Haskell: TVar: orElse

concurrency, ghc, haskell, stm, tvar

Solution

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.)

Problem

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?

Original source