Anything that happens in the STM monad can be undone if something is wrong. If you for example write to a TVal but another thread was doing it at the same time,

Anything that happens in the STM monad can be undone if something is wrong. If you for example write to a TVal but another thread was doing it at the same time, one of the threads won't have its changes go through until the transaction is retried. Of course this is all invisible to the person using STM.