Type annotation to ensure an extensible's variant given extension is matched exhaustively?

Learning
1

I’d like to strengthen the typing discipline of an extensible variant the way we can do it for polymorphic variants. That is via a type annotation.

I believe this is not possible (and suspect it may never be since extensions may not be unique a given structure). But did I maybe miss something ?

More precisely in the to_string function below I’d like to add an annotation that indicates me that B is unhandled.

type t = ..
module M = struct
  type t += A | B
end

let to_string v = match v with
| M.A -> "A"
| _ -> "Unknown"
2

This is indeed not currently possible since extensions are not types.
One option that could work would be to extend the #pattern patterns to extensible variants. Then with pattern negation, your use case could be written:

let to_string v = match v with
| M.A -> "A"
| not #M.t -> "Unknown"
1 Like
3

I googled it, for reference: Pattern negation · Issue #7628 · ocaml/ocaml (github.com)