Re: Fwd: Restricted sums in BoxedRep

[Ryan Yates]

>> related.
>>>>>>>>>
>>>>>>>>> Phrased more succinctly: gc perf dominates large heap / many core
>>>>>>>>> computation in Haskell via sensitivity to allocation volume / mutation
>>>>>>>>> volume (to ensure generational hypothesis stays valid), and providing 
>>>>>>>>> tools
>>>>>>>>> to incrementally reduce the pressure with local changes would be good.
>>>>>>>>>
>>>>>>>>> So I’d propose / suggest that a baby step towards what david asks
>>>>>>>>> would be for us to work out some manner of unboxed tvar/mvar ref 
>>>>>>>>> machinery
>>>>>>>>> that supports unboxed values.
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> On Thu, Oct 15, 2020 at 5:32 AM Andreas Klebinger <
>>>>>>>>> klebinger.andr...@gmx.at> wrote:
>>>>>>>>>
>>>>>>>>>> From a implementors perspective my main questions would be:
>>>>>>>>>>
>>>>>>>>>> * How big is the benefit in practice? How many use cases are
>>>>>>>>>> there?
>>>>>>>>>> * How bad are the costs? (Runtime overhead, rts complexity, ...)
>>>>>>>>>>
>>>>>>>>>> The details of how this would be exposed to a user would are
>>>>>>>>>> important.
>>>>>>>>>> But if the costs are too high for the drawbacks then it becomes a
>>>>>>>>>> moot point.
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> David Feuer schrieb am 14.10.2020 um 22:21:
>>>>>>>>>>
>>>>>>>>>> Forwarded from Andrew Martin below. I think we want more than
>>>>>>>>>> just Maybe (more than one null), but the nesting I described is 
>>>>>>>>>> certainly
>>>>>>>>>> more convenience than necessity.
>>>>>>>>>>
>>>>>>>>>> -- Forwarded message -
>>>>>>>>>> From: Andrew Martin 
>>>>>>>>>> Date: Wed, Oct 14, 2020, 4:14 PM
>>>>>>>>>> Subject: Re: Restricted sums in BoxedRep
>>>>>>>>>> To: David Feuer 
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> You'll have to forward this to the ghc-devs list to share it with
>>>>>>>>>> others since I'm not currently subscribed to it, but I've had this 
>>>>>>>>>> same
>>>>>>>>>> thought before. It is discussed at
>>>>>>>>>> https://github.com/andrewthad/impure-containers/issues/12.
>>>>>>>>>> Here's the relevant excerpt:
>>>>>>>>>>
>>>>>>>>>>> Relatedly, I was thinking the other day that after finishing
>>>>>>>>>>> implementing
>>>>>>>>>>> https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0203-pointer-rep.rst,
>>>>>>>>>>> I should really look at seeing if it's possible to add this
>>>>>>>>>>> maybe-of-a-lifted value trick straight to GHC. I think that with:
>>>>>>>>>>>
>>>>>>>>>>> data RuntimpRep
>>>>>>>>>>>   = BoxedRep Levity
>>>>>>>>>>>   | MaybeBoxedRep Levity
>>>>>>>>>>>   | IntRep
>>>>>>>>>>>   | ...
>>>>>>>>>>>
>>>>>>>>>>> data BuiltinMaybe :: forall (v :: Levity). TYPE v -> TYPE 
>>>>>>>>>>> ('MaybeBoxedRep v)
>>>>>>>>>>>
>>>>>>>>>>> This doesn't have the nesting issues because the kind system
>>>>>>>>>>> prevents nesting. But anyway, back to the original question. I would
>>>>>>>>>>> recommend not using Maybe.Unsafe and using unpacked-maybe
>>>>>>>>>>> instead. The latter is definitely safe, and it only costs an extra 
>>>>>>>>>>> machine
>>>>>>>>>>> word of space in each data constructor it gets used in, and it 
>>>>>>>>>>> doesn't
>>>>>>>>>>> introduce more indirections.
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> On Tue, Oct 13, 2020 at 5:47 PM David Feuer <
>>>>>>>>>> david.fe...@gmail.com> wrote:
>>>>>>>>>>
>>>>>>>>>>> Null pointers are widely known to be a lousy language feature in
>>>>>>>>>>> general, but there are certain situations where they're *really* 
>>>>>>>>>>> useful for
>>>>>>>>>>> compact representation. For example, we define
>>>>>>>>>>>
>>>>>>>>>>> newtype TMVar a = TMVar (TVar (Maybe a))
>>>>>>>>>>>
>>>>>>>>>>> We don't, however, actually use the fact that (Maybe a) is
>>>>>>>>>>> lifted. So we could represent this much more efficiently using 
>>>>>>>>>>> something
>>>>>>>>>>> like
>>>>>>>>>>>
>>>>>>>>>>> newtype TMVar a = TMVar (TVar a)
>>>>>>>>>>>
>>>>>>>>>>> where Nothing is represented by a distinguished "null" pointer.
>>>>>>>>>>>
>>>>>>>>>>> While it's possible to implement this sort of thing in user code
>>>>>>>>>>> (with lots of fuss and care), it's not very nice at all. What I'd 
>>>>>>>>>>> really
>>>>>>>>>>> like to be able to do is represent certain kinds of sums like this 
>>>>>>>>>>> natively.
>>>>>>>>>>>
>>>>>>>>>>> Now that we're getting BoxedRep, I think we can probably make it
>>>>>>>>>>> happen. The trick is to add a special Levity constructor 
>>>>>>>>>>> representing sums
>>>>>>>>>>> of particular shapes. Specifically, we can represent a type like 
>>>>>>>>>>> this if it
>>>>>>>>>>> is a possibly-nested sum which, when flattened into a single sum, 
>>>>>>>>>>> consists
>>>>>>>>>>> of some number of nullary tuples and at most one Lifted or Unlifted 
>>>>>>>>>>> type.
>>>>>>>>>>> Then we can have (inline) primops to convert between the BoxedRep 
>>>>>>>>>>> and the
>>>>>>>>>>> sum-of-sums representations.
>>>>>>>>>>>
>>>>>>>>>>> Anyone have thoughts on details for what the Levity constructor
>>>>>>>>>>> arguments might look like?
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> --
>>>>>>>>>> -Andrew Thaddeus Martin
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> ___
>>>>>>>>>> ghc-devs mailing 
>>>>>>>>>> listghc-devs@haskell.orghttp://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> ___
>>>>>>>>>> ghc-devs mailing list
>>>>>>>>>> ghc-devs@haskell.org
>>>>>>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
>>>>>>>>>>
>>>>>>>>> ___
>>>>> ghc-devs mailing list
>>>>> ghc-devs@haskell.org
>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
>>>>>
>>>>
___
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Re: Fwd: Restricted sums in BoxedRep

[Carter Schonwald]

Ryan: would an unboxed value Mvar, Eg to write StrictMvar (), avoid
creating extra gc pressure / traversal a?

On Thu, Oct 15, 2020 at 4:23 PM Ryan Yates  wrote:

> Ah yes.  In my research work I extended Mutable Constructor fields to
> allow a mutable array field to help with that problem.  This allowed me to
> have a Nil constructor in a sum type and a Node constructor with normal
> fields as well as an array of mutable fields.  Pointer tagging worked as
> expected so a case match on a dereference of a field would not need to
> follow the pointer and no intermediate objects were between Node objects.
>
>
>
> On Thu, Oct 15, 2020 at 4:08 PM David Feuer  wrote:
>
>> This might be lost in the noise for MVar and TVar, but for arrays, there
>> are tremendous advantages to cutting out the extra heap objects.
>>
>> On Thu, Oct 15, 2020, 1:10 PM Ryan Yates  wrote:
>>
>>> I haven't been following this discussion too closely, but in my research
>>> work I found that some of the benefits that I wanted in this direction were
>>> already there with pointer tagging.
>>>
>>> On Thu, Oct 15, 2020 at 12:59 PM David Feuer 
>>> wrote:
>>>
>>>> Yes, that's something quite different. We'd need a whole different heap
>>>> object type for such MVars and TVars. My approach covers the case where the
>>>> unboxed thing can only take on a few values, for some value of "a few"
>>>> which, depending on implementation, may or may not be very small. If the
>>>> nulls point to actual heap objects in pre-allocated pinned memory (say),
>>>> then up to 64 or so might be reasonable. If they point to "invalid" address
>>>> space, then the numbers could go up a good bit.
>>>>
>>>> On Thu, Oct 15, 2020, 12:50 PM Carter Schonwald <
>>>> carter.schonw...@gmail.com> wrote:
>>>>
>>>>> Indeed, I mean things that aren’t pointery, and could be represented
>>>>> by a tvar paired with a mutable byte array or mvar with mutable byte 
>>>>> array,
>>>>> but which we’d want considered as a single heap object from the rts/gc
>>>>> perspective.
>>>>>
>>>>> On Thu, Oct 15, 2020 at 11:58 AM David Feuer 
>>>>> wrote:
>>>>>
>>>>>> Sorry, unlifted, not unboxed...
>>>>>>
>>>>>> On Thu, Oct 15, 2020, 11:57 AM David Feuer 
>>>>>> wrote:
>>>>>>
>>>>>>> Putting unboxed things in TVar, MVar, etc., is part of Andrew
>>>>>>> Martin's accepted BoxedRep proposal.
>>>>>>>
>>>>>>> On Thu, Oct 15, 2020, 11:44 AM Carter Schonwald <
>>>>>>> carter.schonw...@gmail.com> wrote:
>>>>>>>
>>>>>>>> A related idea that came up recently and is perhaps simpler ties
>>>>>>>> into this via the lens of having unboxed Mvars/tvars (even if it’s
>>>>>>>> restricted to just things we can embed in a word64#)
>>>>>>>>
>>>>>>>> This came up in
>>>>>>>> https://gitlab.haskell.org/ghc/ghc/-/issues/18798#note_307410,
>>>>>>>> where viktor had millions of independent mvars holding what’s 
>>>>>>>> essentially a
>>>>>>>> strict unit ()!
>>>>>>>>
>>>>>>>> The motivation in this later scenario is that in high concurrency
>>>>>>>> settings, the less trivial stuff the gc needs to trace under updates, 
>>>>>>>> the
>>>>>>>> better ghc scales.
>>>>>>>>
>>>>>>>> This may not be a use case david has in mind, but certainly seems
>>>>>>>> related.
>>>>>>>>
>>>>>>>> Phrased more succinctly: gc perf dominates large heap / many core
>>>>>>>> computation in Haskell via sensitivity to allocation volume / mutation
>>>>>>>> volume (to ensure generational hypothesis stays valid), and providing 
>>>>>>>> tools
>>>>>>>> to incrementally reduce the pressure with local changes would be good.
>>>>>>>>
>>>>>>>> So I’d propose / suggest that a baby step towards what david asks
>>>>>>>> would be for us to work out some manner of unboxed tvar/mvar ref 
>>>&

Re: Fwd: Restricted sums in BoxedRep

[Ryan Yates]

Ah yes.  In my research work I extended Mutable Constructor fields to allow
a mutable array field to help with that problem.  This allowed me to have a
Nil constructor in a sum type and a Node constructor with normal fields as
well as an array of mutable fields.  Pointer tagging worked as expected so
a case match on a dereference of a field would not need to follow the
pointer and no intermediate objects were between Node objects.



On Thu, Oct 15, 2020 at 4:08 PM David Feuer  wrote:

> This might be lost in the noise for MVar and TVar, but for arrays, there
> are tremendous advantages to cutting out the extra heap objects.
>
> On Thu, Oct 15, 2020, 1:10 PM Ryan Yates  wrote:
>
>> I haven't been following this discussion too closely, but in my research
>> work I found that some of the benefits that I wanted in this direction were
>> already there with pointer tagging.
>>
>> On Thu, Oct 15, 2020 at 12:59 PM David Feuer 
>> wrote:
>>
>>> Yes, that's something quite different. We'd need a whole different heap
>>> object type for such MVars and TVars. My approach covers the case where the
>>> unboxed thing can only take on a few values, for some value of "a few"
>>> which, depending on implementation, may or may not be very small. If the
>>> nulls point to actual heap objects in pre-allocated pinned memory (say),
>>> then up to 64 or so might be reasonable. If they point to "invalid" address
>>> space, then the numbers could go up a good bit.
>>>
>>> On Thu, Oct 15, 2020, 12:50 PM Carter Schonwald <
>>> carter.schonw...@gmail.com> wrote:
>>>
>>>> Indeed, I mean things that aren’t pointery, and could be represented by
>>>> a tvar paired with a mutable byte array or mvar with mutable byte array,
>>>> but which we’d want considered as a single heap object from the rts/gc
>>>> perspective.
>>>>
>>>> On Thu, Oct 15, 2020 at 11:58 AM David Feuer 
>>>> wrote:
>>>>
>>>>> Sorry, unlifted, not unboxed...
>>>>>
>>>>> On Thu, Oct 15, 2020, 11:57 AM David Feuer 
>>>>> wrote:
>>>>>
>>>>>> Putting unboxed things in TVar, MVar, etc., is part of Andrew
>>>>>> Martin's accepted BoxedRep proposal.
>>>>>>
>>>>>> On Thu, Oct 15, 2020, 11:44 AM Carter Schonwald <
>>>>>> carter.schonw...@gmail.com> wrote:
>>>>>>
>>>>>>> A related idea that came up recently and is perhaps simpler ties
>>>>>>> into this via the lens of having unboxed Mvars/tvars (even if it’s
>>>>>>> restricted to just things we can embed in a word64#)
>>>>>>>
>>>>>>> This came up in
>>>>>>> https://gitlab.haskell.org/ghc/ghc/-/issues/18798#note_307410,
>>>>>>> where viktor had millions of independent mvars holding what’s 
>>>>>>> essentially a
>>>>>>> strict unit ()!
>>>>>>>
>>>>>>> The motivation in this later scenario is that in high concurrency
>>>>>>> settings, the less trivial stuff the gc needs to trace under updates, 
>>>>>>> the
>>>>>>> better ghc scales.
>>>>>>>
>>>>>>> This may not be a use case david has in mind, but certainly seems
>>>>>>> related.
>>>>>>>
>>>>>>> Phrased more succinctly: gc perf dominates large heap / many core
>>>>>>> computation in Haskell via sensitivity to allocation volume / mutation
>>>>>>> volume (to ensure generational hypothesis stays valid), and providing 
>>>>>>> tools
>>>>>>> to incrementally reduce the pressure with local changes would be good.
>>>>>>>
>>>>>>> So I’d propose / suggest that a baby step towards what david asks
>>>>>>> would be for us to work out some manner of unboxed tvar/mvar ref 
>>>>>>> machinery
>>>>>>> that supports unboxed values.
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> On Thu, Oct 15, 2020 at 5:32 AM Andreas Klebinger <
>>>>>>> klebinger.andr...@gmx.at> wrote:
>>>>>>>
>>>>>>>> From a implementors perspective my main questions would be:
>>>>>>>>
>>>>>>

Re: Fwd: Restricted sums in BoxedRep

[David Feuer]

This might be lost in the noise for MVar and TVar, but for arrays, there
are tremendous advantages to cutting out the extra heap objects.

On Thu, Oct 15, 2020, 1:10 PM Ryan Yates  wrote:

> I haven't been following this discussion too closely, but in my research
> work I found that some of the benefits that I wanted in this direction were
> already there with pointer tagging.
>
> On Thu, Oct 15, 2020 at 12:59 PM David Feuer 
> wrote:
>
>> Yes, that's something quite different. We'd need a whole different heap
>> object type for such MVars and TVars. My approach covers the case where the
>> unboxed thing can only take on a few values, for some value of "a few"
>> which, depending on implementation, may or may not be very small. If the
>> nulls point to actual heap objects in pre-allocated pinned memory (say),
>> then up to 64 or so might be reasonable. If they point to "invalid" address
>> space, then the numbers could go up a good bit.
>>
>> On Thu, Oct 15, 2020, 12:50 PM Carter Schonwald <
>> carter.schonw...@gmail.com> wrote:
>>
>>> Indeed, I mean things that aren’t pointery, and could be represented by
>>> a tvar paired with a mutable byte array or mvar with mutable byte array,
>>> but which we’d want considered as a single heap object from the rts/gc
>>> perspective.
>>>
>>> On Thu, Oct 15, 2020 at 11:58 AM David Feuer 
>>> wrote:
>>>
>>>> Sorry, unlifted, not unboxed...
>>>>
>>>> On Thu, Oct 15, 2020, 11:57 AM David Feuer 
>>>> wrote:
>>>>
>>>>> Putting unboxed things in TVar, MVar, etc., is part of Andrew Martin's
>>>>> accepted BoxedRep proposal.
>>>>>
>>>>> On Thu, Oct 15, 2020, 11:44 AM Carter Schonwald <
>>>>> carter.schonw...@gmail.com> wrote:
>>>>>
>>>>>> A related idea that came up recently and is perhaps simpler ties into
>>>>>> this via the lens of having unboxed Mvars/tvars (even if it’s restricted 
>>>>>> to
>>>>>> just things we can embed in a word64#)
>>>>>>
>>>>>> This came up in
>>>>>> https://gitlab.haskell.org/ghc/ghc/-/issues/18798#note_307410, where
>>>>>> viktor had millions of independent mvars holding what’s essentially a
>>>>>> strict unit ()!
>>>>>>
>>>>>> The motivation in this later scenario is that in high concurrency
>>>>>> settings, the less trivial stuff the gc needs to trace under updates, the
>>>>>> better ghc scales.
>>>>>>
>>>>>> This may not be a use case david has in mind, but certainly seems
>>>>>> related.
>>>>>>
>>>>>> Phrased more succinctly: gc perf dominates large heap / many core
>>>>>> computation in Haskell via sensitivity to allocation volume / mutation
>>>>>> volume (to ensure generational hypothesis stays valid), and providing 
>>>>>> tools
>>>>>> to incrementally reduce the pressure with local changes would be good.
>>>>>>
>>>>>> So I’d propose / suggest that a baby step towards what david asks
>>>>>> would be for us to work out some manner of unboxed tvar/mvar ref 
>>>>>> machinery
>>>>>> that supports unboxed values.
>>>>>>
>>>>>>
>>>>>>
>>>>>> On Thu, Oct 15, 2020 at 5:32 AM Andreas Klebinger <
>>>>>> klebinger.andr...@gmx.at> wrote:
>>>>>>
>>>>>>> From a implementors perspective my main questions would be:
>>>>>>>
>>>>>>> * How big is the benefit in practice? How many use cases are there?
>>>>>>> * How bad are the costs? (Runtime overhead, rts complexity, ...)
>>>>>>>
>>>>>>> The details of how this would be exposed to a user would are
>>>>>>> important.
>>>>>>> But if the costs are too high for the drawbacks then it becomes a
>>>>>>> moot point.
>>>>>>>
>>>>>>>
>>>>>>> David Feuer schrieb am 14.10.2020 um 22:21:
>>>>>>>
>>>>>>> Forwarded from Andrew Martin below. I think we want more than just
>>>>>>> Maybe (more than one null), but the nesting I described is certainly 
>>>>>&g

Re: Fwd: Restricted sums in BoxedRep

[Ryan Yates]

I haven't been following this discussion too closely, but in my research
work I found that some of the benefits that I wanted in this direction were
already there with pointer tagging.

On Thu, Oct 15, 2020 at 12:59 PM David Feuer  wrote:

> Yes, that's something quite different. We'd need a whole different heap
> object type for such MVars and TVars. My approach covers the case where the
> unboxed thing can only take on a few values, for some value of "a few"
> which, depending on implementation, may or may not be very small. If the
> nulls point to actual heap objects in pre-allocated pinned memory (say),
> then up to 64 or so might be reasonable. If they point to "invalid" address
> space, then the numbers could go up a good bit.
>
> On Thu, Oct 15, 2020, 12:50 PM Carter Schonwald <
> carter.schonw...@gmail.com> wrote:
>
>> Indeed, I mean things that aren’t pointery, and could be represented by a
>> tvar paired with a mutable byte array or mvar with mutable byte array, but
>> which we’d want considered as a single heap object from the rts/gc
>> perspective.
>>
>> On Thu, Oct 15, 2020 at 11:58 AM David Feuer 
>> wrote:
>>
>>> Sorry, unlifted, not unboxed...
>>>
>>> On Thu, Oct 15, 2020, 11:57 AM David Feuer 
>>> wrote:
>>>
>>>> Putting unboxed things in TVar, MVar, etc., is part of Andrew Martin's
>>>> accepted BoxedRep proposal.
>>>>
>>>> On Thu, Oct 15, 2020, 11:44 AM Carter Schonwald <
>>>> carter.schonw...@gmail.com> wrote:
>>>>
>>>>> A related idea that came up recently and is perhaps simpler ties into
>>>>> this via the lens of having unboxed Mvars/tvars (even if it’s restricted 
>>>>> to
>>>>> just things we can embed in a word64#)
>>>>>
>>>>> This came up in
>>>>> https://gitlab.haskell.org/ghc/ghc/-/issues/18798#note_307410, where
>>>>> viktor had millions of independent mvars holding what’s essentially a
>>>>> strict unit ()!
>>>>>
>>>>> The motivation in this later scenario is that in high concurrency
>>>>> settings, the less trivial stuff the gc needs to trace under updates, the
>>>>> better ghc scales.
>>>>>
>>>>> This may not be a use case david has in mind, but certainly seems
>>>>> related.
>>>>>
>>>>> Phrased more succinctly: gc perf dominates large heap / many core
>>>>> computation in Haskell via sensitivity to allocation volume / mutation
>>>>> volume (to ensure generational hypothesis stays valid), and providing 
>>>>> tools
>>>>> to incrementally reduce the pressure with local changes would be good.
>>>>>
>>>>> So I’d propose / suggest that a baby step towards what david asks
>>>>> would be for us to work out some manner of unboxed tvar/mvar ref machinery
>>>>> that supports unboxed values.
>>>>>
>>>>>
>>>>>
>>>>> On Thu, Oct 15, 2020 at 5:32 AM Andreas Klebinger <
>>>>> klebinger.andr...@gmx.at> wrote:
>>>>>
>>>>>> From a implementors perspective my main questions would be:
>>>>>>
>>>>>> * How big is the benefit in practice? How many use cases are there?
>>>>>> * How bad are the costs? (Runtime overhead, rts complexity, ...)
>>>>>>
>>>>>> The details of how this would be exposed to a user would are
>>>>>> important.
>>>>>> But if the costs are too high for the drawbacks then it becomes a
>>>>>> moot point.
>>>>>>
>>>>>>
>>>>>> David Feuer schrieb am 14.10.2020 um 22:21:
>>>>>>
>>>>>> Forwarded from Andrew Martin below. I think we want more than just
>>>>>> Maybe (more than one null), but the nesting I described is certainly more
>>>>>> convenience than necessity.
>>>>>>
>>>>>> -- Forwarded message -
>>>>>> From: Andrew Martin 
>>>>>> Date: Wed, Oct 14, 2020, 4:14 PM
>>>>>> Subject: Re: Restricted sums in BoxedRep
>>>>>> To: David Feuer 
>>>>>>
>>>>>>
>>>>>> You'll have to forward this to the ghc-devs list to share it with
>>>>>> others since I'm not currently subscribed to 

Re: Fwd: Restricted sums in BoxedRep

[David Feuer]

Yes, that's something quite different. We'd need a whole different heap
object type for such MVars and TVars. My approach covers the case where the
unboxed thing can only take on a few values, for some value of "a few"
which, depending on implementation, may or may not be very small. If the
nulls point to actual heap objects in pre-allocated pinned memory (say),
then up to 64 or so might be reasonable. If they point to "invalid" address
space, then the numbers could go up a good bit.

On Thu, Oct 15, 2020, 12:50 PM Carter Schonwald 
wrote:

> Indeed, I mean things that aren’t pointery, and could be represented by a
> tvar paired with a mutable byte array or mvar with mutable byte array, but
> which we’d want considered as a single heap object from the rts/gc
> perspective.
>
> On Thu, Oct 15, 2020 at 11:58 AM David Feuer 
> wrote:
>
>> Sorry, unlifted, not unboxed...
>>
>> On Thu, Oct 15, 2020, 11:57 AM David Feuer  wrote:
>>
>>> Putting unboxed things in TVar, MVar, etc., is part of Andrew Martin's
>>> accepted BoxedRep proposal.
>>>
>>> On Thu, Oct 15, 2020, 11:44 AM Carter Schonwald <
>>> carter.schonw...@gmail.com> wrote:
>>>
>>>> A related idea that came up recently and is perhaps simpler ties into
>>>> this via the lens of having unboxed Mvars/tvars (even if it’s restricted to
>>>> just things we can embed in a word64#)
>>>>
>>>> This came up in
>>>> https://gitlab.haskell.org/ghc/ghc/-/issues/18798#note_307410, where
>>>> viktor had millions of independent mvars holding what’s essentially a
>>>> strict unit ()!
>>>>
>>>> The motivation in this later scenario is that in high concurrency
>>>> settings, the less trivial stuff the gc needs to trace under updates, the
>>>> better ghc scales.
>>>>
>>>> This may not be a use case david has in mind, but certainly seems
>>>> related.
>>>>
>>>> Phrased more succinctly: gc perf dominates large heap / many core
>>>> computation in Haskell via sensitivity to allocation volume / mutation
>>>> volume (to ensure generational hypothesis stays valid), and providing tools
>>>> to incrementally reduce the pressure with local changes would be good.
>>>>
>>>> So I’d propose / suggest that a baby step towards what david asks would
>>>> be for us to work out some manner of unboxed tvar/mvar ref machinery that
>>>> supports unboxed values.
>>>>
>>>>
>>>>
>>>> On Thu, Oct 15, 2020 at 5:32 AM Andreas Klebinger <
>>>> klebinger.andr...@gmx.at> wrote:
>>>>
>>>>> From a implementors perspective my main questions would be:
>>>>>
>>>>> * How big is the benefit in practice? How many use cases are there?
>>>>> * How bad are the costs? (Runtime overhead, rts complexity, ...)
>>>>>
>>>>> The details of how this would be exposed to a user would are important.
>>>>> But if the costs are too high for the drawbacks then it becomes a moot
>>>>> point.
>>>>>
>>>>>
>>>>> David Feuer schrieb am 14.10.2020 um 22:21:
>>>>>
>>>>> Forwarded from Andrew Martin below. I think we want more than just
>>>>> Maybe (more than one null), but the nesting I described is certainly more
>>>>> convenience than necessity.
>>>>>
>>>>> -- Forwarded message -
>>>>> From: Andrew Martin 
>>>>> Date: Wed, Oct 14, 2020, 4:14 PM
>>>>> Subject: Re: Restricted sums in BoxedRep
>>>>> To: David Feuer 
>>>>>
>>>>>
>>>>> You'll have to forward this to the ghc-devs list to share it with
>>>>> others since I'm not currently subscribed to it, but I've had this same
>>>>> thought before. It is discussed at
>>>>> https://github.com/andrewthad/impure-containers/issues/12. Here's the
>>>>> relevant excerpt:
>>>>>
>>>>>> Relatedly, I was thinking the other day that after finishing
>>>>>> implementing
>>>>>> https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0203-pointer-rep.rst,
>>>>>> I should really look at seeing if it's possible to add this
>>>>>> maybe-of-a-lifted value trick straight to GHC. I think that with:
>>>>>>
>>>>&

Re: Fwd: Restricted sums in BoxedRep

[Carter Schonwald]

Indeed, I mean things that aren’t pointery, and could be represented by a
tvar paired with a mutable byte array or mvar with mutable byte array, but
which we’d want considered as a single heap object from the rts/gc
perspective.

On Thu, Oct 15, 2020 at 11:58 AM David Feuer  wrote:

> Sorry, unlifted, not unboxed...
>
> On Thu, Oct 15, 2020, 11:57 AM David Feuer  wrote:
>
>> Putting unboxed things in TVar, MVar, etc., is part of Andrew Martin's
>> accepted BoxedRep proposal.
>>
>> On Thu, Oct 15, 2020, 11:44 AM Carter Schonwald <
>> carter.schonw...@gmail.com> wrote:
>>
>>> A related idea that came up recently and is perhaps simpler ties into
>>> this via the lens of having unboxed Mvars/tvars (even if it’s restricted to
>>> just things we can embed in a word64#)
>>>
>>> This came up in
>>> https://gitlab.haskell.org/ghc/ghc/-/issues/18798#note_307410, where
>>> viktor had millions of independent mvars holding what’s essentially a
>>> strict unit ()!
>>>
>>> The motivation in this later scenario is that in high concurrency
>>> settings, the less trivial stuff the gc needs to trace under updates, the
>>> better ghc scales.
>>>
>>> This may not be a use case david has in mind, but certainly seems
>>> related.
>>>
>>> Phrased more succinctly: gc perf dominates large heap / many core
>>> computation in Haskell via sensitivity to allocation volume / mutation
>>> volume (to ensure generational hypothesis stays valid), and providing tools
>>> to incrementally reduce the pressure with local changes would be good.
>>>
>>> So I’d propose / suggest that a baby step towards what david asks would
>>> be for us to work out some manner of unboxed tvar/mvar ref machinery that
>>> supports unboxed values.
>>>
>>>
>>>
>>> On Thu, Oct 15, 2020 at 5:32 AM Andreas Klebinger <
>>> klebinger.andr...@gmx.at> wrote:
>>>
>>>> From a implementors perspective my main questions would be:
>>>>
>>>> * How big is the benefit in practice? How many use cases are there?
>>>> * How bad are the costs? (Runtime overhead, rts complexity, ...)
>>>>
>>>> The details of how this would be exposed to a user would are important.
>>>> But if the costs are too high for the drawbacks then it becomes a moot
>>>> point.
>>>>
>>>>
>>>> David Feuer schrieb am 14.10.2020 um 22:21:
>>>>
>>>> Forwarded from Andrew Martin below. I think we want more than just
>>>> Maybe (more than one null), but the nesting I described is certainly more
>>>> convenience than necessity.
>>>>
>>>> -- Forwarded message -
>>>> From: Andrew Martin 
>>>> Date: Wed, Oct 14, 2020, 4:14 PM
>>>> Subject: Re: Restricted sums in BoxedRep
>>>> To: David Feuer 
>>>>
>>>>
>>>> You'll have to forward this to the ghc-devs list to share it with
>>>> others since I'm not currently subscribed to it, but I've had this same
>>>> thought before. It is discussed at
>>>> https://github.com/andrewthad/impure-containers/issues/12. Here's the
>>>> relevant excerpt:
>>>>
>>>>> Relatedly, I was thinking the other day that after finishing
>>>>> implementing
>>>>> https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0203-pointer-rep.rst,
>>>>> I should really look at seeing if it's possible to add this
>>>>> maybe-of-a-lifted value trick straight to GHC. I think that with:
>>>>>
>>>>> data RuntimpRep
>>>>>   = BoxedRep Levity
>>>>>   | MaybeBoxedRep Levity
>>>>>   | IntRep
>>>>>   | ...
>>>>>
>>>>> data BuiltinMaybe :: forall (v :: Levity). TYPE v -> TYPE ('MaybeBoxedRep 
>>>>> v)
>>>>>
>>>>> This doesn't have the nesting issues because the kind system prevents
>>>>> nesting. But anyway, back to the original question. I would recommend not
>>>>> using Maybe.Unsafe and using unpacked-maybe instead. The latter is
>>>>> definitely safe, and it only costs an extra machine word of space in each
>>>>> data constructor it gets used in, and it doesn't introduce more
>>>>> indirections.
>>>>>
>>>>
>>>> On Tue, Oct 13, 2

Re: Fwd: Restricted sums in BoxedRep

[David Feuer]

Sorry, unlifted, not unboxed...

On Thu, Oct 15, 2020, 11:57 AM David Feuer  wrote:

> Putting unboxed things in TVar, MVar, etc., is part of Andrew Martin's
> accepted BoxedRep proposal.
>
> On Thu, Oct 15, 2020, 11:44 AM Carter Schonwald <
> carter.schonw...@gmail.com> wrote:
>
>> A related idea that came up recently and is perhaps simpler ties into
>> this via the lens of having unboxed Mvars/tvars (even if it’s restricted to
>> just things we can embed in a word64#)
>>
>> This came up in
>> https://gitlab.haskell.org/ghc/ghc/-/issues/18798#note_307410, where
>> viktor had millions of independent mvars holding what’s essentially a
>> strict unit ()!
>>
>> The motivation in this later scenario is that in high concurrency
>> settings, the less trivial stuff the gc needs to trace under updates, the
>> better ghc scales.
>>
>> This may not be a use case david has in mind, but certainly seems
>> related.
>>
>> Phrased more succinctly: gc perf dominates large heap / many core
>> computation in Haskell via sensitivity to allocation volume / mutation
>> volume (to ensure generational hypothesis stays valid), and providing tools
>> to incrementally reduce the pressure with local changes would be good.
>>
>> So I’d propose / suggest that a baby step towards what david asks would
>> be for us to work out some manner of unboxed tvar/mvar ref machinery that
>> supports unboxed values.
>>
>>
>>
>> On Thu, Oct 15, 2020 at 5:32 AM Andreas Klebinger <
>> klebinger.andr...@gmx.at> wrote:
>>
>>> From a implementors perspective my main questions would be:
>>>
>>> * How big is the benefit in practice? How many use cases are there?
>>> * How bad are the costs? (Runtime overhead, rts complexity, ...)
>>>
>>> The details of how this would be exposed to a user would are important.
>>> But if the costs are too high for the drawbacks then it becomes a moot
>>> point.
>>>
>>>
>>> David Feuer schrieb am 14.10.2020 um 22:21:
>>>
>>> Forwarded from Andrew Martin below. I think we want more than just Maybe
>>> (more than one null), but the nesting I described is certainly more
>>> convenience than necessity.
>>>
>>> -- Forwarded message -
>>> From: Andrew Martin 
>>> Date: Wed, Oct 14, 2020, 4:14 PM
>>> Subject: Re: Restricted sums in BoxedRep
>>> To: David Feuer 
>>>
>>>
>>> You'll have to forward this to the ghc-devs list to share it with others
>>> since I'm not currently subscribed to it, but I've had this same thought
>>> before. It is discussed at
>>> https://github.com/andrewthad/impure-containers/issues/12. Here's the
>>> relevant excerpt:
>>>
>>>> Relatedly, I was thinking the other day that after finishing
>>>> implementing
>>>> https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0203-pointer-rep.rst,
>>>> I should really look at seeing if it's possible to add this
>>>> maybe-of-a-lifted value trick straight to GHC. I think that with:
>>>>
>>>> data RuntimpRep
>>>>   = BoxedRep Levity
>>>>   | MaybeBoxedRep Levity
>>>>   | IntRep
>>>>   | ...
>>>>
>>>> data BuiltinMaybe :: forall (v :: Levity). TYPE v -> TYPE ('MaybeBoxedRep 
>>>> v)
>>>>
>>>> This doesn't have the nesting issues because the kind system prevents
>>>> nesting. But anyway, back to the original question. I would recommend not
>>>> using Maybe.Unsafe and using unpacked-maybe instead. The latter is
>>>> definitely safe, and it only costs an extra machine word of space in each
>>>> data constructor it gets used in, and it doesn't introduce more
>>>> indirections.
>>>>
>>>
>>> On Tue, Oct 13, 2020 at 5:47 PM David Feuer 
>>> wrote:
>>>
>>>> Null pointers are widely known to be a lousy language feature in
>>>> general, but there are certain situations where they're *really* useful for
>>>> compact representation. For example, we define
>>>>
>>>> newtype TMVar a = TMVar (TVar (Maybe a))
>>>>
>>>> We don't, however, actually use the fact that (Maybe a) is lifted. So
>>>> we could represent this much more efficiently using something like
>>>>
>>>> newtype TMVar a = TMVar (TVar a)
>>>&g

Re: Fwd: Restricted sums in BoxedRep

[David Feuer]

Putting unboxed things in TVar, MVar, etc., is part of Andrew Martin's
accepted BoxedRep proposal.

On Thu, Oct 15, 2020, 11:44 AM Carter Schonwald 
wrote:

> A related idea that came up recently and is perhaps simpler ties into this
> via the lens of having unboxed Mvars/tvars (even if it’s restricted to just
> things we can embed in a word64#)
>
> This came up in
> https://gitlab.haskell.org/ghc/ghc/-/issues/18798#note_307410, where
> viktor had millions of independent mvars holding what’s essentially a
> strict unit ()!
>
> The motivation in this later scenario is that in high concurrency
> settings, the less trivial stuff the gc needs to trace under updates, the
> better ghc scales.
>
> This may not be a use case david has in mind, but certainly seems related.
>
>
> Phrased more succinctly: gc perf dominates large heap / many core
> computation in Haskell via sensitivity to allocation volume / mutation
> volume (to ensure generational hypothesis stays valid), and providing tools
> to incrementally reduce the pressure with local changes would be good.
>
> So I’d propose / suggest that a baby step towards what david asks would be
> for us to work out some manner of unboxed tvar/mvar ref machinery that
> supports unboxed values.
>
>
>
> On Thu, Oct 15, 2020 at 5:32 AM Andreas Klebinger <
> klebinger.andr...@gmx.at> wrote:
>
>> From a implementors perspective my main questions would be:
>>
>> * How big is the benefit in practice? How many use cases are there?
>> * How bad are the costs? (Runtime overhead, rts complexity, ...)
>>
>> The details of how this would be exposed to a user would are important.
>> But if the costs are too high for the drawbacks then it becomes a moot
>> point.
>>
>>
>> David Feuer schrieb am 14.10.2020 um 22:21:
>>
>> Forwarded from Andrew Martin below. I think we want more than just Maybe
>> (more than one null), but the nesting I described is certainly more
>> convenience than necessity.
>>
>> -- Forwarded message -
>> From: Andrew Martin 
>> Date: Wed, Oct 14, 2020, 4:14 PM
>> Subject: Re: Restricted sums in BoxedRep
>> To: David Feuer 
>>
>>
>> You'll have to forward this to the ghc-devs list to share it with others
>> since I'm not currently subscribed to it, but I've had this same thought
>> before. It is discussed at
>> https://github.com/andrewthad/impure-containers/issues/12. Here's the
>> relevant excerpt:
>>
>>> Relatedly, I was thinking the other day that after finishing
>>> implementing
>>> https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0203-pointer-rep.rst,
>>> I should really look at seeing if it's possible to add this
>>> maybe-of-a-lifted value trick straight to GHC. I think that with:
>>>
>>> data RuntimpRep
>>>   = BoxedRep Levity
>>>   | MaybeBoxedRep Levity
>>>   | IntRep
>>>   | ...
>>>
>>> data BuiltinMaybe :: forall (v :: Levity). TYPE v -> TYPE ('MaybeBoxedRep v)
>>>
>>> This doesn't have the nesting issues because the kind system prevents
>>> nesting. But anyway, back to the original question. I would recommend not
>>> using Maybe.Unsafe and using unpacked-maybe instead. The latter is
>>> definitely safe, and it only costs an extra machine word of space in each
>>> data constructor it gets used in, and it doesn't introduce more
>>> indirections.
>>>
>>
>> On Tue, Oct 13, 2020 at 5:47 PM David Feuer 
>> wrote:
>>
>>> Null pointers are widely known to be a lousy language feature in
>>> general, but there are certain situations where they're *really* useful for
>>> compact representation. For example, we define
>>>
>>> newtype TMVar a = TMVar (TVar (Maybe a))
>>>
>>> We don't, however, actually use the fact that (Maybe a) is lifted. So we
>>> could represent this much more efficiently using something like
>>>
>>> newtype TMVar a = TMVar (TVar a)
>>>
>>> where Nothing is represented by a distinguished "null" pointer.
>>>
>>> While it's possible to implement this sort of thing in user code (with
>>> lots of fuss and care), it's not very nice at all. What I'd really like to
>>> be able to do is represent certain kinds of sums like this natively.
>>>
>>> Now that we're getting BoxedRep, I think we can probably make it happen.
>>> The trick is to add a special Levity constructor represent

Re: Fwd: Restricted sums in BoxedRep

[Carter Schonwald]

A related idea that came up recently and is perhaps simpler ties into this
via the lens of having unboxed Mvars/tvars (even if it’s restricted to just
things we can embed in a word64#)

This came up in
https://gitlab.haskell.org/ghc/ghc/-/issues/18798#note_307410, where viktor
had millions of independent mvars holding what’s essentially a strict unit
()!

The motivation in this later scenario is that in high concurrency settings,
the less trivial stuff the gc needs to trace under updates, the better ghc
scales.

This may not be a use case david has in mind, but certainly seems related.

Phrased more succinctly: gc perf dominates large heap / many core
computation in Haskell via sensitivity to allocation volume / mutation
volume (to ensure generational hypothesis stays valid), and providing tools
to incrementally reduce the pressure with local changes would be good.

So I’d propose / suggest that a baby step towards what david asks would be
for us to work out some manner of unboxed tvar/mvar ref machinery that
supports unboxed values.



On Thu, Oct 15, 2020 at 5:32 AM Andreas Klebinger 
wrote:

> From a implementors perspective my main questions would be:
>
> * How big is the benefit in practice? How many use cases are there?
> * How bad are the costs? (Runtime overhead, rts complexity, ...)
>
> The details of how this would be exposed to a user would are important.
> But if the costs are too high for the drawbacks then it becomes a moot
> point.
>
>
> David Feuer schrieb am 14.10.2020 um 22:21:
>
> Forwarded from Andrew Martin below. I think we want more than just Maybe
> (more than one null), but the nesting I described is certainly more
> convenience than necessity.
>
> -- Forwarded message -
> From: Andrew Martin 
> Date: Wed, Oct 14, 2020, 4:14 PM
> Subject: Re: Restricted sums in BoxedRep
> To: David Feuer 
>
>
> You'll have to forward this to the ghc-devs list to share it with others
> since I'm not currently subscribed to it, but I've had this same thought
> before. It is discussed at
> https://github.com/andrewthad/impure-containers/issues/12. Here's the
> relevant excerpt:
>
>> Relatedly, I was thinking the other day that after finishing implementing
>> https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0203-pointer-rep.rst,
>> I should really look at seeing if it's possible to add this
>> maybe-of-a-lifted value trick straight to GHC. I think that with:
>>
>> data RuntimpRep
>>   = BoxedRep Levity
>>   | MaybeBoxedRep Levity
>>   | IntRep
>>   | ...
>>
>> data BuiltinMaybe :: forall (v :: Levity). TYPE v -> TYPE ('MaybeBoxedRep v)
>>
>> This doesn't have the nesting issues because the kind system prevents
>> nesting. But anyway, back to the original question. I would recommend not
>> using Maybe.Unsafe and using unpacked-maybe instead. The latter is
>> definitely safe, and it only costs an extra machine word of space in each
>> data constructor it gets used in, and it doesn't introduce more
>> indirections.
>>
>
> On Tue, Oct 13, 2020 at 5:47 PM David Feuer  wrote:
>
>> Null pointers are widely known to be a lousy language feature in general,
>> but there are certain situations where they're *really* useful for compact
>> representation. For example, we define
>>
>> newtype TMVar a = TMVar (TVar (Maybe a))
>>
>> We don't, however, actually use the fact that (Maybe a) is lifted. So we
>> could represent this much more efficiently using something like
>>
>> newtype TMVar a = TMVar (TVar a)
>>
>> where Nothing is represented by a distinguished "null" pointer.
>>
>> While it's possible to implement this sort of thing in user code (with
>> lots of fuss and care), it's not very nice at all. What I'd really like to
>> be able to do is represent certain kinds of sums like this natively.
>>
>> Now that we're getting BoxedRep, I think we can probably make it happen.
>> The trick is to add a special Levity constructor representing sums of
>> particular shapes. Specifically, we can represent a type like this if it is
>> a possibly-nested sum which, when flattened into a single sum, consists of
>> some number of nullary tuples and at most one Lifted or Unlifted type.
>> Then we can have (inline) primops to convert between the BoxedRep and the
>> sum-of-sums representations.
>>
>> Anyone have thoughts on details for what the Levity constructor arguments
>> might look like?
>>
>
>
> --
> -Andrew Thaddeus Martin
>
>
> ___
> ghc-devs mailing 
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>
>
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Re: Fwd: Restricted sums in BoxedRep

[Andreas Klebinger]

From a implementors perspective my main questions would be:

* How big is the benefit in practice? How many use cases are there?
* How bad are the costs? (Runtime overhead, rts complexity, ...)

The details of how this would be exposed to a user would are important.
But if the costs are too high for the drawbacks then it becomes a moot
point.

David Feuer schrieb am 14.10.2020 um 22:21:

Forwarded from Andrew Martin below. I think we want more than just
Maybe (more than one null), but the nesting I described is certainly
more convenience than necessity.

-- Forwarded message -
From: *Andrew Martin* mailto:andrew.thadd...@gmail.com>>
Date: Wed, Oct 14, 2020, 4:14 PM
Subject: Re: Restricted sums in BoxedRep
To: David Feuer mailto:david.fe...@gmail.com>>


You'll have to forward this to the ghc-devs list to share it with
others since I'm not currently subscribed to it, but I've had this
same thought before. It is discussed at
https://github.com/andrewthad/impure-containers/issues/12. Here's the
relevant excerpt:

Relatedly, I was thinking the other day that after finishing
implementing

https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0203-pointer-rep.rst,
I should really look at seeing if it's possible to add this
maybe-of-a-lifted value trick straight to GHC. I think that with:

|data RuntimpRep = BoxedRep Levity | MaybeBoxedRep Levity | IntRep
| ... data BuiltinMaybe :: forall (v :: Levity). TYPE v -> TYPE
('MaybeBoxedRep v) |

This doesn't have the nesting issues because the kind system
prevents nesting. But anyway, back to the original question. I
would recommend not using |Maybe.Unsafe| and using
|unpacked-maybe| instead. The latter is definitely safe, and it
only costs an extra machine word of space in each data constructor
it gets used in, and it doesn't introduce more indirections.


On Tue, Oct 13, 2020 at 5:47 PM David Feuer mailto:david.fe...@gmail.com>> wrote:

Null pointers are widely known to be a lousy language feature in
general, but there are certain situations where they're *really*
useful for compact representation. For example, we define

    newtype TMVar a = TMVar (TVar (Maybe a))

We don't, however, actually use the fact that (Maybe a) is lifted.
So we could represent this much more efficiently using something like

    newtype TMVar a = TMVar (TVar a)

where Nothing is represented by a distinguished "null" pointer.

While it's possible to implement this sort of thing in user code
(with lots of fuss and care), it's not very nice at all. What I'd
really like to be able to do is represent certain kinds of sums
like this natively.

Now that we're getting BoxedRep, I think we can probably make it
happen. The trick is to add a special Levity constructor
representing sums of particular shapes. Specifically, we can
represent a type like this if it is a possibly-nested sum which,
when flattened into a single sum, consists of some number of
nullary tuples and at most one Lifted or Unlifted type.  Then we
can have (inline) primops to convert between the BoxedRep and the
sum-of-sums representations.

Anyone have thoughts on details for what the Levity constructor
arguments might look like?



--
-Andrew Thaddeus Martin


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Fwd: Restricted sums in BoxedRep

[David Feuer]

Forwarded from Andrew Martin below. I think we want more than just Maybe
(more than one null), but the nesting I described is certainly more
convenience than necessity.

-- Forwarded message -
From: Andrew Martin 
Date: Wed, Oct 14, 2020, 4:14 PM
Subject: Re: Restricted sums in BoxedRep
To: David Feuer 


You'll have to forward this to the ghc-devs list to share it with others
since I'm not currently subscribed to it, but I've had this same thought
before. It is discussed at
https://github.com/andrewthad/impure-containers/issues/12. Here's the
relevant excerpt:

> Relatedly, I was thinking the other day that after finishing implementing
> https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0203-pointer-rep.rst,
> I should really look at seeing if it's possible to add this
> maybe-of-a-lifted value trick straight to GHC. I think that with:
>
> data RuntimpRep
>   = BoxedRep Levity
>   | MaybeBoxedRep Levity
>   | IntRep
>   | ...
>
> data BuiltinMaybe :: forall (v :: Levity). TYPE v -> TYPE ('MaybeBoxedRep v)
>
> This doesn't have the nesting issues because the kind system prevents
> nesting. But anyway, back to the original question. I would recommend not
> using Maybe.Unsafe and using unpacked-maybe instead. The latter is
> definitely safe, and it only costs an extra machine word of space in each
> data constructor it gets used in, and it doesn't introduce more
> indirections.
>

On Tue, Oct 13, 2020 at 5:47 PM David Feuer  wrote:

> Null pointers are widely known to be a lousy language feature in general,
> but there are certain situations where they're *really* useful for compact
> representation. For example, we define
>
> newtype TMVar a = TMVar (TVar (Maybe a))
>
> We don't, however, actually use the fact that (Maybe a) is lifted. So we
> could represent this much more efficiently using something like
>
> newtype TMVar a = TMVar (TVar a)
>
> where Nothing is represented by a distinguished "null" pointer.
>
> While it's possible to implement this sort of thing in user code (with
> lots of fuss and care), it's not very nice at all. What I'd really like to
> be able to do is represent certain kinds of sums like this natively.
>
> Now that we're getting BoxedRep, I think we can probably make it happen.
> The trick is to add a special Levity constructor representing sums of
> particular shapes. Specifically, we can represent a type like this if it is
> a possibly-nested sum which, when flattened into a single sum, consists of
> some number of nullary tuples and at most one Lifted or Unlifted type.
> Then we can have (inline) primops to convert between the BoxedRep and the
> sum-of-sums representations.
>
> Anyone have thoughts on details for what the Levity constructor arguments
> might look like?
>


-- 
-Andrew Thaddeus Martin
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Re: Restricted sums in BoxedRep

[David Feuer]

Yes, I think you're getting the gist. Pattern matching with one or two
nulls is just an equality test or three. In practice, we'd want a fixed
number of evenly spaced nulls, allowing jump table techniques to be used
when there are more cases. We'd presumably have a hard limit for the number
of nulls a type is allowed to have.

On Wed, Oct 14, 2020, 10:29 AM Spiwack, Arnaud 
wrote:

> Ok, I believe get it now,
>
> Let's imagine (to take only the simplest case) that we have a `Nullable#
> a` type, such that `Nullable# a = (# (##) | a #)`. What would be the kind
> of `Nullable#`? I imagine that it would be something like `TYPE (BoxedRep
> Lifted) -> TYPE (BoxedRep Nullable)`.
>
> Then you would want to abstract the type of arrays/tvars/whatnot from
> `Type -> Type` to `forall r. TYPE (BoxRep r) -> Type`.
>
> Is that a correct interpretation of your suggestion?
>
> If so, my guess would be that all the above is fine, but I suspect (and
> I'm quite a bit out of my comfort zone here) that there can be considerable
> difficulties in implementing pattern-matching for such a type.
>
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Re: Restricted sums in BoxedRep

[Spiwack, Arnaud]

Ok, I believe get it now,

Let's imagine (to take only the simplest case) that we have a `Nullable# a`
type, such that `Nullable# a = (# (##) | a #)`. What would be the kind of
`Nullable#`? I imagine that it would be something like `TYPE (BoxedRep
Lifted) -> TYPE (BoxedRep Nullable)`.

Then you would want to abstract the type of arrays/tvars/whatnot from `Type
-> Type` to `forall r. TYPE (BoxRep r) -> Type`.

Is that a correct interpretation of your suggestion?

If so, my guess would be that all the above is fine, but I suspect (and I'm
quite a bit out of my comfort zone here) that there can be considerable
difficulties in implementing pattern-matching for such a type.
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Re: Restricted sums in BoxedRep

[David Feuer]

I don't know your terminology, sorry. By "null" I'm referring to something
distinguished, of which there can be more than one. These can be pointers
to statically allocated objects if necessary, though pointers in an unused
address range would probably be nicer. My goal is to be able to shove a
compact representation of something like (# (##) | (##) | (##) | a #) into
an array/MutVar/etc., rather than needing to box it to do so.

On Wed, Oct 14, 2020, 7:45 AM Spiwack, Arnaud 
wrote:

> I may have misunderstood, but my understanding is the following:
>
> - Since a is a boxed type, it can never be the null pointer
> - So I can use a null pointer unambiguously
>
> Let's call this null-pointer expanded type, `Nullable# a`, it is now a
> different sort than `a`, since it can have the null pointer in it. Is that
> still what you wish for? The risk being that combining such a `Nullable# a`
> with another data structure may very well require an additional boxing,
> which is what, I believe, you were trying to avoid.
>
> /Arnaud
>
> On Tue, Oct 13, 2020 at 11:47 PM David Feuer 
> wrote:
>
>> Null pointers are widely known to be a lousy language feature in general,
>> but there are certain situations where they're *really* useful for compact
>> representation. For example, we define
>>
>> newtype TMVar a = TMVar (TVar (Maybe a))
>>
>> We don't, however, actually use the fact that (Maybe a) is lifted. So we
>> could represent this much more efficiently using something like
>>
>> newtype TMVar a = TMVar (TVar a)
>>
>> where Nothing is represented by a distinguished "null" pointer.
>>
>> While it's possible to implement this sort of thing in user code (with
>> lots of fuss and care), it's not very nice at all. What I'd really like to
>> be able to do is represent certain kinds of sums like this natively.
>>
>> Now that we're getting BoxedRep, I think we can probably make it happen.
>> The trick is to add a special Levity constructor representing sums of
>> particular shapes. Specifically, we can represent a type like this if it is
>> a possibly-nested sum which, when flattened into a single sum, consists of
>> some number of nullary tuples and at most one Lifted or Unlifted type.
>> Then we can have (inline) primops to convert between the BoxedRep and the
>> sum-of-sums representations.
>>
>> Anyone have thoughts on details for what the Levity constructor arguments
>> might look like?
>> ___
>> ghc-devs mailing list
>> ghc-devs@haskell.org
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>>
>
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Re: Restricted sums in BoxedRep

[Spiwack, Arnaud]

I don't imagine it would be hard for the GC to check that a pointer is null
before following it. The argument may be, though, that it's too high a
price to pay for the occasional use of a null pointer. (I must add that I
have no opinion, or idea really, on this)

On Wed, Oct 14, 2020 at 1:54 PM Sebastian Graf  wrote:

> I believe Simon told me once that NULL pointers in places we assume
> BoxedRep things are not an option, because the GC assumes it is free to
> follow that pointer. It won't check if it's NULL or not.
> That's also the reason why we lower `LitRubbish` (which we use for absent
> BoxedRep literals) as `()` when going to STG
> 
> -- We can't use NULL, so supply an arbitrary (untyped!) closure that we
> know can be followed by the GC.
>
> Am Mi., 14. Okt. 2020 um 13:46 Uhr schrieb Spiwack, Arnaud <
> arnaud.spiw...@tweag.io>:
>
>> I may have misunderstood, but my understanding is the following:
>>
>> - Since a is a boxed type, it can never be the null pointer
>> - So I can use a null pointer unambiguously
>>
>> Let's call this null-pointer expanded type, `Nullable# a`, it is now a
>> different sort than `a`, since it can have the null pointer in it. Is that
>> still what you wish for? The risk being that combining such a `Nullable# a`
>> with another data structure may very well require an additional boxing,
>> which is what, I believe, you were trying to avoid.
>>
>> /Arnaud
>>
>> On Tue, Oct 13, 2020 at 11:47 PM David Feuer 
>> wrote:
>>
>>> Null pointers are widely known to be a lousy language feature in
>>> general, but there are certain situations where they're *really* useful for
>>> compact representation. For example, we define
>>>
>>> newtype TMVar a = TMVar (TVar (Maybe a))
>>>
>>> We don't, however, actually use the fact that (Maybe a) is lifted. So we
>>> could represent this much more efficiently using something like
>>>
>>> newtype TMVar a = TMVar (TVar a)
>>>
>>> where Nothing is represented by a distinguished "null" pointer.
>>>
>>> While it's possible to implement this sort of thing in user code (with
>>> lots of fuss and care), it's not very nice at all. What I'd really like to
>>> be able to do is represent certain kinds of sums like this natively.
>>>
>>> Now that we're getting BoxedRep, I think we can probably make it happen.
>>> The trick is to add a special Levity constructor representing sums of
>>> particular shapes. Specifically, we can represent a type like this if it is
>>> a possibly-nested sum which, when flattened into a single sum, consists of
>>> some number of nullary tuples and at most one Lifted or Unlifted type.
>>> Then we can have (inline) primops to convert between the BoxedRep and the
>>> sum-of-sums representations.
>>>
>>> Anyone have thoughts on details for what the Levity constructor
>>> arguments might look like?
>>> ___
>>> ghc-devs mailing list
>>> ghc-devs@haskell.org
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
>>>
>> ___
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>>
>
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Re: Restricted sums in BoxedRep

[Sebastian Graf]

I believe Simon told me once that NULL pointers in places we assume
BoxedRep things are not an option, because the GC assumes it is free to
follow that pointer. It won't check if it's NULL or not.
That's also the reason why we lower `LitRubbish` (which we use for absent
BoxedRep literals) as `()` when going to STG

-- We can't use NULL, so supply an arbitrary (untyped!) closure that we
know can be followed by the GC.

Am Mi., 14. Okt. 2020 um 13:46 Uhr schrieb Spiwack, Arnaud <
arnaud.spiw...@tweag.io>:

> I may have misunderstood, but my understanding is the following:
>
> - Since a is a boxed type, it can never be the null pointer
> - So I can use a null pointer unambiguously
>
> Let's call this null-pointer expanded type, `Nullable# a`, it is now a
> different sort than `a`, since it can have the null pointer in it. Is that
> still what you wish for? The risk being that combining such a `Nullable# a`
> with another data structure may very well require an additional boxing,
> which is what, I believe, you were trying to avoid.
>
> /Arnaud
>
> On Tue, Oct 13, 2020 at 11:47 PM David Feuer 
> wrote:
>
>> Null pointers are widely known to be a lousy language feature in general,
>> but there are certain situations where they're *really* useful for compact
>> representation. For example, we define
>>
>> newtype TMVar a = TMVar (TVar (Maybe a))
>>
>> We don't, however, actually use the fact that (Maybe a) is lifted. So we
>> could represent this much more efficiently using something like
>>
>> newtype TMVar a = TMVar (TVar a)
>>
>> where Nothing is represented by a distinguished "null" pointer.
>>
>> While it's possible to implement this sort of thing in user code (with
>> lots of fuss and care), it's not very nice at all. What I'd really like to
>> be able to do is represent certain kinds of sums like this natively.
>>
>> Now that we're getting BoxedRep, I think we can probably make it happen.
>> The trick is to add a special Levity constructor representing sums of
>> particular shapes. Specifically, we can represent a type like this if it is
>> a possibly-nested sum which, when flattened into a single sum, consists of
>> some number of nullary tuples and at most one Lifted or Unlifted type.
>> Then we can have (inline) primops to convert between the BoxedRep and the
>> sum-of-sums representations.
>>
>> Anyone have thoughts on details for what the Levity constructor arguments
>> might look like?
>> ___
>> ghc-devs mailing list
>> ghc-devs@haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
>>
> ___
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> ghc-devs@haskell.org
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Re: Restricted sums in BoxedRep

[Spiwack, Arnaud]

I may have misunderstood, but my understanding is the following:

- Since a is a boxed type, it can never be the null pointer
- So I can use a null pointer unambiguously

Let's call this null-pointer expanded type, `Nullable# a`, it is now a
different sort than `a`, since it can have the null pointer in it. Is that
still what you wish for? The risk being that combining such a `Nullable# a`
with another data structure may very well require an additional boxing,
which is what, I believe, you were trying to avoid.

/Arnaud

On Tue, Oct 13, 2020 at 11:47 PM David Feuer  wrote:

> Null pointers are widely known to be a lousy language feature in general,
> but there are certain situations where they're *really* useful for compact
> representation. For example, we define
>
> newtype TMVar a = TMVar (TVar (Maybe a))
>
> We don't, however, actually use the fact that (Maybe a) is lifted. So we
> could represent this much more efficiently using something like
>
> newtype TMVar a = TMVar (TVar a)
>
> where Nothing is represented by a distinguished "null" pointer.
>
> While it's possible to implement this sort of thing in user code (with
> lots of fuss and care), it's not very nice at all. What I'd really like to
> be able to do is represent certain kinds of sums like this natively.
>
> Now that we're getting BoxedRep, I think we can probably make it happen.
> The trick is to add a special Levity constructor representing sums of
> particular shapes. Specifically, we can represent a type like this if it is
> a possibly-nested sum which, when flattened into a single sum, consists of
> some number of nullary tuples and at most one Lifted or Unlifted type.
> Then we can have (inline) primops to convert between the BoxedRep and the
> sum-of-sums representations.
>
> Anyone have thoughts on details for what the Levity constructor arguments
> might look like?
> ___
> ghc-devs mailing list
> ghc-devs@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
>
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Restricted sums in BoxedRep

[David Feuer]

Null pointers are widely known to be a lousy language feature in general,
but there are certain situations where they're *really* useful for compact
representation. For example, we define

newtype TMVar a = TMVar (TVar (Maybe a))

We don't, however, actually use the fact that (Maybe a) is lifted. So we
could represent this much more efficiently using something like

newtype TMVar a = TMVar (TVar a)

where Nothing is represented by a distinguished "null" pointer.

While it's possible to implement this sort of thing in user code (with lots
of fuss and care), it's not very nice at all. What I'd really like to be
able to do is represent certain kinds of sums like this natively.

Now that we're getting BoxedRep, I think we can probably make it happen.
The trick is to add a special Levity constructor representing sums of
particular shapes. Specifically, we can represent a type like this if it is
a possibly-nested sum which, when flattened into a single sum, consists of
some number of nullary tuples and at most one Lifted or Unlifted type.
Then we can have (inline) primops to convert between the BoxedRep and the
sum-of-sums representations.

Anyone have thoughts on details for what the Levity constructor arguments
might look like?
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