Improving Random
Zemyla
zemyla at gmail.com
Tue May 26 22:47:17 UTC 2020
And can you explain how to take an existing RNG and write it in this new
format?
On Tue, May 26, 2020, 15:54 David Feuer <david.feuer at gmail.com> wrote:
> Could you explain the reasoning behind the deprecations?
>
> On Tue, May 26, 2020, 6:00 AM <dominic at steinitz.org> wrote:
>
>> Hello Libraries,
>>
>> You may recall that following the blog post
>> <https://alexey.kuleshevi.ch/blog/2019/12/21/random-benchmarks/> by
>> @lehins, a group of us (@curiousleo, @lehins and me) invited participation
>> in February
>> <https://mail.haskell.org/pipermail/libraries/2020-February/030261.html>
>> to take this work and apply it to improving the current random library.
>>
>> Our proximate goals were to fix #25
>> <https://github.com/haskell/random/issues/25> (filed in 2015) and #51
>> <https://github.com/haskell/random/issues/51> (filed in 2018). After a
>> lot of discussion and experimentation, we have a proposal that addresses
>> both these issues and also: #26
>> <https://github.com/haskell/random/issues/26>, #44
>> <https://github.com/haskell/random/issues/44>, #53
>> <https://github.com/haskell/random/issues/53>, #55
>> <https://github.com/haskell/random/issues/55>, #58
>> <https://github.com/haskell/random/issues/58> and #59
>> <https://github.com/haskell/random/issues/59>.
>>
>> For backwards compatibility, the proposal retains the old style classes
>> and enhances them. Thus in 1.1 we have
>>
>> class RandomGen g where
>> next :: g -> (Int, g)
>> genRange :: g -> (Int, Int)
>> split :: g -> (g, g)
>> {-# MINIMAL next, split #-}
>>
>> and in 1.2 we have
>>
>> class RandomGen g where
>> next :: g -> (Int, g)
>> genWord8 :: g -> (Word8, g)
>> genWord16 :: g -> (Word16, g)
>> genWord32 :: g -> (Word32, g)
>> genWord64 :: g -> (Word64, g)
>> genWord32R :: Word32 -> g -> (Word32, g)
>> genWord64R :: Word64 -> g -> (Word64, g)
>> genShortByteString :: Int
>> -> g -> (Data.ByteString.Short.Internal.ShortByteString, g)
>> genRange :: g -> (Int, Int)
>> split :: g -> (g, g)
>> {-# MINIMAL split, (genWord32 | genWord64 | next, genRange) #-}
>>
>> and next and genRange are deprecated. This interface is what allows the
>> significantly faster performance as no longer is everything forced to go
>> via Integer.
>>
>> Several new interfaces are introduced and it is recommended that new
>> applications use these and, where feasible, existing applications migrate
>> to using them.
>>
>> The major API addition in this PR is the definition of a new class
>> MonadRandom:
>>
>> -- | 'MonadRandom' is an interface to monadic pseudo-random number generators.
>> class Monad m => MonadRandom g s m | g m -> s where
>> {-# MINIMAL freezeGen,thawGen,(uniformWord32|uniformWord64) #-}
>>
>> type Frozen g = (f :: Type) | f -> g
>> freezeGen :: g s -> m (Frozen g)
>> thawGen :: Frozen g -> m (g s)
>>
>> uniformWord32 :: g s -> m Word32 -- default implementation in terms of uniformWord64
>> uniformWord64 :: g s -> m Word64 -- default implementation in terms of uniformWord32
>> -- plus methods for other word sizes and for byte strings
>> -- all have default implementations so the MINIMAL pragma holds
>>
>> Conceptually, in MonadRandom g s m, g s is the type of the generator, s
>> is the state type, and m the underlying monad. Via the functional
>> dependency g m -> s, the state type is determined by the generator and
>> monad.
>>
>> Frozen is the type of the generator's state "at rest". It is defined as
>> an injective type family via f -> g, so there is no ambiguity as to
>> which g any Frozen g belongs to.
>>
>> This definition is generic enough to accommodate, for example, the Gen
>> type from mwc-random, which itself abstracts over the underlying
>> primitive monad and state token. This is the full instance declaration
>> (provided here as an example - this instance is not part of random as
>> random does not depend on mwc-random):
>>
>> instance (s ~ PrimState m, PrimMonad m) => MonadRandom MWC.Gen s m where
>> type Frozen MWC.Gen = MWC.Seed
>> freezeGen = MWC.save
>> thawGen = MWC.restore
>>
>> uniformWord8 = MWC.uniform
>> uniformWord16 = MWC.uniform
>> uniformWord32 = MWC.uniform
>> uniformWord64 = MWC.uniform
>> uniformShortByteString n g = unsafeSTToPrim (genShortByteStringST n (MWC.uniform g))
>>
>> Pure random number generators can also be made instances of this class
>> providing a uniform interface to both pure and stateful random number
>> generators. An instance for the standard number generator StdGen is
>> provided.
>>
>> The Random typeclass has conceptually been split into Uniform and
>> UniformRange. The Random typeclass is still included for backwards
>> compatibility. Uniform is for types where it is possible to sample from
>> the type's entire domain; UniformRange is for types where one can sample
>> from a specified range:
>>
>> class Uniform a where
>> uniformM :: MonadRandom g s m => g s -> m a
>>
>> class UniformRange a where
>> uniformRM :: MonadRandom g s m => (a, a) -> g s -> m a
>>
>> The proposal is a breaking change but the changes are not very intrusive
>> and we have PRs ready for the affected downstream libraries:
>>
>> - requires base >= 4.10 (GHC-8.2)
>> - StdGen is no longer an instance of Read
>> - randomIO and randomRIO were extracted from the Random class into
>> separate functions
>>
>> In addition, there may be import clashes with new functions, e.g. uniform
>> and uniformR.
>>
>> Further explanatory details may be found here
>> <https://github.com/idontgetoutmuch/random/blob/v1.2-release-notes/RELEASE-NOTES-v1.2.md#api-changes>
>> and the PR for the proposed new version is here
>> <https://github.com/haskell/random/pull/61>.
>>
>> Here are some benchmarks run on a 3.1 GHz Intel Core i7. The full
>> benchmarks can be run using e.g. stack bench. The benchmarks are
>> measured in milliseconds per 100,000 generations. In some cases, the
>> performance is over x1000(!) times better; the minimum performance increase
>> for the types listed below is more than x35.
>>
>> | Name | Mean (1.1) | Mean (1.2) | Improvement|
>> | ----------------------- | ---------- | ---------- | ---------- |
>> | pure/random/Float | 30 | 0.03 | 1038|
>> | pure/random/Double | 52 | 0.03 | 1672|
>> | pure/random/Integer | 43 | 0.33 | 131|
>> | pure/uniform/Word8 | 14 | 0.03 | 422|
>> | pure/uniform/Word16 | 13 | 0.03 | 375|
>> | pure/uniform/Word32 | 21 | 0.03 | 594|
>> | pure/uniform/Word64 | 42 | 0.03 | 1283|
>> | pure/uniform/Word | 44 | 0.03 | 1491|
>> | pure/uniform/Int8 | 15 | 0.03 | 511|
>> | pure/uniform/Int16 | 15 | 0.03 | 507|
>> | pure/uniform/Int32 | 22 | 0.03 | 749|
>> | pure/uniform/Int64 | 44 | 0.03 | 1405|
>> | pure/uniform/Int | 43 | 0.03 | 1512|
>> | pure/uniform/Char | 17 | 0.49 | 35|
>> | pure/uniform/Bool | 18 | 0.03 | 618|
>> | pure/uniform/CChar | 14 | 0.03 | 485|
>> | pure/uniform/CSChar | 14 | 0.03 | 455|
>> | pure/uniform/CUChar | 13 | 0.03 | 448|
>> | pure/uniform/CShort | 14 | 0.03 | 473|
>> | pure/uniform/CUShort | 13 | 0.03 | 457|
>> | pure/uniform/CInt | 21 | 0.03 | 737|
>> | pure/uniform/CUInt | 21 | 0.03 | 742|
>> | pure/uniform/CLong | 43 | 0.03 | 1544|
>> | pure/uniform/CULong | 42 | 0.03 | 1460|
>> | pure/uniform/CPtrdiff | 43 | 0.03 | 1494|
>> | pure/uniform/CSize | 43 | 0.03 | 1475|
>> | pure/uniform/CWchar | 22 | 0.03 | 785|
>> | pure/uniform/CSigAtomic | 21 | 0.03 | 749|
>> | pure/uniform/CLLong | 43 | 0.03 | 1554|
>> | pure/uniform/CULLong | 42 | 0.03 | 1505|
>> | pure/uniform/CIntPtr | 43 | 0.03 | 1476|
>> | pure/uniform/CUIntPtr | 42 | 0.03 | 1463|
>> | pure/uniform/CIntMax | 43 | 0.03 | 1535|
>> | pure/uniform/CUIntMax | 42 | 0.03 | 1493|
>>
>>
>>
>> Dominic Steinitz
>> dominic at steinitz.org
>> http://idontgetoutmuch.org
>> Twitter: @idontgetoutmuch
>>
>>
>>
>>
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