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This means you + can write code that appears to issue requests and immediately receive their + responses, but process the requests jointly all at once. +category: Algorithms +homepage: https://github.com/google/hs-batching#readme +bug-reports: https://github.com/google/hs-batching/issues +author: Andrew Pritchard +maintainer: Andrew Pritchard +copyright: 2019-2021 Google LLC +license: Apache-2.0 +license-file: LICENSE +build-type: Simple +extra-source-files: + CHANGELOG.md + +source-repository head + type: git + location: https://github.com/google/hs-batching + +library + exposed-modules: + Control.Batching + other-modules: + Paths_batching + hs-source-dirs: + src + build-depends: + base >=4.12 && <4.16 + , primitive >=0.1 && <0.8 + , short-vec >=0.1 && <0.2 + , sint >=0.1 && <0.2 + default-language: Haskell2010 + +test-suite Batching-test + type: exitcode-stdio-1.0 + main-is: Main.hs + other-modules: + Paths_batching + hs-source-dirs: + test + build-depends: + QuickCheck + , base >=4.12 && <4.16 + , batching + , primitive >=0.1 && <0.8 + , short-vec + , sint >=0.1 && <0.2 + , test-framework + , test-framework-quickcheck2 + default-language: Haskell2010 diff --git a/src/Control/Batching.hs b/src/Control/Batching.hs new file mode 100644 --- /dev/null +++ b/src/Control/Batching.hs @@ -0,0 +1,274 @@ +-- Copyright 2019-2021 Google LLC +-- +-- Licensed under the Apache License, Version 2.0 (the "License"); +-- you may not use this file except in compliance with the License. +-- You may obtain a copy of the License at +-- +-- http://www.apache.org/licenses/LICENSE-2.0 +-- +-- Unless required by applicable law or agreed to in writing, software +-- distributed under the License is distributed on an "AS IS" BASIS, +-- WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. +-- See the License for the specific language governing permissions and +-- limitations under the License. + +{-# LANGUAGE AllowAmbiguousTypes #-} +{-# LANGUAGE BangPatterns #-} +{-# LANGUAGE DataKinds #-} +{-# LANGUAGE FlexibleContexts #-} +{-# LANGUAGE GADTs #-} +{-# LANGUAGE KindSignatures #-} +{-# LANGUAGE MagicHash #-} +{-# LANGUAGE RankNTypes #-} +{-# LANGUAGE ScopedTypeVariables #-} +{-# LANGUAGE TypeApplications #-} +{-# LANGUAGE TypeOperators #-} + +-- | An 'Applicative' for deferring "requests" to handle them all in bulk. + +module Control.Batching + ( Batching, request, batchRequest, runBatching, runBatching_ + ) where + +import Control.Applicative (Applicative(..)) +import Control.Monad.ST (runST) +import Data.Foldable (sequenceA_, toList) +import qualified Data.Foldable as F +import Data.Functor.Identity (Identity(..)) +import GHC.TypeNats (type (+), Nat) + +import qualified Data.Primitive.Array as A +import Data.SInt (SInt(unSInt), reifySInt, withSInt, sintVal, addSInt) +import Data.Vec.Short (Vec) +import qualified Data.Vec.Short as Vec + +-- Quick-and-dirty Vec builder standin: a list. +-- +-- Supports O(1) cons, O(n) conversion to Vec, and O(m) prepend of length-m to +-- length-n. +-- +-- Note we won't ever left-associate appends, because we design the 'Batching' +-- type to avoid it, so O(m) prepend won't lead to any super-linear behavior. +newtype VecBuilder (n :: Nat) a = VecBuilder + { _vbContents :: [a] + } + +nil :: VecBuilder 0 a +nil = VecBuilder [] + +cons :: a -> VecBuilder n a -> VecBuilder (1+n) a +cons x (VecBuilder xs) = VecBuilder (x:xs) + +vbToVec' :: SInt n -> VecBuilder n a -> Vec n a +vbToVec' n (VecBuilder c) = reifySInt n $ Vec.fromList c + +-- Quick-and-dirty Vec iterator standin: a list. +-- +-- Supports O(1) uncons, O(m) bulk-uncons (split) of length m, and O(n) +-- conversion from Vec. +-- +-- Note we won't ever recursively split, since splits are only generated by +-- 'batchRequest', and not by ('<*>'); so O(m) split won't lead to any +-- super-linear behavior. +newtype VecView (n :: Nat) a = VecView { _unVecView :: [a] } + +uncons :: VecView (1+n) a -> (a, VecView n a) +uncons (VecView (x:xs)) = (x, VecView xs) +uncons _ = error "Internal error: invalid VecView." + +split + :: forall m n a. SInt m -> VecView (m+n) a -> (VecView m a, VecView n a) +split m (VecView xs) = + let (ma, na) = splitAt (unSInt m) xs + in (VecView ma, VecView na) + +vvFromVec :: Vec n a -> VecView n a +vvFromVec = VecView . toList + +-- TODO(awpr): consider factoring out a @SizedBatch n rq rs a@ type and +-- providing a scoped API for accessing it, e.g.: +-- +-- withSizedBatch +-- :: Batching rq rs a +-- -> (forall n. KnownNat n => SizedBatch n rq rs a -> r) +-- -> r +-- getBatchRequests :: SizedBatch n rq rs a -> Vec n rq +-- putBatchResponses :: SizedBatch n rq rs a -> Vec n rs -> a + +-- -- Start with a simple version: contains the requests and a function to -- +-- consume the responses. +-- data Batching0 rq rs a = forall n. Batching0 +-- { n :: SInt n +-- , requests :: VecBuilder n rq +-- , cont :: VecView n rs -> a +-- } +-- +-- -- First transformation: actually use the curried (<**>) type internally so +-- -- that we can right-associate all of the request prepends. +-- newtype Batching1 rq rs a = Batching1 +-- { _unBatching1 :: forall r. Batching0 rq rs (a -> r) -> Batching0 rq rs r +-- } +-- +-- -- Second transformation: turn the above function inside out, to enable the +-- -- third transformation. +-- newtype Batching2 rq rs a = Batching2 +-- { _unBatching2 +-- :: forall z r. (Batching0 rq rs r -> z) -> (Batching0 rq rs (a -> r) -> z) +-- } +-- +-- -- Third transformation: curry both of the function types above to get rid +-- -- of a GADT constructor. +-- newtype Batching rq rs a = Batching +-- { _unBatching +-- :: forall z r +-- . (forall n. SInt n -> (VecView n rs -> r) -> VecBuilder n rq -> z) +-- -> (forall m. SInt m -> (VecView m rs -> a -> r) -> VecBuilder m rq -> z) +-- } +-- +-- Now we have a newtype Batching that: +-- - takes a continuation for what to do with a VecBuilder of requests and a +-- VecView-consuming function returning @r@, with matching lengths. +-- - takes a VecView-consuming function for a (smaller) VecView returning +-- @a -> r@. +-- - takes a (smaller) VecBuilder of requests. +-- - wraps the VecView-consuming function with code to consume a prefix and +-- apply away the @a@ parameter. +-- - prepends some of its own requests to the VecBuilder. +-- - passes them on to the continuation and returns its result. + +-- | The bulk request-response Applicative. +-- +-- A value of type @Batching rq rs a@ describes a computation that gathers some +-- number of @rq@ request values, expects the same number of @rs@ response +-- values, and ultimately returns an @a@ result value derived from the +-- responses. +-- +-- This can be used to apply an offline resource allocation algorithm to code +-- written as if allocation requests were satisfied incrementally. +-- +-- This synergizes well with @-XApplicativeDo@, which allows using do-notation +-- for this type, as long as requests do not depend on earlier responses. +newtype Batching rq rs a = Batching +-- This is essentially the same as the Ap type from +-- Control.Applicative.Free.Fast, but specialized to lists of requests and +-- responses. + { _unBulk + :: forall z r + . (forall n. SInt n -> (VecView n rs -> r) -> VecBuilder n rq -> z) + -> (forall m. SInt m -> (VecView m rs -> a -> r) -> VecBuilder m rq -> z) + } + +instance Functor (Batching rq rs) where + fmap f (Batching go) = Batching $ \k m g rqs -> go k m (\vv x -> g vv (f x)) rqs + {-# INLINE fmap #-} + +instance Applicative (Batching rq rs) where + pure x = Batching $ \k m g rqs -> k m (`g` x) rqs + {-# INLINE pure #-} + Batching f <*> Batching x = + Batching $ \k m g rqs -> f (x k) m (\vv h -> g vv . h) rqs + {-# INLINE (<*>) #-} + liftA2 f (Batching x) (Batching y) = Batching $ \k m g rqs -> + x (y k) m (\vv a b -> g vv (f a b)) rqs + {-# INLINE liftA2 #-} + +-- | Issue one request and retrieve its response. +request :: rq -> Batching rq rs rs +request rq = Batching $ \k n g !rqs -> k + (sintVal `addSInt` n) + -- Consume our response off the front of the VecView and pass it to the + -- result function. + (\rss -> let !(rs, rss') = uncons rss in g rss' rs) + -- Add our request to the front of the VecBuilder. + (rq `cons` rqs) +{-# INLINE request #-} + +-- Specialized implementation for issuing Traversables of requests at once. +-- The goal here is to separate the traversal gathering requests from the +-- traversal building up the result, so we don't have to allocate a bunch of +-- memory to hold a closure that will build the resulting data structure while +-- we're handling the requests. +newtype BulkCont rq rs a = BulkCont + { _bcCont :: A.Array rs -> Int -> (Int, a) + } + +instance Functor (BulkCont rq rs) where + fmap f (BulkCont k) = BulkCont (\a i -> + let !(i', x) = k a i + in (i', f x)) + {-# INLINE fmap #-} + +instance Applicative (BulkCont rq rs) where + pure x = BulkCont (\_ i -> (i, x)) + {-# INLINE pure #-} + + BulkCont kf <*> BulkCont kx = BulkCont (\a i -> + let !(i', f) = kf a i + !(i'', x) = kx a i' + in (i'', f x)) + {-# INLINE (<*>) #-} + + liftA2 f (BulkCont kx) (BulkCont ky) = BulkCont (\a i -> + let !(i', x) = kx a i + !(i'', y) = ky a i' + in (i'', f x y)) + {-# INLINE liftA2 #-} + +rqBulkCont :: rq -> BulkCont rq rs rs +rqBulkCont _ = BulkCont $ \a i -> + case A.indexArrayM a i of ((), x) -> (1 + i, x) +{-# INLINE rqBulkCont #-} + +vvToArray :: forall n a. SInt n -> VecView n a -> A.Array a +vvToArray n (VecView v) = runST $ + A.newArray (unSInt n) undefined >>= \arr -> do + sequenceA_ $ zipWith (A.writeArray arr) [0..] v + A.unsafeFreezeArray arr + +-- | Issue a Traversable of requests and retrieve their responses. +batchRequest :: forall t rq rs. Traversable t => t rq -> Batching rq rs (t rs) +batchRequest rqs0 = + let !rqs = \rest -> F.foldr (:) rest rqs0 + !n = length rqs0 + in withSInt n $ \sn -> + Batching $ \k m g (VecBuilder rqs1) -> k + (sn `addSInt` m) + (\rss -> + let !(rss0, rss1) = split sn rss + !(_, rssT) = + _bcCont (traverse rqBulkCont rqs0) (vvToArray sn rss0) 0 + in g rss1 rssT) + (VecBuilder (rqs rqs1)) +{-# INLINE batchRequest #-} + +-- | Given an allocator function in any 'Functor', run a 'Batching' computation. +runBatching + :: Functor f + => (forall n. Vec n rq -> f (Vec n rs)) + -> Batching rq rs a -> f a +runBatching f (Batching go) = go + (\n q rqs -> q . vvFromVec <$> f (vbToVec' n rqs)) + sintVal + (const id) + nil +{-# INLINE runBatching #-} + +-- | Like 'runBatching', but without a 'Functor' (or implicitly in 'Identity'). +runBatching_ :: (forall n. Vec n rq -> Vec n rs) -> Batching rq rs a -> a +runBatching_ f = runIdentity . runBatching (Identity . f) +{-# INLINE runBatching_ #-} + +-- TODO(awpr): consider adding a Batched monad that supports many batches of +-- requests: +-- +-- type Batched rq rs a = Free (Batching rq rs) a +-- +-- -- Use ApplicativeDo inside this to group requests into a batch. +-- batch :: Batching rq rs a -> Batched rq rs a +-- +-- -- Note this requires Monad rather than just Functor, since it can have +-- multiple batches. +-- runBatched +-- :: Monad f +-- => (forall n. KnownNat n => Vec n rq -> f (Vec n rs)) +-- -> Batched rq rs a -> f a diff --git a/test/Main.hs b/test/Main.hs new file mode 100644 --- /dev/null +++ b/test/Main.hs @@ -0,0 +1,52 @@ +-- Copyright 2021 Google LLC +-- +-- Licensed under the Apache License, Version 2.0 (the "License"); +-- you may not use this file except in compliance with the License. +-- You may obtain a copy of the License at +-- +-- http://www.apache.org/licenses/LICENSE-2.0 +-- +-- Unless required by applicable law or agreed to in writing, software +-- distributed under the License is distributed on an "AS IS" BASIS, +-- WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. +-- See the License for the specific language governing permissions and +-- limitations under the License. + +{-# LANGUAGE RankNTypes #-} +{-# LANGUAGE ScopedTypeVariables #-} +{-# LANGUAGE TypeApplications #-} + +module Main where + +import Data.Functor.Const (Const(..)) + +import Test.Framework (defaultMain) +import Test.Framework.Providers.QuickCheck2 (testProperty) +import Test.QuickCheck ((===), forAllBlind, arbitrary) + +import Data.Vec.Short (rev) + +import Control.Batching + +main :: IO () +main = defaultMain + [ testProperty "deferred map" $ forAllBlind (arbitrary @(Int -> Int)) $ + \f xs -> + runBatching_ (fmap f) (traverse request xs) === map f xs + + , testProperty "deferred map2" $ forAllBlind (arbitrary @(Int -> Int)) $ + \f xss -> + runBatching_ (fmap f) (traverse batchRequest xss) === + map (map f) xss + + , testProperty "roundabout reverse" $ \ (xs :: [Int]) -> + runBatching_ rev (traverse request xs) === reverse xs + + , testProperty "list?" $ \ (xs :: [Int]) -> + runBatching (\rqs -> [rqs, rev rqs]) (traverse request xs) === + [xs, reverse xs] + + , testProperty "silliness" $ \ (xs :: [Int]) -> + runBatching (\rqs -> Const (length rqs)) (traverse request xs) === + Const (length xs) + ]