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.