I'm trying to understand why some code isn't behaving as I'd expect, and to determine whether it's a bug or not (and where that bug might be). Here's the simplest version of the code: import Data.ByteString.Internal (inlinePerformIO) import qualified Data.Vector as V import qualified Data.Vector.Mutable as VM main :: IO () main = do vm <- VM.new 1 VM.write vm 0 'A' let x = inlinePerformIO $ VM.write vm 0 'B' x `seq` (V.freeze vm >>= print) A more complete example is available on lpaste[1]. The problem is that I would expect the output to be "B", but in fact "A" is still printed. From the longer paste that I linked to, you can see that: * When using unsafePerformIO and unsafeDupablePerformIO, the semantics work as I would have expected: "B" is printed after forcing evaluation of the result. * If I add a `VM.read vm 0` call after the write, it also works. * Using IORef, the behavior is also as I would have expected: as soon as the result is evaluated, the reference is updated. I'm testing on GHC 7.8.3, Ubuntu 64-bit, and compiling with -O2. I'm curious if anyone has an idea as to why there is this difference in behavior. Michael [1] http://lpaste.net/108483