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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.

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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.

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.

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