OCaml corebuild cannot link to Core.Std

My problem is similar to this, however, in my case .ocamlinit is set.

Here is my ocaml version.

mymac:Desktop myusr$ ocaml --version
The OCaml toplevel, version 4.08.1

Here is my opam version.

mymac:Desktop myusr$ opam --version
2.0.5

Here is my opam switch.

mymac:Desktop myusr$ opam switch
#  switch   compiler                    description
→  4.08.1   ocaml-base-compiler.4.08.1  4.08.1
   default  ocaml-base-compiler.4.08.1  default

Here's my .ocamlinit:

mymac:Desktop myusr$ cat ~/.ocamlinit
(* ## added by OPAM user-setup for ocamltop / base ## 3ec62baf6f9c219ae06d9814069da862 ## you can edit, but keep this line *)
#use "topfind";;
(* ## end of OPAM user-setup addition for ocamltop / base ## keep this line *)
#thread;;
#require "core.top";;
#require "core.syntax";;

Here is the evidence that I already have core installed.

mymac:Desktop myusr$ opam install core utop
[NOTE] Package utop is already installed (current version is 2.4.1).
[NOTE] Package core is already installed (current version is v0.12.3).

Here is the sum.ml file from Real World OCaml:

open Core.Std

let rev read_and_accumulate accum =
      let line = In_channel.input_line In_channel.stdin in
      match line with
      | None -> accum
      | Some x -> read_and_accumulate (accum +. Float.of_string x)

let () =
  printf "Total: %F\n" (read_and_accumulate 0.)

Here's what happens when I try to build it with corebuild:

mymac:Desktop myusr$ corebuild sum.native
+ ocamlfind ocamlc -c -w A-4-33-40-41-42-43-34-44 -strict-sequence -g -bin-annot -short-paths -thread -package core -ppx 'ppx-jane -as-ppx' -o sum.cmo sum.ml
File "sum.ml", line 1, characters 5-13:
1 | open Core.Std
         ^^^^^^^^
Error: Unbound module Core.Std
Command exited with code 2.
Hint: Recursive traversal of subdirectories was not enabled for this build,
  as the working directory does not look like an ocamlbuild project (no
  '_tags' or 'myocamlbuild.ml' file). If you have modules in subdirectories,
  you should add the option "-r" or create an empty '_tags' file.

  To enable recursive traversal for some subdirectories only, you can use the
  following '_tags' file:

      true: -traverse
      <dir1> or <dir2>: traverse

Compilation unsuccessful after building 2 targets (1 cached) in 00:00:00.

Why isn't corebuild linking to the core library? How can I fix this?

Using modern Core

Since the admission of Dune and module aliases, it is no longer needed to have an extra Std submodule, therefore Janestreet removed it (after a two-year-long deprecation). Therefore, now we're writing

open Core

instead of

open Core.Std (* no longer works *)

The same is true with Core_kernel et alas.

Since OCaml and Janesteet have moved since that time a lot, the newer version of RWO was created with updated examples. It is still a work in progress but looks quite close to be ready. So you can switch to it.

Sticking to older versions of OCaml

If you would like to use the first version of Real World OCaml, then you have to choose a version of OCaml and Janesteet's Core library which are known to be compatible. I failed to find any authoritative recommendations on which version it is better to use with the old book. So I would suggest using OCaml 4.02.3. Then you can install core as usual with opam install core (it should install version 113.33.03), and as far as I remember, it should work with the old book. If you or anyone else is having problems with this version please tell me in the comments section, and I will update this recommendation.

Using modern Core

Sticking to older versions of OCaml

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