Is the "else" part of orElse called when a transaction is retried due to another transaction writing to a TVar it had read, or only when retry is explicitly called?
If you have
orElse a b
then b is only run if retry is called explicitly in a. Otherwise orElse would essentially become nondeterministic. (The rerunning of transactions that is done by the STM runtime is transparent and should not effect the outcome of any computation.)
来源：https://stackoverflow.com/questions/10101044/haskell-tvar-orelse