STM is useless with full immutability: it is safe in Haskell because effects (including mutation) can be controlled/packaged into contained areas, via abstracti

STM is useless with full immutability: it is safe in Haskell because effects (including mutation) can be controlled/packaged into contained areas, via abstractions like monads. That said, one downside of STM is that it isn't as efficient as some other schemes especially with high contention, as everything gets serialized through the metadata for tracking transactions. It's an awesome tool, but not a panacea (like with any other technology). (I'm sure you know this, but others reading the thread may not.)