Discussion: New atomic IORef functions

[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 <drkoster at qq.com> 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 <david.feuer at gmail.com> 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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