Discussion: New atomic IORef functions

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Discussion: New atomic IORef functions

David Feuer
I have proposed[1] the replacement of the atomicModifyMutVar# primop, and the addition of two cheaper but less capable ones. It seems likely that the proposal will succeed, but that the GHC steering committee will leave the question of user interface changes to the libraries list. I would like to open the discussion here.

The new primops lead naturally to several thin wrappers:

-- Atomically replace the IORef contents
-- with the first component of the result of
-- applying the function to the old contents.
-- Return the old value and the result of
-- applying the function, without forcing the latter.
--
-- atomicModifyIORef ref f = do
--   (_old, ~(_new, res)) <- atomicModifyIORef2Lazy ref f
--   return res
atomicModifyIORef2Lazy
  :: IORef a -> (a -> (a, b)) -> IO (a, (a, b))

-- Atomically replace the IORef contents
-- with the result of applying the function
-- to the old contents. Return the old and
-- new contents without forcing the latter.
atomicModifyIORefLazy_
  :: IORef a -> (a -> a) -> IO (a, a)

-- Atomically replace the IORef contents
-- with the given value and return the old
-- contents.
--
-- atomicWriteIORef ref x = void (atomicSwapIORef ref x)
atomicSwapIORef
  :: IORef a -> a -> IO a

Based on the code I've read that uses atomicModifyIORef, I believe that the complete laziness of atomicModifyIORef2Lazy and atomicModifyIORefLazy_ is very rarely desirable. I therefore believe we should also (or perhaps instead?) offer stricter versions:

atomicModifyIORef2
  :: IORef a -> (a -> (a, b)) -> IO (a, (a, b))
atomicModifyIORef2 ref f = do
  r@(_old, (_new, _res)) <- atomicModifyIORef2Lazy ref f
  return r

atomicModifyIORef_
  :: IORef a -> (a -> a) -> IO (a, a)
atomicModifyIORef_ ref f = do
  r@(_old, !_new) <- atomicModifyIORefLazy_ ref f
  return r

The classic atomicModifyIORef also admits a less gratuitously lazy version:

atomicModifyIORefNGL
  :: IORef a -> (a -> (a,b)) -> IO b
atomicModifyIORefNGL ref f = do
  (_old, (_new, res)) <- atomicModifyIORef2 ref f
  return res

Should we add that as well (with a better name)? Should we even consider *replacing* the current atomicModifyIORef with that version? That could theoretically break existing code, but I suspect it would do so very rarely. If we don't change the existing atomicModifyIORef now, I think we should consider deprecating it: it's very easy to accidentally use it too lazily.

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Re: Discussion: New atomic IORef functions

David Feuer
Whoops! I left out the proposal link:


Also, what I called atomicModifyIORef_ below should really be called something like atomicModifyIORef'_, since it forces a polymorphic value.

Another thing to note: the underlying atomicModifyMutVar2# primop actually supports more than just pairs. It can handle triples, solos, and any other record types whose first components are lifted:

atomicModifyIORefSoloLazy
  :: IORef a -> (a -> Solo a) -> IO (Solo a)

atomicModifyIORefSolo
  :: IORef a -> (a -> Solo a) -> IO a

atomicModifyIORef3, atomicModifyIORef3Lazy
  :: IORef a -> (a -> (a, b, c)) -> IO (a, b, c)

etc.

Should we add any such?

On Sat, Jul 7, 2018, 2:35 PM David Feuer <[hidden email]> wrote:
I have proposed[1] the replacement of the atomicModifyMutVar# primop, and the addition of two cheaper but less capable ones. It seems likely that the proposal will succeed, but that the GHC steering committee will leave the question of user interface changes to the libraries list. I would like to open the discussion here.

The new primops lead naturally to several thin wrappers:

-- Atomically replace the IORef contents
-- with the first component of the result of
-- applying the function to the old contents.
-- Return the old value and the result of
-- applying the function, without forcing the latter.
--
-- atomicModifyIORef ref f = do
--   (_old, ~(_new, res)) <- atomicModifyIORef2Lazy ref f
--   return res
atomicModifyIORef2Lazy
  :: IORef a -> (a -> (a, b)) -> IO (a, (a, b))

-- Atomically replace the IORef contents
-- with the result of applying the function
-- to the old contents. Return the old and
-- new contents without forcing the latter.
atomicModifyIORefLazy_
  :: IORef a -> (a -> a) -> IO (a, a)

-- Atomically replace the IORef contents
-- with the given value and return the old
-- contents.
--
-- atomicWriteIORef ref x = void (atomicSwapIORef ref x)
atomicSwapIORef
  :: IORef a -> a -> IO a

Based on the code I've read that uses atomicModifyIORef, I believe that the complete laziness of atomicModifyIORef2Lazy and atomicModifyIORefLazy_ is very rarely desirable. I therefore believe we should also (or perhaps instead?) offer stricter versions:

atomicModifyIORef2
  :: IORef a -> (a -> (a, b)) -> IO (a, (a, b))
atomicModifyIORef2 ref f = do
  r@(_old, (_new, _res)) <- atomicModifyIORef2Lazy ref f
  return r

atomicModifyIORef_
  :: IORef a -> (a -> a) -> IO (a, a)
atomicModifyIORef_ ref f = do
  r@(_old, !_new) <- atomicModifyIORefLazy_ ref f
  return r

The classic atomicModifyIORef also admits a less gratuitously lazy version:

atomicModifyIORefNGL
  :: IORef a -> (a -> (a,b)) -> IO b
atomicModifyIORefNGL ref f = do
  (_old, (_new, res)) <- atomicModifyIORef2 ref f
  return res

Should we add that as well (with a better name)? Should we even consider *replacing* the current atomicModifyIORef with that version? That could theoretically break existing code, but I suspect it would do so very rarely. If we don't change the existing atomicModifyIORef now, I think we should consider deprecating it: it's very easy to accidentally use it too lazily.

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Re: Discussion: New atomic IORef functions

winter

I believe new variations should always be motivated by use-case if there're too many choices, the lazy behavior of old `atomicModifyIORef` is justified by some cases the modifying functions are lazy in its argument, thus a lazy version could win by not forcing previous thunks, we'd want to keep its behavior as how it's documented.

As for tuples more than pairs, they're not really needed, user can always squeeze their product into `b` component.

IMHO only the addition of `atomicModifyIORef_` is sensible in the context of base, other APIs may go to package like primitives. But if you have a motivated use case with `atomicModifyIORef2`, etc. Please tell me.


On 2018年07月08日 03:09, David Feuer wrote:
Whoops! I left out the proposal link:


Also, what I called atomicModifyIORef_ below should really be called something like atomicModifyIORef'_, since it forces a polymorphic value.

Another thing to note: the underlying atomicModifyMutVar2# primop actually supports more than just pairs. It can handle triples, solos, and any other record types whose first components are lifted:

atomicModifyIORefSoloLazy
  :: IORef a -> (a -> Solo a) -> IO (Solo a)

atomicModifyIORefSolo
  :: IORef a -> (a -> Solo a) -> IO a

atomicModifyIORef3, atomicModifyIORef3Lazy
  :: IORef a -> (a -> (a, b, c)) -> IO (a, b, c)

etc.

Should we add any such?

On Sat, Jul 7, 2018, 2:35 PM David Feuer <[hidden email]> wrote:
I have proposed[1] the replacement of the atomicModifyMutVar# primop, and the addition of two cheaper but less capable ones. It seems likely that the proposal will succeed, but that the GHC steering committee will leave the question of user interface changes to the libraries list. I would like to open the discussion here.

The new primops lead naturally to several thin wrappers:

-- Atomically replace the IORef contents
-- with the first component of the result of
-- applying the function to the old contents.
-- Return the old value and the result of
-- applying the function, without forcing the latter.
--
-- atomicModifyIORef ref f = do
--   (_old, ~(_new, res)) <- atomicModifyIORef2Lazy ref f
--   return res
atomicModifyIORef2Lazy
  :: IORef a -> (a -> (a, b)) -> IO (a, (a, b))

-- Atomically replace the IORef contents
-- with the result of applying the function
-- to the old contents. Return the old and
-- new contents without forcing the latter.
atomicModifyIORefLazy_
  :: IORef a -> (a -> a) -> IO (a, a)

-- Atomically replace the IORef contents
-- with the given value and return the old
-- contents.
--
-- atomicWriteIORef ref x = void (atomicSwapIORef ref x)
atomicSwapIORef
  :: IORef a -> a -> IO a

Based on the code I've read that uses atomicModifyIORef, I believe that the complete laziness of atomicModifyIORef2Lazy and atomicModifyIORefLazy_ is very rarely desirable. I therefore believe we should also (or perhaps instead?) offer stricter versions:

atomicModifyIORef2
  :: IORef a -> (a -> (a, b)) -> IO (a, (a, b))
atomicModifyIORef2 ref f = do
  r@(_old, (_new, _res)) <- atomicModifyIORef2Lazy ref f
  return r

atomicModifyIORef_
  :: IORef a -> (a -> a) -> IO (a, a)
atomicModifyIORef_ ref f = do
  r@(_old, !_new) <- atomicModifyIORefLazy_ ref f
  return r

The classic atomicModifyIORef also admits a less gratuitously lazy version:

atomicModifyIORefNGL
  :: IORef a -> (a -> (a,b)) -> IO b
atomicModifyIORefNGL ref f = do
  (_old, (_new, res)) <- atomicModifyIORef2 ref f
  return res

Should we add that as well (with a better name)? Should we even consider *replacing* the current atomicModifyIORef with that version? That could theoretically break existing code, but I suspect it would do so very rarely. If we don't change the existing atomicModifyIORef now, I think we should consider deprecating it: it's very easy to accidentally use it too lazily.


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Re: Discussion: New atomic IORef functions

David Feuer
In reply to this post by David Feuer
All the fundamental functions I've defined can be usefully used in the base library. I think that's one good reason to put them there. Another is that while the basic array operations have traditionally been exposed through array, vector, and primitive, and the basic TVar operations have been exposed through stm, the basic IORef and MVar operations have been exposed through base (except, for some reason, casMutVar#). I see no reason to change that.

I never proposed a function that forces the previous value unnecessarily, so I don't know why you're complaining about that. The extra laziness I don't like is in the pair result; none of the uses I've seen thus can far make intentional use of that. That's why I tend to think atomicModifyIORef (as it exists today) is almost never what people actually want.

Squeezing into a second component leads to extra allocation in what may be a performance-critical function; that said, I'm willing to hold off on higher tuples for now.

On Sun, Jul 8, 2018, 3:36 AM winter <[hidden email]> wrote:

I believe new variations should always be motivated by use-case if there're too many choices, the lazy behavior of old `atomicModifyIORef` is justified by some cases the modifying functions are lazy in its argument, thus a lazy version could win by not forcing previous thunks, we'd want to keep its behavior as how it's documented.

As for tuples more than pairs, they're not really needed, user can always squeeze their product into `b` component.

IMHO only the addition of `atomicModifyIORef_` is sensible in the context of base, other APIs may go to package like primitives. But if you have a motivated use case with `atomicModifyIORef2`, etc. Please tell me.


On 2018年07月08日 03:09, David Feuer wrote:
Whoops! I left out the proposal link:


Also, what I called atomicModifyIORef_ below should really be called something like atomicModifyIORef'_, since it forces a polymorphic value.

Another thing to note: the underlying atomicModifyMutVar2# primop actually supports more than just pairs. It can handle triples, solos, and any other record types whose first components are lifted:

atomicModifyIORefSoloLazy
  :: IORef a -> (a -> Solo a) -> IO (Solo a)

atomicModifyIORefSolo
  :: IORef a -> (a -> Solo a) -> IO a

atomicModifyIORef3, atomicModifyIORef3Lazy
  :: IORef a -> (a -> (a, b, c)) -> IO (a, b, c)

etc.

Should we add any such?

On Sat, Jul 7, 2018, 2:35 PM David Feuer <[hidden email]> wrote:
I have proposed[1] the replacement of the atomicModifyMutVar# primop, and the addition of two cheaper but less capable ones. It seems likely that the proposal will succeed, but that the GHC steering committee will leave the question of user interface changes to the libraries list. I would like to open the discussion here.

The new primops lead naturally to several thin wrappers:

-- Atomically replace the IORef contents
-- with the first component of the result of
-- applying the function to the old contents.
-- Return the old value and the result of
-- applying the function, without forcing the latter.
--
-- atomicModifyIORef ref f = do
--   (_old, ~(_new, res)) <- atomicModifyIORef2Lazy ref f
--   return res
atomicModifyIORef2Lazy
  :: IORef a -> (a -> (a, b)) -> IO (a, (a, b))

-- Atomically replace the IORef contents
-- with the result of applying the function
-- to the old contents. Return the old and
-- new contents without forcing the latter.
atomicModifyIORefLazy_
  :: IORef a -> (a -> a) -> IO (a, a)

-- Atomically replace the IORef contents
-- with the given value and return the old
-- contents.
--
-- atomicWriteIORef ref x = void (atomicSwapIORef ref x)
atomicSwapIORef
  :: IORef a -> a -> IO a

Based on the code I've read that uses atomicModifyIORef, I believe that the complete laziness of atomicModifyIORef2Lazy and atomicModifyIORefLazy_ is very rarely desirable. I therefore believe we should also (or perhaps instead?) offer stricter versions:

atomicModifyIORef2
  :: IORef a -> (a -> (a, b)) -> IO (a, (a, b))
atomicModifyIORef2 ref f = do
  r@(_old, (_new, _res)) <- atomicModifyIORef2Lazy ref f
  return r

atomicModifyIORef_
  :: IORef a -> (a -> a) -> IO (a, a)
atomicModifyIORef_ ref f = do
  r@(_old, !_new) <- atomicModifyIORefLazy_ ref f
  return r

The classic atomicModifyIORef also admits a less gratuitously lazy version:

atomicModifyIORefNGL
  :: IORef a -> (a -> (a,b)) -> IO b
atomicModifyIORefNGL ref f = do
  (_old, (_new, res)) <- atomicModifyIORef2 ref f
  return res

Should we add that as well (with a better name)? Should we even consider *replacing* the current atomicModifyIORef with that version? That could theoretically break existing code, but I suspect it would do so very rarely. If we don't change the existing atomicModifyIORef now, I think we should consider deprecating it: it's very easy to accidentally use it too lazily.


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Re: Discussion: New atomic IORef functions

winter
In reply to this post by David Feuer
But if you force the pair result, you have to evaluate modifying function isn't it? That's sometime unwanted when you have an very expensive f here, let's say a deep binary search which might not depend previous result.

I think that is also why current atomicModifyIORef is designed this way, so I'd rather keep it the old way.



------------------ Original ------------------
From: David Feuer <[hidden email]>
Date: Thu,Jul 12,2018 0:51 AM
To: winter <[hidden email]>
Cc: Haskell Libraries <[hidden email]>
Subject: Re: Discussion: New atomic IORef functions

All the fundamental functions I've defined can be usefully used in the base library. I think that's one good reason to put them there. Another is that while the basic array operations have traditionally been exposed through array, vector, and primitive, and the basic TVar operations have been exposed through stm, the basic IORef and MVar operations have been exposed through base (except, for some reason, casMutVar#). I see no reason to change that.

I never proposed a function that forces the previous value unnecessarily, so I don't know why you're complaining about that. The extra laziness I don't like is in the pair result; none of the uses I've seen thus can far make intentional use of that. That's why I tend to think atomicModifyIORef (as it exists today) is almost never what people actually want.

Squeezing into a second component leads to extra allocation in what may be a performance-critical function; that said, I'm willing to hold off on higher tuples for now.

On Sun, Jul 8, 2018, 3:36 AM winter <[hidden email]> wrote:

I believe new variations should always be motivated by use-case if there're too many choices, the lazy behavior of old `atomicModifyIORef` is justified by some cases the modifying functions are lazy in its argument, thus a lazy version could win by not forcing previous thunks, we'd want to keep its behavior as how it's documented.

As for tuples more than pairs, they're not really needed, user can always squeeze their product into `b` component.

IMHO only the addition of `atomicModifyIORef_` is sensible in the context of base, other APIs may go to package like primitives. But if you have a motivated use case with `atomicModifyIORef2`, etc. Please tell me.


On 2018年07月08日 03:09, David Feuer wrote:
Whoops! I left out the proposal link:


Also, what I called atomicModifyIORef_ below should really be called something like atomicModifyIORef'_, since it forces a polymorphic value.

Another thing to note: the underlying atomicModifyMutVar2# primop actually supports more than just pairs. It can handle triples, solos, and any other record types whose first components are lifted:

atomicModifyIORefSoloLazy
  :: IORef a -> (a -> Solo a) -> IO (Solo a)

atomicModifyIORefSolo
  :: IORef a -> (a -> Solo a) -> IO a

atomicModifyIORef3, atomicModifyIORef3Lazy
  :: IORef a -> (a -> (a, b, c)) -> IO (a, b, c)

etc.

Should we add any such?

On Sat, Jul 7, 2018, 2:35 PM David Feuer <[hidden email]> wrote:
I have proposed[1] the replacement of the atomicModifyMutVar# primop, and the addition of two cheaper but less capable ones. It seems likely that the proposal will succeed, but that the GHC steering committee will leave the question of user interface changes to the libraries list. I would like to open the discussion here.

The new primops lead naturally to several thin wrappers:

-- Atomically replace the IORef contents
-- with the first component of the result of
-- applying the function to the old contents.
-- Return the old value and the result of
-- applying the function, without forcing the latter.
--
-- atomicModifyIORef ref f = do
--   (_old, ~(_new, res)) <- atomicModifyIORef2Lazy ref f
--   return res
atomicModifyIORef2Lazy
  :: IORef a -> (a -> (a, b)) -> IO (a, (a, b))

-- Atomically replace the IORef contents
-- with the result of applying the function
-- to the old contents. Return the old and
-- new contents without forcing the latter.
atomicModifyIORefLazy_
  :: IORef a -> (a -> a) -> IO (a, a)

-- Atomically replace the IORef contents
-- with the given value and return the old
-- contents.
--
-- atomicWriteIORef ref x = void (atomicSwapIORef ref x)
atomicSwapIORef
  :: IORef a -> a -> IO a

Based on the code I've read that uses atomicModifyIORef, I believe that the complete laziness of atomicModifyIORef2Lazy and atomicModifyIORefLazy_ is very rarely desirable. I therefore believe we should also (or perhaps instead?) offer stricter versions:

atomicModifyIORef2
  :: IORef a -> (a -> (a, b)) -> IO (a, (a, b))
atomicModifyIORef2 ref f = do
  r@(_old, (_new, _res)) <- atomicModifyIORef2Lazy ref f
  return r

atomicModifyIORef_
  :: IORef a -> (a -> a) -> IO (a, a)
atomicModifyIORef_ ref f = do
  r@(_old, !_new) <- atomicModifyIORefLazy_ ref f
  return r

The classic atomicModifyIORef also admits a less gratuitously lazy version:

atomicModifyIORefNGL
  :: IORef a -> (a -> (a,b)) -> IO b
atomicModifyIORefNGL ref f = do
  (_old, (_new, res)) <- atomicModifyIORef2 ref f
  return res

Should we add that as well (with a better name)? Should we even consider *replacing* the current atomicModifyIORef with that version? That could theoretically break existing code, but I suspect it would do so very rarely. If we don't change the existing atomicModifyIORef now, I think we should consider deprecating it: it's very easy to accidentally use it too lazily.


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Re: Discussion: New atomic IORef functions

David Feuer
I'm not committed to changing atomicModifyIORef. I'm much more
interested in adding atomicModifyIORef2, atomicModifyIORef'_, and
atomicSwapIORef. I do think it would be helpful to get a pair-strict
version of atomicModifyIORef (atomicModifyIORefP?), but I guess it's
not horrible if users have to write their own with atomicModifyIORef2.
The lazy atomicModifyIORef2Lazy and atomicModifyIORefLazy_ are pretty
optional: I see their laziness as more an implementation detail than
an essential feature.

On Wed, Jul 11, 2018 at 2:22 PM, Dr.Koster <[hidden email]> wrote:

> But if you force the pair result, you have to evaluate modifying function
> isn't it? That's sometime unwanted when you have an very expensive f here,
> let's say a deep binary search which might not depend previous result.
>
> I think that is also why current atomicModifyIORef is designed this way, so
> I'd rather keep it the old way.
>
> 发自我的iPhone
>
>
> ------------------ Original ------------------
> From: David Feuer <[hidden email]>
> Date: Thu,Jul 12,2018 0:51 AM
> To: winter <[hidden email]>
> Cc: Haskell Libraries <[hidden email]>
> Subject: Re: Discussion: New atomic IORef functions
>
> All the fundamental functions I've defined can be usefully used in the base
> library. I think that's one good reason to put them there. Another is that
> while the basic array operations have traditionally been exposed through
> array, vector, and primitive, and the basic TVar operations have been
> exposed through stm, the basic IORef and MVar operations have been exposed
> through base (except, for some reason, casMutVar#). I see no reason to
> change that.
>
> I never proposed a function that forces the previous value unnecessarily, so
> I don't know why you're complaining about that. The extra laziness I don't
> like is in the pair result; none of the uses I've seen thus can far make
> intentional use of that. That's why I tend to think atomicModifyIORef (as it
> exists today) is almost never what people actually want.
>
> Squeezing into a second component leads to extra allocation in what may be a
> performance-critical function; that said, I'm willing to hold off on higher
> tuples for now.
>
> On Sun, Jul 8, 2018, 3:36 AM winter <[hidden email]> wrote:
>>
>> I believe new variations should always be motivated by use-case if
>> there're too many choices, the lazy behavior of old `atomicModifyIORef` is
>> justified by some cases the modifying functions are lazy in its argument,
>> thus a lazy version could win by not forcing previous thunks, we'd want to
>> keep its behavior as how it's documented.
>>
>> As for tuples more than pairs, they're not really needed, user can always
>> squeeze their product into `b` component.
>>
>> IMHO only the addition of `atomicModifyIORef_` is sensible in the context
>> of base, other APIs may go to package like primitives. But if you have a
>> motivated use case with `atomicModifyIORef2`, etc. Please tell me.
>>
>>
>> On 2018年07月08日 03:09, David Feuer wrote:
>>
>> Whoops! I left out the proposal link:
>>
>> https://github.com/ghc-proposals/ghc-proposals/pull/149
>>
>> Also, what I called atomicModifyIORef_ below should really be called
>> something like atomicModifyIORef'_, since it forces a polymorphic value.
>>
>> Another thing to note: the underlying atomicModifyMutVar2# primop actually
>> supports more than just pairs. It can handle triples, solos, and any other
>> record types whose first components are lifted:
>>
>> atomicModifyIORefSoloLazy
>>   :: IORef a -> (a -> Solo a) -> IO (Solo a)
>>
>> atomicModifyIORefSolo
>>   :: IORef a -> (a -> Solo a) -> IO a
>>
>> atomicModifyIORef3, atomicModifyIORef3Lazy
>>   :: IORef a -> (a -> (a, b, c)) -> IO (a, b, c)
>>
>> etc.
>>
>> Should we add any such?
>>
>> On Sat, Jul 7, 2018, 2:35 PM David Feuer <[hidden email]> wrote:
>>>
>>> I have proposed[1] the replacement of the atomicModifyMutVar# primop, and
>>> the addition of two cheaper but less capable ones. It seems likely that the
>>> proposal will succeed, but that the GHC steering committee will leave the
>>> question of user interface changes to the libraries list. I would like to
>>> open the discussion here.
>>>
>>> The new primops lead naturally to several thin wrappers:
>>>
>>> -- Atomically replace the IORef contents
>>> -- with the first component of the result of
>>> -- applying the function to the old contents.
>>> -- Return the old value and the result of
>>> -- applying the function, without forcing the latter.
>>> --
>>> -- atomicModifyIORef ref f = do
>>> --   (_old, ~(_new, res)) <- atomicModifyIORef2Lazy ref f
>>> --   return res
>>> atomicModifyIORef2Lazy
>>>   :: IORef a -> (a -> (a, b)) -> IO (a, (a, b))
>>>
>>> -- Atomically replace the IORef contents
>>> -- with the result of applying the function
>>> -- to the old contents. Return the old and
>>> -- new contents without forcing the latter.
>>> atomicModifyIORefLazy_
>>>   :: IORef a -> (a -> a) -> IO (a, a)
>>>
>>> -- Atomically replace the IORef contents
>>> -- with the given value and return the old
>>> -- contents.
>>> --
>>> -- atomicWriteIORef ref x = void (atomicSwapIORef ref x)
>>> atomicSwapIORef
>>>   :: IORef a -> a -> IO a
>>>
>>> Based on the code I've read that uses atomicModifyIORef, I believe that
>>> the complete laziness of atomicModifyIORef2Lazy and atomicModifyIORefLazy_
>>> is very rarely desirable. I therefore believe we should also (or perhaps
>>> instead?) offer stricter versions:
>>>
>>> atomicModifyIORef2
>>>   :: IORef a -> (a -> (a, b)) -> IO (a, (a, b))
>>> atomicModifyIORef2 ref f = do
>>>   r@(_old, (_new, _res)) <- atomicModifyIORef2Lazy ref f
>>>   return r
>>>
>>> atomicModifyIORef_
>>>   :: IORef a -> (a -> a) -> IO (a, a)
>>> atomicModifyIORef_ ref f = do
>>>   r@(_old, !_new) <- atomicModifyIORefLazy_ ref f
>>>   return r
>>>
>>> The classic atomicModifyIORef also admits a less gratuitously lazy
>>> version:
>>>
>>> atomicModifyIORefNGL
>>>   :: IORef a -> (a -> (a,b)) -> IO b
>>> atomicModifyIORefNGL ref f = do
>>>   (_old, (_new, res)) <- atomicModifyIORef2 ref f
>>>   return res
>>>
>>> Should we add that as well (with a better name)? Should we even consider
>>> *replacing* the current atomicModifyIORef with that version? That could
>>> theoretically break existing code, but I suspect it would do so very rarely.
>>> If we don't change the existing atomicModifyIORef now, I think we should
>>> consider deprecating it: it's very easy to accidentally use it too lazily.
>>
>>
>>
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Re: Discussion: New atomic IORef functions

winter
OK then, here's some subtle things i want to shout out if you're going to make the change ; )

1. The proposed naming is just not great, how about these:

atomicExchangeIORef :: IORef a -> (a -> (a, b)) -> IO (a, (a, b))
atomicExchangeIORef' :: IORef a -> (a -> (a, b)) -> IO (a, (a, b))

atomicApplyIORef :: IORef a -> (a -> a) -> IO (a, a)
atomicApplyIORef :: IORef a -> (a -> a) -> IO (a, a)

It may not that great but it's more informative IMO.

3. atomicExchangeIORef should not force the tuple, e.g. don't pattern match on the tuple result, instead let users choose to force or not. (May worth adding some document, I'd be happy to help)

4. atomicExchangeIORef' will not only force the tuple, but also force both `a` and `b` part to match atomicModifyIORef' 's semantics.

5. atomicApplyIORef(and atomicApplyIORef') deserve its own primop, since we can skip building selector thunks all together.


On 2018年07月12日 02:31, David Feuer wrote:

> I'm not committed to changing atomicModifyIORef. I'm much more
> interested in adding atomicModifyIORef2, atomicModifyIORef'_, and
> atomicSwapIORef. I do think it would be helpful to get a pair-strict
> version of atomicModifyIORef (atomicModifyIORefP?), but I guess it's
> not horrible if users have to write their own with atomicModifyIORef2.
> The lazy atomicModifyIORef2Lazy and atomicModifyIORefLazy_ are pretty
> optional: I see their laziness as more an implementation detail than
> an essential feature.
>
> On Wed, Jul 11, 2018 at 2:22 PM, Dr.Koster <[hidden email]> wrote:
>> But if you force the pair result, you have to evaluate modifying function
>> isn't it? That's sometime unwanted when you have an very expensive f here,
>> let's say a deep binary search which might not depend previous result.
>>
>> I think that is also why current atomicModifyIORef is designed this way, so
>> I'd rather keep it the old way.
>>
>> 发自我的iPhone
>>
>>
>> ------------------ Original ------------------
>> From: David Feuer <[hidden email]>
>> Date: Thu,Jul 12,2018 0:51 AM
>> To: winter <[hidden email]>
>> Cc: Haskell Libraries <[hidden email]>
>> Subject: Re: Discussion: New atomic IORef functions
>>
>> All the fundamental functions I've defined can be usefully used in the base
>> library. I think that's one good reason to put them there. Another is that
>> while the basic array operations have traditionally been exposed through
>> array, vector, and primitive, and the basic TVar operations have been
>> exposed through stm, the basic IORef and MVar operations have been exposed
>> through base (except, for some reason, casMutVar#). I see no reason to
>> change that.
>>
>> I never proposed a function that forces the previous value unnecessarily, so
>> I don't know why you're complaining about that. The extra laziness I don't
>> like is in the pair result; none of the uses I've seen thus can far make
>> intentional use of that. That's why I tend to think atomicModifyIORef (as it
>> exists today) is almost never what people actually want.
>>
>> Squeezing into a second component leads to extra allocation in what may be a
>> performance-critical function; that said, I'm willing to hold off on higher
>> tuples for now.
>>
>> On Sun, Jul 8, 2018, 3:36 AM winter <[hidden email]> wrote:
>>> I believe new variations should always be motivated by use-case if
>>> there're too many choices, the lazy behavior of old `atomicModifyIORef` is
>>> justified by some cases the modifying functions are lazy in its argument,
>>> thus a lazy version could win by not forcing previous thunks, we'd want to
>>> keep its behavior as how it's documented.
>>>
>>> As for tuples more than pairs, they're not really needed, user can always
>>> squeeze their product into `b` component.
>>>
>>> IMHO only the addition of `atomicModifyIORef_` is sensible in the context
>>> of base, other APIs may go to package like primitives. But if you have a
>>> motivated use case with `atomicModifyIORef2`, etc. Please tell me.
>>>
>>>
>>> On 2018年07月08日 03:09, David Feuer wrote:
>>>
>>> Whoops! I left out the proposal link:
>>>
>>> https://github.com/ghc-proposals/ghc-proposals/pull/149
>>>
>>> Also, what I called atomicModifyIORef_ below should really be called
>>> something like atomicModifyIORef'_, since it forces a polymorphic value.
>>>
>>> Another thing to note: the underlying atomicModifyMutVar2# primop actually
>>> supports more than just pairs. It can handle triples, solos, and any other
>>> record types whose first components are lifted:
>>>
>>> atomicModifyIORefSoloLazy
>>>    :: IORef a -> (a -> Solo a) -> IO (Solo a)
>>>
>>> atomicModifyIORefSolo
>>>    :: IORef a -> (a -> Solo a) -> IO a
>>>
>>> atomicModifyIORef3, atomicModifyIORef3Lazy
>>>    :: IORef a -> (a -> (a, b, c)) -> IO (a, b, c)
>>>
>>> etc.
>>>
>>> Should we add any such?
>>>
>>> On Sat, Jul 7, 2018, 2:35 PM David Feuer <[hidden email]> wrote:
>>>> I have proposed[1] the replacement of the atomicModifyMutVar# primop, and
>>>> the addition of two cheaper but less capable ones. It seems likely that the
>>>> proposal will succeed, but that the GHC steering committee will leave the
>>>> question of user interface changes to the libraries list. I would like to
>>>> open the discussion here.
>>>>
>>>> The new primops lead naturally to several thin wrappers:
>>>>
>>>> -- Atomically replace the IORef contents
>>>> -- with the first component of the result of
>>>> -- applying the function to the old contents.
>>>> -- Return the old value and the result of
>>>> -- applying the function, without forcing the latter.
>>>> --
>>>> -- atomicModifyIORef ref f = do
>>>> --   (_old, ~(_new, res)) <- atomicModifyIORef2Lazy ref f
>>>> --   return res
>>>> atomicModifyIORef2Lazy
>>>>    :: IORef a -> (a -> (a, b)) -> IO (a, (a, b))
>>>>
>>>> -- Atomically replace the IORef contents
>>>> -- with the result of applying the function
>>>> -- to the old contents. Return the old and
>>>> -- new contents without forcing the latter.
>>>> atomicModifyIORefLazy_
>>>>    :: IORef a -> (a -> a) -> IO (a, a)
>>>>
>>>> -- Atomically replace the IORef contents
>>>> -- with the given value and return the old
>>>> -- contents.
>>>> --
>>>> -- atomicWriteIORef ref x = void (atomicSwapIORef ref x)
>>>> atomicSwapIORef
>>>>    :: IORef a -> a -> IO a
>>>>
>>>> Based on the code I've read that uses atomicModifyIORef, I believe that
>>>> the complete laziness of atomicModifyIORef2Lazy and atomicModifyIORefLazy_
>>>> is very rarely desirable. I therefore believe we should also (or perhaps
>>>> instead?) offer stricter versions:
>>>>
>>>> atomicModifyIORef2
>>>>    :: IORef a -> (a -> (a, b)) -> IO (a, (a, b))
>>>> atomicModifyIORef2 ref f = do
>>>>    r@(_old, (_new, _res)) <- atomicModifyIORef2Lazy ref f
>>>>    return r
>>>>
>>>> atomicModifyIORef_
>>>>    :: IORef a -> (a -> a) -> IO (a, a)
>>>> atomicModifyIORef_ ref f = do
>>>>    r@(_old, !_new) <- atomicModifyIORefLazy_ ref f
>>>>    return r
>>>>
>>>> The classic atomicModifyIORef also admits a less gratuitously lazy
>>>> version:
>>>>
>>>> atomicModifyIORefNGL
>>>>    :: IORef a -> (a -> (a,b)) -> IO b
>>>> atomicModifyIORefNGL ref f = do
>>>>    (_old, (_new, res)) <- atomicModifyIORef2 ref f
>>>>    return res
>>>>
>>>> Should we add that as well (with a better name)? Should we even consider
>>>> *replacing* the current atomicModifyIORef with that version? That could
>>>> theoretically break existing code, but I suspect it would do so very rarely.
>>>> If we don't change the existing atomicModifyIORef now, I think we should
>>>> consider deprecating it: it's very easy to accidentally use it too lazily.
>>>
>>>
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>>>
>>>
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Re: Discussion: New atomic IORef functions

David Feuer
In reply to this post by David Feuer
On Wed, Jul 11, 2018, 11:32 PM winter <[hidden email]> wrote:
OK then, here's some subtle things i want to shout out if you're going to make the change ; )

1. The proposed naming is just not great, how about these:

atomicExchangeIORef :: IORef a -> (a -> (a, b)) -> IO (a, (a, b))
atomicExchangeIORef' :: IORef a -> (a -> (a, b)) -> IO (a, (a, b))

atomicApplyIORef :: IORef a -> (a -> a) -> IO (a, a)
atomicApplyIORef :: IORef a -> (a -> a) -> IO (a, a)

It may not that great but it's more informative IMO.

I agree that my names aren't great, but I think yours are worse.


3. atomicExchangeIORef should not force the tuple, e.g. don't pattern match on the tuple result, instead let users choose to force or not. (May worth adding some document, I'd be happy to help)

I generally disagree. Being lazy in the tuple is pretty much an invitation to memory leaks. I think the lazy version should ideally also be available (it's what the new primop will provide, after all), but I don't think it should be the obvious thing for people to reach for.


4. atomicExchangeIORef' will not only force the tuple, but also force both `a` and `b` part to match atomicModifyIORef' 's semantics.

I don't see the need for this. Users can easily force exactly what they want using atomicModifyIORef2 (or if they really want, atomicModifyIORef2Lazy).


5. atomicApplyIORef(and atomicApplyIORef') deserve its own primop, since we can skip building selector thunks all together.

Yes, I've implemented that primop already, and it will be added. I'm still looking for an expert to help implement the simplest of the lot, atomicSwapMutVar#, as efficiently as possible.

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Re: Discussion: New atomic IORef functions

winter
> I generally disagree. Being lazy in the tuple is pretty much an invitation to memory leaks. I think the lazy version should ideally also be available (it's what the new primop will provide, after all), but I don't think it should be the obvious thing for people to reach for.

let's put lazy modifying cases aside(which is not that rare),  I'm pretty sure haskell forks will use strict version from time to time, it's not the lack of ' make a funtion more easy to reach, it's really just a matter of document,  why breaking consistency with original atomicModifyIORef/atomicModifyIORef'  ? I expect the old mnemonic still works on new ones.

> I don't see the need for this. Users can easily force exactly what they want using atomicModifyIORef2 (or if they really want, atomicModifyIORef2Lazy).

Exactly the same reason with above, if people have to force the a and b part to stop leakage, then this strict version is not really doing a good job IMHO, I just don't see the value that you have already forced the modification function, but then stopped at a half place where left a thunk inside the IORef.



On 2018年07月12日 13:07, David Feuer wrote:
On Wed, Jul 11, 2018, 11:32 PM winter <[hidden email]> wrote:
OK then, here's some subtle things i want to shout out if you're going to make the change ; )

1. The proposed naming is just not great, how about these:

atomicExchangeIORef :: IORef a -> (a -> (a, b)) -> IO (a, (a, b))
atomicExchangeIORef' :: IORef a -> (a -> (a, b)) -> IO (a, (a, b))

atomicApplyIORef :: IORef a -> (a -> a) -> IO (a, a)
atomicApplyIORef :: IORef a -> (a -> a) -> IO (a, a)

It may not that great but it's more informative IMO.

I agree that my names aren't great, but I think yours are worse.


3. atomicExchangeIORef should not force the tuple, e.g. don't pattern match on the tuple result, instead let users choose to force or not. (May worth adding some document, I'd be happy to help)

I generally disagree. Being lazy in the tuple is pretty much an invitation to memory leaks. I think the lazy version should ideally also be available (it's what the new primop will provide, after all), but I don't think it should be the obvious thing for people to reach for.


4. atomicExchangeIORef' will not only force the tuple, but also force both `a` and `b` part to match atomicModifyIORef' 's semantics.

I don't see the need for this. Users can easily force exactly what they want using atomicModifyIORef2 (or if they really want, atomicModifyIORef2Lazy).


5. atomicApplyIORef(and atomicApplyIORef') deserve its own primop, since we can skip building selector thunks all together.

Yes, I've implemented that primop already, and it will be added. I'm still looking for an expert to help implement the simplest of the lot, atomicSwapMutVar#, as efficiently as possible.


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