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.

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.