> Implication of Using STM Running I/O Inside STM— There is a strict boundary between the STM world and the ZIO world. This boundary propagates even deeper because we are not allowed to execute arbitrary effects in the STM universe. Performing side effects and I/O operations inside a transaction is problematic. In the STM the only effect that exists is the STM itself. We cannot print something or launch a missile inside a transaction as it will nondeterministically get printed on every reties that transaction does that.
Does Zio actually offer any protection here, or is it just telling the reader that they're on their own and should be wary of footguns?
STM happens inside the STM monad while regular effects happen in the ZIO monad. If you try to do ZIO effects inside an STM transaction you'll get a type error.
Scala doesn't enforce purity like Haskell though so it wont stop you if you call some normal Scala or Java code with side effects. In practice its not a problem because you're wrapping any effectful outside APIs before introducing them into your code.
I am not super familiar with it, great question - my guess would be its the latter! Does Haskell's provide protections?