> I'm not even sure how to avoid writing explicit type annotations for Int, String, and [Int] there; maybe OCaml makes it easy It does make it easy. OCaml has

> I'm not even sure how to avoid writing explicit type annotations for Int, String, and [Int] there; maybe OCaml makes it easy It does make it easy. OCaml has a full matrix of types: - Nominal product types (records) - Nominal sum types (variants) - Structural product types (objects) - Structural sum types (polymorphic variants) You could actually always use structural types and never have to actually define types. But OCaml people actually really like the nominal types because their typing rules are stricter and easier for people to understand. But the structural types can come in really handy, e.g. they give you a very convenient way to encode data between libraries that don't need to know about each other. E.g. there is an effort to add JSON-structured output to the OCaml compiler itself. This is using a polymorphic variant type to represent the JSON data type, to avoid having a dependency on any particular JSON library, but still being compatible with popular OCaml JSON libraries ( https://ocaml-community.github.io/yojson/yojson/Yojson/index.html#type-of-the-json-tree https://ocaml-community.github.io/yojson/yojson/Yojson/index... ).