RFE: openLocalStateIO

[dmwit]

One initializes acid by opening up a checkpoint of the state, and providing an initial value if there's no checkpoint, with

openLocalState :: Sane st => st -> IO (AcidState st)

This works well for applications with a sensible static default state; but in general programs may want to do some computation to produce an initial state. In particular, for random number generator seeding, one often has to do some IO to pick a suitable initial seed. One would like to avoid doing this IO unless it's necessary (in the case of seeding a PRNG the call may block until enough entropy is available, for example, and in any case consumes a limited resource). Something like the following is already possible:

main = do
    seed <- unsafeInterleaveIO pickASeed
    acid <- openLocalState seed
    -- etc.

However, this is a bit awkward; uses of unsafe* are naturally a point of contention. A more ideal solution would be to have an initializer with a type like

openLocalStateIO :: Sane st => IO st -> IO (AcidState st)

available in the acid-state API, which promises to execute its argument only when the state can't be found on disk. IO-variants of other similar functions might be welcome for other users, too.