(Introduced in OCaml 4.02.2, extended in 4.03)
Some syntactic constructions are accepted during parsing and rejected during type checking. These syntactic constructions can therefore not be used directly in vanilla OCaml. However, -ppx rewriters and other external tools can exploit this parser leniency to extend the language with these new syntactic constructions by rewriting them to vanilla constructions.
(Introduced in OCaml 4.02.2)
Operator names starting with a # character and containing more than one # character are reserved for extensions.
(Introduced in OCaml 4.03)
Int and float literals followed by an one-letter identifier in the range [g..z∣ G..Z] are extension-only literals.