM ref) = writeSTRef ref . unSTGen
   {-# INLINE overwriteGen #-}
 
+instance RandomGen g => ThawedGen (STGen g) (ST s) where
+  thawGen (STGen g) = newSTGenM g
+
 
 -- | Applies a pure operation to the wrapped pseudo-random number generator.
 --
@@ -636,7 +645,6 @@ instance RandomGen g => StatefulGen (TGenM g) STM where
 instance RandomGen g => FrozenGen (TGen g) STM where
   type MutableGen (TGen g) STM = TGenM g
   freezeGen = fmap TGen . readTVar . unTGenM
-  thawGen (TGen g) = newTGenM g
   modifyGen (TGenM ref) f = do
     g <- readTVar ref
     let (a, TGen g') = f (TGen g)
@@ -646,6 +654,9 @@ instance RandomGen g => FrozenGen (TGen g) STM where
   overwriteGen (TGenM ref) = writeTVar ref . unTGen
   {-# INLINE overwriteGen #-}
 
+instance RandomGen g => ThawedGen (TGen g) STM where
+  thawGen (TGen g) = newTGenM g
+
 
 -- | Applies a pure operation to the wrapped pseudo-random number generator.
 --
@@ -797,19 +808,17 @@ applyTGen f (TGenM tvar) = do
 --
 -- === @FrozenGen@
 --
--- `FrozenGen` gives us ability to use any stateful pseudo-random number generator in its
--- immutable form, if one exists that is. This concept is commonly known as a seed, which
--- allows us to save and restore the actual mutable state of a pseudo-random number
--- generator. The biggest benefit that can be drawn from a polymorphic access to a
--- stateful pseudo-random number generator in a frozen form is the ability to serialize,
--- deserialize and possibly even use the stateful generator in a pure setting without
--- knowing the actual type of a generator ahead of time. For example we can write a
--- function that accepts a frozen state of some pseudo-random number generator and
--- produces a short list with random even integers.
+-- `FrozenGen` gives us ability to use most of stateful pseudo-random number generator in
+-- its immutable form, if one exists that is.  The biggest benefit that can be drawn from
+-- a polymorphic access to a stateful pseudo-random number generator in a frozen form is
+-- the ability to serialize, deserialize and possibly even use the stateful generator in a
+-- pure setting without knowing the actual type of a generator ahead of time. For example
+-- we can write a function that accepts a frozen state of some pseudo-random number
+-- generator and produces a short list with random even integers.
 --
 -- >>> import Data.Int (Int8)
 -- >>> :{
--- myCustomRandomList :: FrozenGen f m => f -> m [Int8]
+-- myCustomRandomList :: ThawedGen f m => f -> m [Int8]
 -- myCustomRandomList f =
 --   withMutableGen_ f $ \gen -> do
 --     len <- uniformRM (5, 10) gen

test/Spec.hs

@@ -94,7 +94,7 @@ main =
     , uniformSpec (Proxy :: Proxy (Word8, Word16, Word32, Word64, Word))
     , uniformSpec (Proxy :: Proxy (Int8, Word8, Word16, Word32, Word64, Word))
     , uniformSpec (Proxy :: Proxy (Int8, Int16, Word8, Word16, Word32, Word64, Word))
-    , Stateful.statefulSpec
+    , Stateful.statefulGenSpec
     ]
 
 floatTests :: TestTree