The module Bytes is an alias for module Stdlib__Bytes, which is missing error in OCaml 4.14

[aqjune]

(This is a repost of this thread in ocaml discuss because Github Issue seems like a more releveant place)

In OCaml 4.14, running Env.fold_modules in REPL may break module aliasing, causing this funny error:

my_desktop: ~/ $ ocaml
OCaml version 4.14.0
Enter #help;; for help.

# #directory "+compiler-libs";;
# #load "ocamlcommon.cma";;
# Env.fold_modules (fun s p md () -> ()) (Some (Longident.Lident "Bytes"))  !Toploop.toplevel_env ();;
- : unit = ()
# Bytes.get;;
Error: The module Bytes is an alias for module Stdlib__Bytes, which is missing

The result of opam list is as follows:

# Packages matching: installed
# Name                      # Installed # Synopsis
base-bigarray               base
base-threads                base
base-unix                   base
camlp-streams               5.0.1       The Stream and Genlex libraries for use with Camlp4 and Camlp5
camlp5                      8.00.03     pinned to version 8.00.03
conf-perl                   2           Virtual package relying on perl
conf-perl-ipc-system-simple 3           Virtual package relying on perl's IPC::System::Simple
conf-perl-string-shellquote 3           Virtual package relying on perl's String::ShellQuote
dune                        3.7.1       Fast, portable, and opinionated build system
num                         1.4         The legacy Num library for arbitrary-precision integer and rational arithmetic
ocaml                       4.14.0      The OCaml compiler (virtual package)
ocaml-base-compiler         4.14.0      Official release 4.14.0
ocaml-config                2           OCaml Switch Configuration
ocaml-options-vanilla       1           Ensure that OCaml is compiled with no special options enabled
ocamlfind                   1.9.6       A library manager for OCaml

[nojb]

From a distance this doesn't look like a bug, but rather an artifact of the fact that you are loading ocamlcommon.cma which is already linked into the toplevel. If you remove the #load directive, things should work as expected, I think.

[Octachron]

A possibly smaller reproduction case would be

#directory "+compiler-libs";;
#load "ocamlcommon.cma";;
let x = Env.lookup_value ~use:true ~loc:Location.none Longident.(Ldot(Lident "Bytes", "get"))
    !Toploop.toplevel_env;;

which fails with a cannot_scrape_alias error because the persistent environment loaded by #load "ocamlcommon.cma"
has not been initialized.

Since Env is expunged from the standard REPL, using just #directory is not enough. A better choice would be to use utop-full or build a custom not-expunged REPL.

[aqjune]

Hi @Octachron , thanks for the info.
Currently we are using a preprocessor written in camlp5, and unfortunately utop (as well as utop-full) doesn't seem to successfully deal with it.
I would like to build a custom REPL that supports Env, but I wonder how to do that. I tried ocamlmktop -o ocaml-full ocamlcommon.cma, but it raises errors that seem to be related to what @nojb said:

File ".../_opam/lib/ocaml/compiler-libs/ocamlcommon.cma(Config)", line 1:
Error (warning 31 [module-linked-twice]): files .../_opam/lib/ocaml/compiler-libs/ocamlcommon.cma(Config) and .../_o
am/lib/ocaml/compiler-libs/ocamlcommon.cma(Config) both define a module named Config
...

[nojb]

I would like to build a custom REPL that supports Env, but I wonder how to do that. I tried ocamlmktop -o ocaml-full ocamlcommon.cma,

I think ocamlmktop -o ocaml-full should suffice (it already links against ocamlcommon.cma).

[aqjune]

Thanks @nojb , it worked! :)

[aqjune]

I think ocamlmktop -o ocaml-full should suffice (it already links against ocamlcommon.cma).

This was a small surprise to me, because when the original ocaml REPL was used, without #load "ocamlcommon.cma";; I was seeing Error: Reference to undefined global `Env' at the list line.
How does ocamlmktop resolve the undefined global error?

[Octachron]

What happens is that in the standard REPL, compiler-libs modules are removed from the table of global symbols by using an external tool (in order to not pollute the REPL environment with compiler-libs). REPLs created with ocamlmktop don't go though this step and thus compiler-libs modules are available by default.

[aqjune]

Hi all, thanks for comments.
I have one more question to ask - can I prevent Topfind.load_deeply .. from loading ocamlcommon.cma?

For example, if I try this in the ocaml-full:

# Topdirs.dir_use Format.std_formatter "topfind";;
(snip)
# Topfind.load_deeply ["camlp5"];;
.../_opam/lib/ocaml/compiler-libs: added to search path
.../_opam/lib/ocaml/compiler-libs/ocamlcommon.cma: loaded <- this again loads ocamlcommon.cma
(snip)
# let x = Env.lookup_value ~use:true ~loc:Location.none Longident.(Ldot(Lident "Bytes", "get")) !Toploop.toplevel_env;;
Exception:
Env.Error
 (Env.Lookup_error
   ({Location.loc_start =
      {Lexing.pos_fname = "_none_"; pos_lnum = 0; pos_bol = 0; pos_cnum = -1};
     loc_end =
      {Lexing.pos_fname = "_none_"; pos_lnum = 0; pos_bol = 0; pos_cnum = -1};
     loc_ghost = true},
   <abstr>,
   Env.Cannot_scrape_alias (Longident.Lident "Bytes", Path.Pident <abstr>))).

I can still see the Cannot_scrape_alias error.
Thanks..!

[nojb]

I have one more question to ask - can I prevent Topfind.load_deeply .. from loading ocamlcommon.cma?

Topfind is not part of the compiler, it is part of https://github.com/ocaml/ocamlfind and it uses META files to find all the dependencies of a given package. At first sight (but I am not familiar with the details), it looks like it should have a list of packages that it should not load even if they appear as dependencies because they are already linked into the toplevel: ocamlcommon, ocamlbytecomp, ocamltoplevel. Perhaps worth opening an issue over at https://github.com/ocaml/ocamlfind/issues?

[nojb]

I have one more question to ask - can I prevent Topfind.load_deeply .. from loading ocamlcommon.cma?

Topfind is not part of the compiler, it is part of https://github.com/ocaml/ocamlfind and it uses META files to find all the dependencies of a given package. At first sight (but I am not familiar with the details), it looks like it should have a list of packages that it should not load even if they appear as dependencies because they are already linked into the toplevel: ocamlcommon, ocamlbytecomp, ocamltoplevel. Perhaps worth opening an issue over at https://github.com/ocaml/ocamlfind/issues?

Actually, scratch that, it seems the ability to keep some modules from being loaded already exists:

Topfind.don't_load ["compiler-libs.common"]

[aqjune]

Actually, scratch that, it seems the ability to keep some modules from being loaded already exists:

Topfind.don't_load ["compiler-libs.common"]

Oh, I must have looked at the functions that Topfind provides first. That was very helpful, thanks!