Simon Peyton-Jones wrote:
>
> | In the meantime, I'm glad we have got a new function
> | atomicModifyIORef
> | which I for one will use, when it gets into GHC's regular release.
>
> Just before this gets out of the door... any chance of calling it
>
> modifyIORef
>
> and documenting that it's atomic
My vote goes for
modifyIORef :: IORef a -> (a -> (a,b)) -> IO b
modifyIORef_ :: IORef a -> (a -> a) -> IO a
modifyIORef_ ioRef fn = modifyIORef ioRef (\ a0 -> let a1 = fn a0 in (a1,a1))
The names are similar to those of GHC's modifyMVar and modifyMVar_.
I think there's some sense in providing modifyIORef_ as well as
modifyIORef, as I think for some implementations it will be even faster,
since it's unnecessary to create thunks for (fst (fn a0)) and
(snd (fn a0)). But you lot know a lot more about writing Haskell
compilers than I do . . .
Thus you can implement a counter returning integers 1,2,3,...
via
newCounter :: IO (IO Integer)
newCounter = do
ioRef <- newIORef 0
return (modifyIORef_ ioRef (+1))
I don't think it's necessary for the name to include "atomic".
We are unlikely ever to want a non-atomic version of atomicModifyIORef,
especially as such a thing would be quite dangerous, and you can roll
your own from readIORef and writeIORef.
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