It is very easy to solve. Two solutions: * Implement open types for the language. OCaml did this in 4.02, and it is very useful. * Heterogenous containers: htt

It is very easy to solve. Two solutions: * Implement open types for the language. OCaml did this in 4.02, and it is very useful. * Heterogenous containers: https://github.com/MLton/mlton/blob/master/lib/mlton/basic/het-container.fun https://github.com/MLton/mlton/blob/master/lib/mlton/basic/h... It has its uses, albeit it is rare such a use comes into play.