I had the same reaction. The only part of this that I could read was a little surprising: pop :: CallstackIO () pop = modify popFrame wher

I had the same reaction. The only part of this that I could read was a little surprising: pop :: CallstackIO () pop = modify popFrame where popFrame (_:xs) = xs popFrame xs = xs Not that it's very onerous to implement, but I would expect these "pop" semantics to be imported from some more general type (or trait, or whatever Haskell uses for type composition). That said, I don't know what `modify` means.