Bluefin is what the Haskell world calls an “effect system”. I released the first version in early 2024, basing it closely on effectful by Andrzej Rybczak. Similarly to effectful, Bluefin is an “IO-wrapper” (or “analytic”) effect system, meaning that it is a lightweight wrapper around Haskell’s IO type1. The major difference between effectful and Bluefin is that in effectful the presence of effects is indicated by constraints in the type system (a familiar and traditional approach) whereas in Bluefin the presence of effects is indicated by value level arguments.