I haven't yet looked much into effect systems so can't really talk about them but I think even with something like MTL you can achieve a decent level composabil

I haven't yet looked much into effect systems so can't really talk about them but I think even with something like MTL you can achieve a decent level composability/modularity where you build up a library of capabilities that you can then use to pick and choose effects from, eg.: type FooContext m = (GetTime m, GenUUID m, Log m, Auth m, ReadDB m, WriteS3 m, BarApi m, etc...) foo :: (FooContext m) => UUID -> m Foo The main downside is ergonomics, ie. the amount of boilerplate needed, but personally I don't think that's a high price to pay.