I'm not willing to fight too hard for it, but it feels more like the "right" solution than retaining a cut-and-paste copy of the same code and bifurcating further on each argument you want to consider such a degree of freedom. Like I say, I’m not against allowing polymorphism over unlifted-but-boxed types, and I can see the advantages. But it’s a separate proposal in its own right. Simon From: Edward Kmett [mailto:ekmett@gmail.com] Sent: 08 September 2015 09:30 To: Simon Marlow Cc: Simon Peyton Jones; Ryan Newton; Johan Tibell; Manuel M T Chakravarty; Chao-Hong Chen; ghc-devs; Ryan Scott; Ryan Yates Subject: Re: ArrayArrays Once you start to include all the other primitive types there is a bit more of an explosion. MVar#, TVar#, MutVar#, Small variants, etc. can all be modified to carry unlifted content. Being able to be parametric over that choice would permit a number of things in user land to do the same thing with an open-ended set of design possibilities that are rather hard to contemplate in advance. e.g. being able to abstract over them could let you just use a normal (,) to carry around unlifted parametric data types or being able to talk about [MVar# s a] drastically reducing the number of one off data types we need to invent. If you can talk about the machinery mentioned above then you can have typeclasses parameterized on an argument that could be either unlifted or lifted. I'm not willing to fight too hard for it, but it feels more like the "right" solution than retaining a cut-and-paste copy of the same code and bifurcating further on each argument you want to consider such a degree of freedom. As such it seems like a pretty big win for a comparatively minor change to the levity polymorphism machinery. -Edward On Tue, Sep 8, 2015 at 3:40 AM, Simon Marlow <marlowsd@gmail.com<mailto:marlowsd@gmail.com>> wrote: This would be very cool, however it's questionable whether it's worth it. Without any unlifted kind, we need - ArrayArray# - a set of new/read/write primops for every element type, either built-in or made from unsafeCoerce# With the unlifted kind, we would need - ArrayArray# - one set of new/read/write primops With levity polymorphism, we would need - none of this, Array# can be used So having an unlifted kind already kills a lot of the duplication, polymorphism only kills a bit more. Cheers Simon On 08/09/2015 00:14, Edward Kmett wrote: Assume we had the ability to talk about Levity in a new way and instead of just: data Levity = Lifted | Unlifted type * = TYPE 'Lifted type # = TYPE 'Unlifted we replace had a more nuanced notion of TYPE parameterized on another data type: data Levity = Lifted | Unlifted data Param = Composite | Simple Levity and we parameterized TYPE with a Param rather than Levity. Existing strange representations can continue to live in TYPE 'Composite (# Int# , Double #) :: TYPE 'Composite and we don't support parametricity in there, just like, currently we don't allow parametricity in #. We can include the undefined example from Richard's talk: undefined :: forall (v :: Param). v and ultimately lift it into his pi type when it is available just as before. But we could let consider TYPE ('Simple 'Unlifted) as a form of 'parametric #' covering unlifted things we're willing to allow polymorphism over because they are just pointers to something in the heap, that just happens to not be able to be _|_ or a thunk. In this setting, recalling that above, I modified Richard's TYPE to take a Param instead of Levity, we can define a type alias for things that live as a simple pointer to a heap allocated object: type GC (l :: Levity) = TYPE ('Simple l) type * = GC 'Lifted and then we can look at existing primitives generalized: Array# :: forall (l :: Levity) (a :: GC l). a -> GC 'Unlifted MutableArray# :: forall (l :: Levity) (a :: GC l). * -> a -> GC 'Unlifted SmallArray# :: forall (l :: Levity) (a :: GC l). a -> GC 'Unlifted SmallMutableArray# :: forall (l :: Levity) (a :: GC l). * -> a -> GC 'Unlifted MutVar# :: forall (l :: Levity) (a :: GC l). * -> a -> GC 'Unlifted MVar# :: forall (l :: Levity) (a :: GC l). * -> a -> GC 'Unlifted Weak#, StablePtr#, StableName#, etc. all can take similar modifications. Recall that an ArrayArray# was just an Array# hacked up to be able to hold onto the subset of # that is collectable. Almost all of the operations on these data types can work on the more general kind of argument. newArray# :: forall (s :: *) (l :: Levity) (a :: GC l). Int# -> a -> State# s -> (# State# s, MutableArray# s a #) writeArray# :: forall (s :: *) (l :: Levity) (a :: GC l). MutableArray# s a -> Int# -> a -> State# s -> State# s readArray# :: forall (s :: *) (l :: Levity) (a :: GC l). MutableArray# s a -> Int# -> State# s -> (# State# s, a #) etc. Only a couple of our existing primitives _can't_ generalize this way. The one that leaps to mind is atomicModifyMutVar, which would need to stay constrained to only work on arguments in *, because of the way it operates. With that we can still talk about MutableArray# s Int but now we can also talk about: MutableArray# s (MutableArray# s Int) without the layer of indirection through a box in * and without an explosion of primops. The same newFoo, readFoo, writeFoo machinery works for both kinds. The struct machinery doesn't get to take advantage of this, but it would let us clean house elsewhere in Prim and drastically improve the range of applicability of the existing primitives with nothing more than a small change to the levity machinery. I'm not attached to any of the names above, I coined them just to give us a concrete thing to talk about. Here I'm only proposing we extend machinery in GHC.Prim this way, but an interesting 'now that the barn door is open' question is to consider that our existing Haskell data types often admit a similar form of parametricity and nothing in principle prevents this from working for Maybe or [] and once you permit inference to fire across all of GC l then it seems to me that you'd start to get those same capabilities there as well when LevityPolymorphism was turned on. -Edward On Mon, Sep 7, 2015 at 5:56 PM, Simon Peyton Jones <simonpj@microsoft.com<mailto:simonpj@microsoft.com> <mailto:simonpj@microsoft.com<mailto:simonpj@microsoft.com>>> wrote: This could make the menagerie of ways to pack {Small}{Mutable}Array{Array}# references into a {Small}{Mutable}Array{Array}#' actually typecheck soundly, reducing the need for folks to descend into the use of the more evil structure primitives we're talking about, and letting us keep a few more principles around us.____ __ __ I’m lost. Can you give some concrete examples that illustrate how levity polymorphism will help us?____ Simon____ __ __ *From:*Edward Kmett [mailto:ekmett@gmail.com<mailto:ekmett@gmail.com> <mailto:ekmett@gmail.com<mailto:ekmett@gmail.com>>] *Sent:* 07 September 2015 21:17 *To:* Simon Peyton Jones *Cc:* Ryan Newton; Johan Tibell; Simon Marlow; Manuel M T Chakravarty; Chao-Hong Chen; ghc-devs; Ryan Scott; Ryan Yates *Subject:* Re: ArrayArrays____ __ __ I had a brief discussion with Richard during the Haskell Symposium about how we might be able to let parametricity help a bit in reducing the space of necessarily primops to a slightly more manageable level. ____ __ __ Notably, it'd be interesting to explore the ability to allow parametricity over the portion of # that is just a gcptr.____ __ __ We could do this if the levity polymorphism machinery was tweaked a bit. You could envision the ability to abstract over things in both * and the subset of # that are represented by a gcptr, then modifying the existing array primitives to be parametric in that choice of levity for their argument so long as it was of a "heap object" levity.____ __ __ This could make the menagerie of ways to pack {Small}{Mutable}Array{Array}# references into a {Small}{Mutable}Array{Array}#' actually typecheck soundly, reducing the need for folks to descend into the use of the more evil structure primitives we're talking about, and letting us keep a few more principles around us.____ __ __ Then in the cases like `atomicModifyMutVar#` where it needs to actually be in * rather than just a gcptr, due to the constructed field selectors it introduces on the heap then we could keep the existing less polymorphic type.____ __ __ -Edward____ __ __ On Mon, Sep 7, 2015 at 9:59 AM, Simon Peyton Jones <simonpj@microsoft.com<mailto:simonpj@microsoft.com> <mailto:simonpj@microsoft.com<mailto:simonpj@microsoft.com>>> wrote:____ It was fun to meet and discuss this.____ ____ Did someone volunteer to write a wiki page that describes the proposed design? And, I earnestly hope, also describes the menagerie of currently available array types and primops so that users can have some chance of picking the right one?!____ ____ Thanks____ ____ Simon____ ____ *From:*ghc-devs [mailto:ghc-devs-bounces@haskell.org<mailto:ghc-devs-bounces@haskell.org> <mailto:ghc-devs-bounces@haskell.org<mailto:ghc-devs-bounces@haskell.org>>] *On Behalf Of *Ryan Newton *Sent:* 31 August 2015 23:11 *To:* Edward Kmett; Johan Tibell *Cc:* Simon Marlow; Manuel M T Chakravarty; Chao-Hong Chen; ghc-devs; Ryan Scott; Ryan Yates *Subject:* Re: ArrayArrays____ ____ Dear Edward, Ryan Yates, and other interested parties -- ____ ____ So when should we meet up about this?____ ____ May I propose the Tues afternoon break for everyone at ICFP who is interested in this topic? We can meet out in the coffee area and congregate around Edward Kmett, who is tall and should be easy to find ;-).____ ____ I think Ryan is going to show us how to use his new primops for combined array + other fields in one heap object?____ ____ On Sat, Aug 29, 2015 at 9:24 PM Edward Kmett <ekmett@gmail.com<mailto:ekmett@gmail.com> <mailto:ekmett@gmail.com<mailto:ekmett@gmail.com>>> wrote:____ Without a custom primitive it doesn't help much there, you have to store the indirection to the mask.____ ____ With a custom primitive it should cut the on heap root-to-leaf path of everything in the HAMT in half. A shorter HashMap was actually one of the motivating factors for me doing this. It is rather astoundingly difficult to beat the performance of HashMap, so I had to start cheating pretty badly. ;)____ ____ -Edward____ ____ On Sat, Aug 29, 2015 at 5:45 PM, Johan Tibell <johan.tibell@gmail.com<mailto:johan.tibell@gmail.com> <mailto:johan.tibell@gmail.com<mailto:johan.tibell@gmail.com>>> wrote:____ I'd also be interested to chat at ICFP to see if I can use this for my HAMT implementation.____ ____ On Sat, Aug 29, 2015 at 3:07 PM, Edward Kmett <ekmett@gmail.com<mailto:ekmett@gmail.com> <mailto:ekmett@gmail.com<mailto:ekmett@gmail.com>>> wrote:____ Sounds good to me. Right now I'm just hacking up composable accessors for "typed slots" in a fairly lens-like fashion, and treating the set of slots I define and the 'new' function I build for the data type as its API, and build atop that. This could eventually graduate to template-haskell, but I'm not entirely satisfied with the solution I have. I currently distinguish between what I'm calling "slots" (things that point directly to another SmallMutableArrayArray# sans wrapper) and "fields" which point directly to the usual Haskell data types because unifying the two notions meant that I couldn't lift some coercions out "far enough" to make them vanish.____ ____ I'll be happy to run through my current working set of issues in person and -- as things get nailed down further -- in a longer lived medium than in personal conversations. ;)____ ____ -Edward____ ____ On Sat, Aug 29, 2015 at 7:59 AM, Ryan Newton <rrnewton@gmail.com<mailto:rrnewton@gmail.com> <mailto:rrnewton@gmail.com<mailto:rrnewton@gmail.com>>> wrote:____ I'd also love to meet up at ICFP and discuss this. I think the array primops plus a TH layer that lets (ab)use them many times without too much marginal cost sounds great. And I'd like to learn how we could be either early users of, or help with, this infrastructure.____ ____ CC'ing in Ryan Scot and Omer Agacan who may also be interested in dropping in on such discussions @ICFP, and Chao-Hong Chen, a Ph.D. student who is currently working on concurrent data structures in Haskell, but will not be at ICFP.____ ____ ____ On Fri, Aug 28, 2015 at 7:47 PM, Ryan Yates <fryguybob@gmail.com<mailto:fryguybob@gmail.com> <mailto:fryguybob@gmail.com<mailto:fryguybob@gmail.com>>> wrote:____ I completely agree. I would love to spend some time during ICFP and friends talking about what it could look like. My small array for STM changes for the RTS can be seen here [1]. It is on a branch somewhere between 7.8 and 7.10 and includes irrelevant STM bits and some confusing naming choices (sorry), but should cover all the details needed to implement it for a non-STM context. The biggest surprise for me was following small array too closely and having a word/byte offset miss-match [2]. [1]: https://github.com/fryguybob/ghc/compare/ghc-htm-bloom...fryguybob:ghc-htm-m... [2]: https://ghc.haskell.org/trac/ghc/ticket/10413 Ryan____ On Fri, Aug 28, 2015 at 10:09 PM, Edward Kmett <ekmett@gmail.com<mailto:ekmett@gmail.com> <mailto:ekmett@gmail.com<mailto:ekmett@gmail.com>>> wrote: > I'd love to have that last 10%, but its a lot of work to get there and more > importantly I don't know quite what it should look like. > > On the other hand, I do have a pretty good idea of how the primitives above > could be banged out and tested in a long evening, well in time for 7.12. And > as noted earlier, those remain useful even if a nicer typed version with an > extra level of indirection to the sizes is built up after. > > The rest sounds like a good graduate student project for someone who has > graduate students lying around. Maybe somebody at Indiana University who has > an interest in type theory and parallelism can find us one. =) > > -Edward > > On Fri, Aug 28, 2015 at 8:48 PM, Ryan Yates <fryguybob@gmail.com<mailto:fryguybob@gmail.com> <mailto:fryguybob@gmail.com<mailto:fryguybob@gmail.com>>> wrote: >> >> I think from my perspective, the motivation for getting the type >> checker involved is primarily bringing this to the level where users >> could be expected to build these structures. it is reasonable to >> think that there are people who want to use STM (a context with >> mutation already) to implement a straight forward data structure that >> avoids extra indirection penalty. There should be some places where >> knowing that things are field accesses rather then array indexing >> could be helpful, but I think GHC is good right now about handling >> constant offsets. In my code I don't do any bounds checking as I know >> I will only be accessing my arrays with constant indexes. I make >> wrappers for each field access and leave all the unsafe stuff in >> there. When things go wrong though, the compiler is no help. Maybe >> template Haskell that generates the appropriate wrappers is the right >> direction to go. >> There is another benefit for me when working with these as arrays in >> that it is quite simple and direct (given the hoops already jumped >> through) to play with alignment. I can ensure two pointers are never >> on the same cache-line by just spacing things out in the array. >> >> On Fri, Aug 28, 2015 at 7:33 PM, Edward Kmett <ekmett@gmail.com<mailto:ekmett@gmail.com> <mailto:ekmett@gmail.com<mailto:ekmett@gmail.com>>> wrote: >> > They just segfault at this level. ;) >> > >> > Sent from my iPhone >> > >> > On Aug 28, 2015, at 7:25 PM, Ryan Newton <rrnewton@gmail.com<mailto:rrnewton@gmail.com> <mailto:rrnewton@gmail.com<mailto:rrnewton@gmail.com>>> wrote: >> > >> > You presumably also save a bounds check on reads by hard-coding the >> > sizes? >> > >> > On Fri, Aug 28, 2015 at 3:39 PM, Edward Kmett <ekmett@gmail.com<mailto:ekmett@gmail.com> <mailto:ekmett@gmail.com<mailto:ekmett@gmail.com>>> wrote: >> >> >> >> Also there are 4 different "things" here, basically depending on two >> >> independent questions: >> >> >> >> a.) if you want to shove the sizes into the info table, and >> >> b.) if you want cardmarking. >> >> >> >> Versions with/without cardmarking for different sizes can be done >> >> pretty >> >> easily, but as noted, the infotable variants are pretty invasive. >> >> >> >> -Edward >> >> >> >> On Fri, Aug 28, 2015 at 6:36 PM, Edward Kmett <ekmett@gmail.com<mailto:ekmett@gmail.com> <mailto:ekmett@gmail.com<mailto:ekmett@gmail.com>>> wrote: >> >>> >> >>> Well, on the plus side you'd save 16 bytes per object, which adds up >> >>> if >> >>> they were small enough and there are enough of them. You get a bit >> >>> better >> >>> locality of reference in terms of what fits in the first cache line of >> >>> them. >> >>> >> >>> -Edward >> >>> >> >>> On Fri, Aug 28, 2015 at 6:14 PM, Ryan Newton <rrnewton@gmail.com<mailto:rrnewton@gmail.com> <mailto:rrnewton@gmail.com<mailto:rrnewton@gmail.com>>> >> >>> wrote: >> >>>> >> >>>> Yes. And for the short term I can imagine places we will settle with >> >>>> arrays even if it means tracking lengths unnecessarily and >> >>>> unsafeCoercing >> >>>> pointers whose types don't actually match their siblings. >> >>>> >> >>>> Is there anything to recommend the hacks mentioned for fixed sized >> >>>> array >> >>>> objects *other* than using them to fake structs? (Much to >> >>>> derecommend, as >> >>>> you mentioned!) >> >>>> >> >>>> On Fri, Aug 28, 2015 at 3:07 PM Edward Kmett <ekmett@gmail.com<mailto:ekmett@gmail.com> <mailto:ekmett@gmail.com<mailto:ekmett@gmail.com>>> >> >>>> wrote: >> >>>>> >> >>>>> I think both are useful, but the one you suggest requires a lot more >> >>>>> plumbing and doesn't subsume all of the usecases of the other. >> >>>>> >> >>>>> -Edward >> >>>>> >> >>>>> On Fri, Aug 28, 2015 at 5:51 PM, Ryan Newton <rrnewton@gmail.com<mailto:rrnewton@gmail.com> <mailto:rrnewton@gmail.com<mailto:rrnewton@gmail.com>>> >> >>>>> wrote: >> >>>>>> >> >>>>>> So that primitive is an array like thing (Same pointed type, >> >>>>>> unbounded >> >>>>>> length) with extra payload. >> >>>>>> >> >>>>>> I can see how we can do without structs if we have arrays, >> >>>>>> especially >> >>>>>> with the extra payload at front. But wouldn't the general solution >> >>>>>> for >> >>>>>> structs be one that that allows new user data type defs for # >> >>>>>> types? >> >>>>>> >> >>>>>> >> >>>>>> >> >>>>>> On Fri, Aug 28, 2015 at 4:43 PM Edward Kmett <ekmett@gmail.com<mailto:ekmett@gmail.com> <mailto:ekmett@gmail.com<mailto:ekmett@gmail.com>>> >> >>>>>> wrote: >> >>>>>>> >> >>>>>>> Some form of MutableStruct# with a known number of words and a >> >>>>>>> known >> >>>>>>> number of pointers is basically what Ryan Yates was suggesting >> >>>>>>> above, but >> >>>>>>> where the word counts were stored in the objects themselves. >> >>>>>>> >> >>>>>>> Given that it'd have a couple of words for those counts it'd >> >>>>>>> likely >> >>>>>>> want to be something we build in addition to MutVar# rather than a >> >>>>>>> replacement. >> >>>>>>> >> >>>>>>> On the other hand, if we had to fix those numbers and build info >> >>>>>>> tables that knew them, and typechecker support, for instance, it'd >> >>>>>>> get >> >>>>>>> rather invasive. >> >>>>>>> >> >>>>>>> Also, a number of things that we can do with the 'sized' versions >> >>>>>>> above, like working with evil unsized c-style arrays directly >> >>>>>>> inline at the >> >>>>>>> end of the structure cease to be possible, so it isn't even a pure >> >>>>>>> win if we >> >>>>>>> did the engineering effort. >> >>>>>>> >> >>>>>>> I think 90% of the needs I have are covered just by adding the one >> >>>>>>> primitive. The last 10% gets pretty invasive. >> >>>>>>> >> >>>>>>> -Edward >> >>>>>>> >> >>>>>>> On Fri, Aug 28, 2015 at 5:30 PM, Ryan Newton <rrnewton@gmail.com<mailto:rrnewton@gmail.com> <mailto:rrnewton@gmail.com<mailto:rrnewton@gmail.com>>> >> >>>>>>> wrote: >> >>>>>>>> >> >>>>>>>> I like the possibility of a general solution for mutable structs >> >>>>>>>> (like Ed said), and I'm trying to fully understand why it's hard. >> >>>>>>>> >> >>>>>>>> So, we can't unpack MutVar into constructors because of object >> >>>>>>>> identity problems. But what about directly supporting an >> >>>>>>>> extensible set of >> >>>>>>>> unlifted MutStruct# objects, generalizing (and even replacing) >> >>>>>>>> MutVar#? That >> >>>>>>>> may be too much work, but is it problematic otherwise? >> >>>>>>>> >> >>>>>>>> Needless to say, this is also critical if we ever want best in >> >>>>>>>> class >> >>>>>>>> lockfree mutable structures, just like their Stm and sequential >> >>>>>>>> counterparts. >> >>>>>>>> >> >>>>>>>> On Fri, Aug 28, 2015 at 4:43 AM Simon Peyton Jones >> >>>>>>>> <simonpj@microsoft.com<mailto:simonpj@microsoft.com> <mailto:simonpj@microsoft.com<mailto:simonpj@microsoft.com>>> wrote: >> >>>>>>>>> >> >>>>>>>>> At the very least I'll take this email and turn it into a short >> >>>>>>>>> article. >> >>>>>>>>> >> >>>>>>>>> Yes, please do make it into a wiki page on the GHC Trac, and >> >>>>>>>>> maybe >> >>>>>>>>> make a ticket for it. >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> Thanks >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> Simon >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> From: Edward Kmett [mailto:ekmett@gmail.com<mailto:ekmett@gmail.com> <mailto:ekmett@gmail.com<mailto:ekmett@gmail.com>>] >> >>>>>>>>> Sent: 27 August 2015 16:54 >> >>>>>>>>> To: Simon Peyton Jones >> >>>>>>>>> Cc: Manuel M T Chakravarty; Simon Marlow; ghc-devs >> >>>>>>>>> Subject: Re: ArrayArrays >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> An ArrayArray# is just an Array# with a modified invariant. It >> >>>>>>>>> points directly to other unlifted ArrayArray#'s or ByteArray#'s. >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> While those live in #, they are garbage collected objects, so >> >>>>>>>>> this >> >>>>>>>>> all lives on the heap. >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> They were added to make some of the DPH stuff fast when it has >> >>>>>>>>> to >> >>>>>>>>> deal with nested arrays. >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> I'm currently abusing them as a placeholder for a better thing. >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> The Problem >> >>>>>>>>> >> >>>>>>>>> ----------------- >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> Consider the scenario where you write a classic doubly-linked >> >>>>>>>>> list >> >>>>>>>>> in Haskell. >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> data DLL = DLL (IORef (Maybe DLL) (IORef (Maybe DLL) >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> Chasing from one DLL to the next requires following 3 pointers >> >>>>>>>>> on >> >>>>>>>>> the heap. >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> DLL ~> IORef (Maybe DLL) ~> MutVar# RealWorld (Maybe DLL) ~> >> >>>>>>>>> Maybe >> >>>>>>>>> DLL ~> DLL >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> That is 3 levels of indirection. >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> We can trim one by simply unpacking the IORef with >> >>>>>>>>> -funbox-strict-fields or UNPACK >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> We can trim another by adding a 'Nil' constructor for DLL and >> >>>>>>>>> worsening our representation. >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> data DLL = DLL !(IORef DLL) !(IORef DLL) | Nil >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> but now we're still stuck with a level of indirection >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> DLL ~> MutVar# RealWorld DLL ~> DLL >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> This means that every operation we perform on this structure >> >>>>>>>>> will >> >>>>>>>>> be about half of the speed of an implementation in most other >> >>>>>>>>> languages >> >>>>>>>>> assuming we're memory bound on loading things into cache! >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> Making Progress >> >>>>>>>>> >> >>>>>>>>> ---------------------- >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> I have been working on a number of data structures where the >> >>>>>>>>> indirection of going from something in * out to an object in # >> >>>>>>>>> which >> >>>>>>>>> contains the real pointer to my target and coming back >> >>>>>>>>> effectively doubles >> >>>>>>>>> my runtime. >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> We go out to the MutVar# because we are allowed to put the >> >>>>>>>>> MutVar# >> >>>>>>>>> onto the mutable list when we dirty it. There is a well defined >> >>>>>>>>> write-barrier. >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> I could change out the representation to use >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> data DLL = DLL (MutableArray# RealWorld DLL) | Nil >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> I can just store two pointers in the MutableArray# every time, >> >>>>>>>>> but >> >>>>>>>>> this doesn't help _much_ directly. It has reduced the amount of >> >>>>>>>>> distinct >> >>>>>>>>> addresses in memory I touch on a walk of the DLL from 3 per >> >>>>>>>>> object to 2. >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> I still have to go out to the heap from my DLL and get to the >> >>>>>>>>> array >> >>>>>>>>> object and then chase it to the next DLL and chase that to the >> >>>>>>>>> next array. I >> >>>>>>>>> do get my two pointers together in memory though. I'm paying for >> >>>>>>>>> a card >> >>>>>>>>> marking table as well, which I don't particularly need with just >> >>>>>>>>> two >> >>>>>>>>> pointers, but we can shed that with the "SmallMutableArray#" >> >>>>>>>>> machinery added >> >>>>>>>>> back in 7.10, which is just the old array code a a new data >> >>>>>>>>> type, which can >> >>>>>>>>> speed things up a bit when you don't have very big arrays: >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> data DLL = DLL (SmallMutableArray# RealWorld DLL) | Nil >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> But what if I wanted my object itself to live in # and have two >> >>>>>>>>> mutable fields and be able to share the sme write barrier? >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> An ArrayArray# points directly to other unlifted array types. >> >>>>>>>>> What >> >>>>>>>>> if we have one # -> * wrapper on the outside to deal with the >> >>>>>>>>> impedence >> >>>>>>>>> mismatch between the imperative world and Haskell, and then just >> >>>>>>>>> let the >> >>>>>>>>> ArrayArray#'s hold other arrayarrays. >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> data DLL = DLL (MutableArrayArray# RealWorld) >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> now I need to make up a new Nil, which I can just make be a >> >>>>>>>>> special >> >>>>>>>>> MutableArrayArray# I allocate on program startup. I can even >> >>>>>>>>> abuse pattern >> >>>>>>>>> synonyms. Alternately I can exploit the internals further to >> >>>>>>>>> make this >> >>>>>>>>> cheaper. >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> Then I can use the readMutableArrayArray# and >> >>>>>>>>> writeMutableArrayArray# calls to directly access the preceding >> >>>>>>>>> and next >> >>>>>>>>> entry in the linked list. >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> So now we have one DLL wrapper which just 'bootstraps me' into a >> >>>>>>>>> strict world, and everything there lives in #. >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> next :: DLL -> IO DLL >> >>>>>>>>> >> >>>>>>>>> next (DLL m) = IO $ \s -> case readMutableArrayArray# s of >> >>>>>>>>> >> >>>>>>>>> (# s', n #) -> (# s', DLL n #) >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> >> >>>>>>>>> It turns out GHC is quite happy to optimize all of that code to >> >>>>>>>>> keep things unboxed. The 'DLL' wrappers get removed pretty >> >>>>>>>>> easily when they >> >>>>>>>>> are known strict and you chain