[rendaw]

It looks like "effect" as in impure functions in a functional language? I.e. a new way of dealing with effects (global/hidden state mutations) in a language that makes the pure-impure distinction. I'm not entirely sure.

I thought it was going to be something like contracts or dependent types or something.

[adastra22]

No. It is essentially resumable exceptions. You throw an exception saying “I need a MyAlgebraicType” and the effect handler catches the exception, generates the value, and returns execution to the place the exception was called from.

[helix278]

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.