ocaml/parsing/VIPs.md

VIPs

A VIP is a common syntax error, for which a good error message should be given.

Structures versus signatures

Everything that is allowed in a structure but forbidden in a signature, or vice-versa, is a VIP. For instance, writing:

  exception A = B

is allowed in a structure, but forbidden in a signature. (Here, we might wish to make the error message depend on the lookahead token; the token = suggests that the user confuses a structure and a signature.)

Similarly, writing struct where sig is expected, or vice-versa, is probably a common mistake.