Complaining about with default in OCaml - ocaml

Question

The following code from Real World OCaml Chapter 17: Data Serialization with S-Expressions page 339 is not compiling on utop:
type http_server_config = { web_root: string;
port: int with default(80);
addr: string with default("localhost");
} [##deriving sexp] ;;
It is complaining about the with in port: int with default(80);
Thanks!

Answer 1

In recent versions of the Janestreet's Core library, all syntax extensions have been converted from camlp4 extensions to ppx extensions. Consequently, you unfortunately need to adapt the syntax of all examples using syntax extensions in Real World OCaml.
Fortunately, contrarily to camlp4 extensions, ppx extensions cannot wildly modify OCaml syntax. At most they can use a slightly extended syntax compared to vanilla OCaml, wich adds extension nodes and attributes.
In particular, this means that since field:type with … is not synctatically valid for vanilla OCaml, it is also not a valid syntax with the ppx_sexp_conv extension enabled. In your case, the default value annotation needs to be written as an attribute of the corresponding record fields:
type http_server_config = {
web_root: string;
port: int [#default 80];
addr: string [#default "localhost"];
} [##deriving sexp] ;;
Note that in order to work in utop you need first to have required the
ppx_sexp_conv extension and opened the default runtime module:
#require "ppx_sexp_conv";;
open Sexplib.Std ;;
type http_server_config = {
web_root: string;
port: int [#default 80];
addr: string [#default "localhost"];
} [##deriving sexp] ;;

Answer 2

The examples in Real World OCaml assume you have Core installed, which includes some syntax extensions. I followed the installation instructions here and I still see the same problem you're seeing.
When I tried to install the syntax extending module, I saw this:
utop # #camlp4o;;
utop was built without camlp4 support.
So, I conclude that it's a little bit tricky to set up the environment that Real World OCaml is expecting.
At any rate, #Basile_Starynkevitch is correct. The code is using syntax extensions that aren't supported in your utop.