Simon Marlow writes:
We currently have
Foreign.Marshal.Unsafe.unsafeLocalState :: IO a -> a unsafeLocalState = unsafePerformIO
however, unsafePerformIO in GHC is rather expensive, because it tries to avoid the IO being executed more than once when there are multiple processors. However, for the cases that we claim unsafeLocalState is to be used for, it would be safe to use the cheaper unsafeDupablePerformIO instead.
So code using unsafeLocalState as documented will go faster? Sounds like a clear win. That's part of the reason for documenting interfaces. (That code misusing it will get broken is a bonus.) It might make sense to also export unsafeDupablePerformIO from System.IO.Unsafe, for single-threaded uses that don't conform to the unsafeLocalState contract.