toplevel: show hints for the "undefined global" error (#10647)

3 files changed, 41 insertions, 0 deletions

diff --git a/.depend b/.depend
index 6f49bd4755..422aab3479 100644
--- a/.depend
+++ b/.depend
@@ -6305,6 +6305,7 @@ toplevel/genprintval.cmi : \
     typing/env.cmi
 toplevel/topcommon.cmo : \
     typing/typedtree.cmi \
+    bytecomp/symtable.cmi \
     parsing/printast.cmi \
     typing/predef.cmi \
     parsing/pprintast.cmi \
@@ -6332,6 +6333,7 @@ toplevel/topcommon.cmo : \
     toplevel/topcommon.cmi
 toplevel/topcommon.cmx : \
     typing/typedtree.cmx \
+    bytecomp/symtable.cmx \
     parsing/printast.cmx \
     typing/predef.cmx \
     parsing/pprintast.cmx \
diff --git a/Changes b/Changes
index a60c88a612..66a4d127d3 100644
--- a/Changes
+++ b/Changes
@@ -244,6 +244,9 @@ Working version
 - #11774: ocamlyacc: fail if there is an I/O error
   (Demi Marie Obenour)
 
+- #10647: Show hints for the "undefined global" error in the toplevel
+  (Wiktor Kuchta, review by Gabriel Scherer)
+
 ### Manual and documentation:
 
 - #9430, #11291: Document the general desugaring rules for binding operators.
diff --git a/toplevel/topcommon.ml b/toplevel/topcommon.ml
index fc2336e8d3..dcee7bbff5 100644
--- a/toplevel/topcommon.ml
+++ b/toplevel/topcommon.ml
@@ -380,3 +380,39 @@ let try_run_directive ppf dir_name pdir_arg =
             dir_name dir_type arg_type;
           false
   end
+
+(* Overriding exception printers with toplevel-specific ones *)
+
+let loading_hint_printer ppf s =
+  Symtable.report_error ppf (Symtable.Undefined_global s);
+  let find_with_ext ext =
+    try Some (Load_path.find_uncap (s ^ ext)) with Not_found -> None
+  in
+  fprintf ppf
+    "@.Hint: @[\
+     This means that the interface of a module is loaded, \
+     but its implementation is not.@,";
+  (* Filenames don't have to correspond to module names,
+     especially for archives (cmas), which bundle multiple modules.
+     But very often they do. *)
+  begin match List.find_map find_with_ext [".cma"; ".cmo"] with
+  | Some path ->
+    fprintf ppf
+      "Found %s @,in the load paths. \
+       @,Did you mean to load it using @,#load \"%s\" \
+       @,or by passing it as an argument to the toplevel?"
+       path (Filename.basename path)
+  | None ->
+    fprintf ppf
+      "Did you mean to load a compiled implementation of the module \
+       @,using #load or by passing it as an argument to the toplevel?"
+  end;
+  fprintf ppf "@]"
+
+let () =
+  Location.register_error_of_exn
+    (function
+      | Symtable.Error (Symtable.Undefined_global s) ->
+        Some (Location.error_of_printer_file loading_hint_printer s)
+      | _ -> None
+    )