If I am inside a STM whose transaction fail, and I retry as part of the normal control flow (no STM collision etc..), I might want to indicate to someone outside a way to take corrective action.
If it's purely 'outgoing', then my STM can still be replayed purely.
How might I perform outgoing IO from a STM retry ?
Has anyone encountered this ? How evil is this ?
If you are sure that the performed IO does not affect the invariants of STM, you can use
unsafeIOToSTM. Use it with great care, as all the unsafe stuff.
I do, however, wonder if you really need that. It's impossible to tell from the question. If you can, you should avoid it.