Sorry, Haskell’s “monad transformer library”. One of the earliest approaches to composability of multiple monadic effects. It’s pretty similar to an algebraic effect system allowing you to write effectual computations with types like `(Error m, Nondet m, WithState App m) => m ()` to indicate a computation that returns nothing but must be executed with access to error handling, nondeterminism, and access to the App type as state.

There are a few drawbacks to it, but it is a pretty simple way to get 80% of the ergonomics of algebraic effects (in Haskell).