...

The problem was whether DiffArrays should be thread-safe in the Concurrent Haskell sense, which means protecting access to the DiffArray with an MVar.

Can you give some estimate of the cost of using an MVar in this way?

It depends a lot on whether the cost of using the MVar is dwarfed by other costs in the relevant operations. For example, Handles are protected by MVars but this isn't a big deal because you don't usually do lots of tiny operations on a Handle (well, it bites you if you try to implement 'cat' using hGetChar/hPutChar). Compared to an IORef, modifying an MVar is many times more expensive. takeMVar/putMVar are implemented as out-of-line calls, but they could be partially inlined to cover the cases where they don't need to block if efficiency is really critical.

Could some cheaper mechanism be provided for the special case of a mutex?

My guess is that it wouldn't be worth having a special mutex type instead of (MVar ()). Cheers, Simon