Error message missing information about type inclusion

[jctis]

Hi,

This looks like a bug in the error message:

$ cat foo.ml

type t = < x : int >

class c = object method x = 3 method y = true end

let o = new c
$ cat foo.mli
type t = < x : int >

class c : object method x : int method y : bool end

val o : t
$ ocamlc -c foo.mli && ocamlc -c foo.ml
File "foo.ml", line 1:
Error: The implementation foo.ml does not match the interface foo.cmi:
       Values do not match: val o : c is not included in val o : t
       File "foo.mli", line 5, characters 0-9: Expected declaration
       File "foo.ml", line 6, characters 4-5: Actual declaration
$

I guess I understand why type c is not included in t. However, the last two lines are a bit confusing since the Expected declaration as well as the Actual declaration are empty. I would expect the compiler to tell me the method y cannot be hidden in the type of o.

Yours,

Judicaël Courant

[gasche]

I don't see a bug here:

• the locations reported appear to be correct, they point to the lines let o = ... (in foo.ml) and val o : ... (in foo.mli)
• the type of o is a subtype of t, but in OCaml subtyping is not inferred, you have to use an explicit cast

You could fix foo.ml with let o = (new c :> t).

$ ocaml
# #use "foo.ml";;
type t = < x : int >
class c : object method x : int method y : bool end
val o : c = <obj>
# let o' = (o :> t);;
val o' : t = <obj>

does that make sense?

[jctis]

You are right, I overlooked what the compiler pointed at.

I however expected a more informative message: I would like the compiler to tell me why c is not included in t, more precisely I would like it to tell me that row y of c is missing in t (of course my example is trivial, but the one I stumbled upon was more complex). That is what the compiler does when you incorrectly use subtyping for modules:

$ cat barfoo.ml 

module X(Y: sig val x : int val y : int end) = struct end

module Z = X(struct let x = 3 end)
  
$ ocamlc -c barfoo.ml 
File "barfoo.ml", line 4, characters 13-33:
4 | module Z = X(struct let x = 3 end)
                 ^^^^^^^^^^^^^^^^^^^^
Error: Signature mismatch:
       Modules do not match:
         sig val x : int end
       is not included in
         sig val x : int val y : int end
       The value `y' is required but not provided
       File "barfoo.ml", line 2, characters 28-39: Expected declaration

[lpw25]

#10170 and it's incoming follow-up PR should support that

[jctis]

Indeed. Thanks!

[antalsz]

Hi @jctis – the new PR #10407 should address this once it's merged. I've added this particular error as a test case (testsuite/tests/typing-modules/pr10399.ml) so that we can see this :-)

[jctis]

Hi @antalsz
The message in your test case is much more informative. Thanks :-)