GHC.Conc modifyTVar primitive

[dmbarbour]

I would like modifyTVar as a 'write-only' primitive supported by GHC's STM.

The semantic definition is:

modifyTVar :: TVar a -> (a -> a) -> STM () modifyTVar v f = readTVar v >>= writeTVar v . f

However, explicitly reading then writing the TVar introduces unnecessary interference between transactions. The value held by the TVar does not affect the behavior of the transaction. modifyTVar should not interfere with other transactions that only writeTVar or modifyTVar.

Even if a non-interfering implementation isn't feasible due to the underlying implementation of GHC's STM, providing the function would provide a point for a transparent upgrade in the future.

[simonpj]

Tim Harris writes: not sure if this is what the original authors intend, but one option would be to execute all these modifications at commit-time.

I.e. the log entry would be <tvar, f> pairs for these tvars, rather than <tvar, old, new> triples.

At commit time these entries would be expanded to <tvar, old, new> triples, and then attempt to be committed in the normal way.

If there is a subsequent conflict on one of these tvars then "f" would be re-evaluated and then the commit re-started.

There'd be some details to work through if the same tvar is read/written directly in the same tx.