Ah, sorry. The question still holds, though it's less obvious what it means with this effect stack. Perhaps a more obvious example would be failing, stateful co

Ah, sorry. The question still holds, though it's less obvious what it means with this effect stack. Perhaps a more obvious example would be failing, stateful computation written as follows as an order-free restricted type in Haskell ( MonadPlus , MonadState s ) => m a Given some concrete choices of monads to witness those effects like MaybeT and StateT there are two "orders" we can stack them. MaybeT (StateT s Identity) a StateT s (MaybeT Identity) a where Identity forms the "effect free" base of our stack. While functions which operate on types with the order-free signature work for each stack, ultimately the stacks represent different combined effects. In particular, the `StateT s (MaybeT Identity)` stack throws away state if the computation fails while `MaybeT (StateT s Identity)` does not. run1 :: StateT s (MaybeT Identity) a -> s -> Maybe (s, a) -- failure captures the result state run2 :: MaybeT (StateT s Identity) a -> s -> (s, Maybe a) -- result state always produced, failure captured -- only in the result value So, ultimately the semantics of your effect set depend upon the ordering of your effects. At least in Haskell. I'd be interested to see how Koka handles it.