On 25 June 2004 11:30, Daan Leijen wrote:
> Is there anyone who knows why "atomicModifyIORef" has type:
>
> IORef a -> (a -> (a, b)) -> IO b (1)
>
> Instead of:
>
> IORef a -> (a -> a) -> IO a (2)
>
>
> It seems to me that returning the old value is always good
> enough right? Here is an implementation of "atomicModifyIORef"
> with the current type in terms of a function "proposedModifyIORef"
> with type (2).
>
> atomicModifyIORef :: IORef a -> (a -> (a,b)) -> IO b
> atomicModifyIORef ref f
> = do old <- proposedModifyIORef ref (fst . f)
> return (snd (f old))
Yes, but in your version you apply f to the old value twice, potentially
duplicating an expensive computation.
atomicModifyIORef isn't part of the FFI spec, BTW.
Cheers,
Simon
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