A mutable variable in a mutation capable monad (IO) holding a Unboxed value. This allows fast modification because of unboxed storage.

Multithread Consistency Notes

In general, any value that straddles a machine word cannot be guaranteed to be consistently read from another thread without a lock. GHC heap objects are always machine word aligned, therefore, a IORef is also word aligned. On a 64-bit platform, writing a 64-bit aligned type from one thread and reading it from another thread should give consistent old or new value. The same holds true for 32-bit values on a 32-bit platform.