Yes, this is well known, good catch! If you read McBride's paper "Clowns to the Left of Me, Jokers to the Right" (https://www.cis.upenn.edu/~cis39903/static/cl

Yes, this is well known, good catch! If you read McBride's paper "Clowns to the Left of Me, Jokers to the Right" (https://www.cis.upenn.edu/~cis39903/static/clowns.pdf https://www.cis.upenn.edu/~cis39903/static/clowns.pdf) is exactly about systematically deriving the type of defunctionalized stacks for the fold over an ADT. The ordinary derivative is then the special case where you are just mapping a function that doesn't change the type.