Record updates on OCaml types with private fields

Record updates on OCaml types with private fields

Goal: clarify whether updating a non-private field is permitted when the record type’s signature declares another field private.

OCaml’s private modifier blocks external mutation of that field, but it is unclear whether updating a sibling field is rejected because the operation copies the private field into a new record value. The answer may depend on whether the record type is concrete or abstract in the signature.

Version assumption: OCaml 5.x; private behavior is reported as stable since OCaml 3.12.

• Does updating field y via record-update syntax compile when field x is private and y is mutable?
• If it compiles, is x copied unchanged, or does the update count as modifying a private field?
• Does fresh record construction outside the module face the same restriction?