[mercurial]

> The module system in OCaml sounds very nice (and we all know what the "O" for!). But there's still a bias towards a sort of static-ness in FP. For example, the use of abstract data types where a Java programmer may use a class hierarchy. Clients cannot extend an ADT: I can't make my own List in Haskell and pass it off to a function.

Depends on what your function accepts. If it takes explicitly a list, you're screwed, but it clearly was never intended to be generic. If it accepts something Foldable or Traversable, just make sure your data structure has an instance for these type classes.

In OCaml, you can have objects and inheritance if you absolutely want to, but you can get a lot out of structural typing before going there. If you want extensible ADTs, you can, but you need to plan for it by using polymorphic variants [1] at the expense of some safety.

1: https://realworldocaml.org/v1/en/html/variants.html - scroll down to the "Polymorphic variants" section