README.md

ocamlfind ocamlc -package camlp4.extend,camlp4.quotations -syntax camlp4o -c pa_hello.ml
ocamlc -pp "camlp4o ./pa_hello.cmo" hello.ml

hello.ml

let () = print_endline (hello "hello world")

pa_hello.ml

module Id : Camlp4.Sig.Id = struct
  let name = "HelloWorld"
  let version = "1.0"
end

module Make (Syntax : Camlp4.Sig.Camlp4Syntax) = struct
include Syntax

EXTEND Syntax.Gram
  expr:
    [[ "hello"; x = expr -> x ]]
  ;
END
end

module M = Camlp4.Register.OCamlSyntaxExtension(Id)(Make)