OCaml, F#, Haskell, Elm, ... (anything with lightweight notation to define sum types [sometimes with only 1 variant], and a module system to limit who can const

OCaml, F#, Haskell, Elm, ... (anything with lightweight notation to define sum types [sometimes with only 1 variant], and a module system to limit who can construct them) There's a whole well-reviewed book on exactly this: https://pragprog.com/book/swdddf/domain-modeling-made-functional https://pragprog.com/book/swdddf/domain-modeling-made-functi...