Consider implementing atomicModifyMutVar# in Haskell

I believe we could implement atomicModifyMutVar# in Haskell, rather than making it a primop. This seems like a simpler/cleaner approach (among other things, it gets a perfectly legitimate type signature!), but we'd have to see how it affects performance. I think something like this should work:

atomicModifyMutVar# :: MutVar# s a -> (a -> (a,b)) -> State# s -> (# State# s, b #)
atomicModifyMutVar# mv f s =
  case readMutVar# mv s of { (# s', old #) ->
  let f_old = f old in
  case casMutVar# mv old (fst $ f_old) s of { (# s', unchanged, new #) ->
  case unchanged of
    0# -> (# s', snd f_old #)
    _ -> atomicModifyMutVar# mv f s' }}

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Trac field	Value
Version	8.2.1-rc2
Type	Task
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Runtime System
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