Unbelievably, I actually used this in a project when I found myself thinking that I needed a combination of a forward and backward propagating state monad. [1]

Unbelievably, I actually used this in a project when I found myself thinking that I needed a combination of a forward and backward propagating state monad. [1] https://github.com/ElementsProject/simplicity/blob/35627fc49ad96fcb844cca72ffbc69b6e934cb4c/Haskell/Core/Simplicity/LibSecp256k1/Spec.hs#L548 https://github.com/ElementsProject/simplicity/blob/35627fc49...