But entirely definable in user code, so an effect is essentially a set of possibly impure operations you can perform (like I/O or exception throwing), and a fun

But entirely definable in user code, so an effect is essentially a set of possibly impure operations you can perform (like I/O or exception throwing), and a function that exhibits that effect has access to those operations. Of course the call sites then also exhibit that effect, unless they provide implementations of the effect operations.