[Caml-list] Bug or useless error message

["Christoph Höger" <christoph.hoeger@tu-berlin.de>]

Dear all,

right now I am unable to produce a working example of that problem, so
please bear with me.

Basically, I just patched ppx_deriving_yojson to generate polymorphic
functions for recursive types, i.e.

type 'a foo = ...
 and bar = int foo [@@deriving yojson]

becomes

let rec foo_of_yojson : 'a. (json -> [`Ok of 'a, `Error of string]) ->
json -> [`Ok of 'a, `Error of string] = ...
and bar_of_yojson = ...

this does, indeed, work in the test cases.

If I apply this method to my (large) real world module, I get a strange
error:

Error: Signature mismatch:
       ...
       Values do not match:
         val typedef_struct_of_yojson :
           (Yojson.Safe.json -> [ `Error of string | `Ok of 'a ]) ->
           Yojson.Safe.json ->
           [ `Error of string | `Ok of 'a typedef_struct ]
       is not included in
         val typedef_struct_of_yojson :
           (Yojson.Safe.json -> [ `Error of string | `Ok of 'a ]) ->
           Yojson.Safe.json ->
           [ `Error of string | `Ok of 'a typedef_struct ]

Syntactically, the types are the same, but obviously they are not equal
in their internal representation.

I can workaround this problem by hiding this particular function from
the interface (yes, it only occurs in one function out of ~300).
However, this message bugs me: Is something like this expected when
using explicit generalization?

regards,

Christoph

-- 
Christoph Höger

Technische Universität Berlin
Fakultät IV - Elektrotechnik und Informatik
Übersetzerbau und Programmiersprachen

Sekr. TEL12-2, Ernst-Reuter-Platz 7, 10587 Berlin

Tel.: +49 (30) 314-24890
E-Mail: christoph.hoeger@tu-berlin.de