[tome]

Oh, I meant it's impossible to mix ST with actual exceptions as implemented in the RTS, rather than with ExceptT which simulates exceptions in pure code (like StateT simulates mutable state in pure code).

You're right, through a stroke of luck it's possible to use `ExceptT e ST r` and either handle the exception part first, to get `ST r`, or handle the ST part first to get `Either e r`, so in that sense you can "mix" exceptions and ST. That doesn't work for all transformers though. If you have `Stream (Of a) ST r` then you must consume the stream first. You can't run the ST part an get a pure stream `Stream (Of a) Identity r`. So in that sense ST can't be mixed with other effects. Bluefin does allow you to do that, though.