Hi friends, I am writing a library that has Handle-like values for resources that are not thread-safe (foreign state from a non-thread-safe C library). In other words, a resource can only be "used" by one Haskell thread at a time. Unless I take steps, if one shares the resource between concurrent Haskell threads, bad things will happen at runtime (even with a single-threaded runtime). Naturally, I want consumers to use my library safely, so I am considering making it safe for all by pessimistically enforcing safe usage with MVars. Is it common/expected that Haskell libraries enforce safe access like this? MVars are not free from cost, but are they cheap (i.e. almost as fast as no forced synchronization) as long as no blocking/rescheduling occurs? Thanks, -- Thomas Koster