simplexmq/spec/modules/Simplex/Messaging/TMap.md

Simplex.Messaging.TMap

STM-safe concurrent map (TVar (Map k a)).

Source: TMap.hs

lookupInsert / lookupDelete

Atomic swap operations using stateTVar + alterF. lookupInsert returns the previous value (if any) while inserting the new one; lookupDelete returns the value while removing it. Both are single STM operations — no window between lookup and modification.

union

Left-biased: the passed-in Map wins on key conflicts. union additions tmap overwrites existing keys in tmap with values from additions.

alterF

The STM action f runs inside the same STM transaction. If f retries, the entire alterF retries. If f has side effects via other TVars, they compose atomically with the map modification.