Auto-load custom printers in the OCaml toplevel.

[yallop]

Ctypes comes with a subpackage ctypes.top that provides custom printing for C types and values. At present ctypes.top needs to be explicitly loaded into the OCaml top level.

This PR auto-loads the custom printers whenever the ctypes package is loaded in the OCaml top level, making custom printing available by default.

Before:

# #require "ctypes";;
[...]
/home/jeremy/.opam/4.03.0/lib/ctypes/ctypes.cma: loaded
# Ctypes.(int @-> int @-> returning string);;
- : (int -> int -> string) Ctypes_static.fn =
Ctypes_static.Function (Ctypes_static.Primitive Ctypes_primitive_types.Int,
 Ctypes_static.Function (Ctypes_static.Primitive Ctypes_primitive_types.Int,
  Ctypes_static.Returns
   (Ctypes_static.View
     {Ctypes_static.read = <fun>; write = <fun>; format_typ = None;
      format = None;
      ty =
       Ctypes_static.Pointer
        (Ctypes_static.Primitive Ctypes_primitive_types.Char)})))

After:

# #require "ctypes";;
[...]
/home/jeremy/.opam/4.03.0/lib/ctypes/ctypes.cma: loaded
/home/jeremy/.opam/4.03.0/lib/ctypes/ctypes-top.cma: loaded
# Ctypes.(int @-> int @-> returning string);;
- : (int -> int -> string) Ctypes.fn = char*(int, int)

The ctypes.top subpackage is retained for compatibility reasons.

[dbuenzli]

Somehow I thought that way at a certain point but changed my mind see here dbuenzli/gg#1 for discussion.

I think that you should keep a way to load the library without side effects on the toplevel and it's more logical to keep the bare name for that and keep the convention of having packages PKG.top for toplevel features.

Now a reasonable enhanced toplevel would load the .top by convention if it exists and provide a way to prevent it from loading and use the bare library instead.

I really prefer if the bare package name, side-effect wise, denotes exactly the same as using the library.

FWIW this is the convention I implement in all my packages.

[dbuenzli]

I forgot. Another benefit is that if you structure your packages the way I mentioned you can keep the idea that subpackages exactly correspond to cm[x]a files. Something I find reasonable in the long term for simplifying the build eco-system.

We should rather try convince toplevels to enforce conventions about loading and preventing to load PKG.top packages.

[yallop]

Thanks, @dbuenzli. A toplevel convention to load .top packages by default would be useful.

The main problem identified in dbuenzli/gg#1 is the automatic and silent execution of open Gg. I can see how that would be a problem. But there's nothing like that here; the .top package installs printers, but does nothing else.