3 ms·

Isn't mixing of effects exactly what monad transformers are for? AFAICT you want an `ExceptT e ST` for some exception type `e`. https://hackage.haskell.org/pac

• codebje 2y ago
  Yes, but transformers have a few drawbacks: the order of stacking alters behaviour, and you need to write n^2 instances for n transformers. Compare ExceptT e (StateT m a) and StateT (ExceptT e m a): if you just want your computation to have state and exceptions the difference shouldn’t matter.
  • throw156754228 2y ago
    Also their other well known problem: you lose the program state if an exception is thrown in the monad above.
    • tome 2y ago
      Yeah, that's basically the same problem as `StateT s (ExceptT e m)` but for `StateT s m` where `m` throws exceptions.
  • dllthomas 2y ago
    > if you just want your computation to have state and exceptions the difference shouldn’t matter But... you don't just want that. You almost certainly care whether state changes are discarded when an exception is thrown. I don't claim that the types there are the most obvious or natural way to specify that, but there is a meaningful difference that shouldn't be handwaved away.
    • codebje 2y ago
      The fact that with transformer stacks you can wind up with state changes being discarded on exception if you get the stack arranged in a particular order isn't what I'd call a _feature_ of transformer stacks :-) If effect A is invoked before effect B, effect A should happen and stay that way: if I update state before an exception is thrown, the update should persist. If I send a network message before an exception is thrown, the network message is still sent. If I launch the nukes before an exception is thrown, the missiles are still flying. If I want to batch up state updates to only happen together as a unit, I'll use a transaction effect.
  • dullcrisp 2y ago
    I might have this wrong but I think if you want state and exceptions you probably want StateT (ExceptT e m a). The alternative would be to have state or exceptions, i.e. when you have an exception you no longer have state (which might be a legitimate type in some cases).
    • gloria_mundi 2y ago
      Remember that transformers are "inside-out", i.e. `StateT (ExceptT e m) a` is isomorphic to `m (Except e (State a))`. If we want to keep state if an exception occurs, you need a `m (State (Except e a))` which is `ExceptT e (StateT m) a`.
      • dullcrisp 2y ago
        Yeah I could never keep this straight
        • dllthomas 2y ago
          The way I remembered it, before I internalized it, was to think about applying the run functions one at a time. runSomethingT will take a `SomethingT ... m ... a` and give you some kind of `m (... a)`.
• tome 2y ago
  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.