I have written a small library for supporting one-shot without using unsfePerformIO... The library uses SYSV semaphores under linux to make sure the functional argument of "once" is only ever run once. It uses the ProcessID as the key for the semaphore, so will even enforce the once-only property accross different Haskell threads. Some semaphore functions are also exported, allowing other constraints to be used (for example, once only over multiple processes by using a constant ID rather than the processID. I have attached the source for the library incase anyone is interested. If people think it is useful I could put it up on a website (let me know). Also attached is an example, which can be compiled with: ghc -o test NamedSem.hs Test.hs -package posix Keean.