[Caml-list] Bug in typing polymorphic variants found

[John Max Skaller <skaller@ozemail.com.au>]

>
>
>> Question: I previously found a serious bug with polymorphic
>> variants in Ocaml 3.02.  The code that triggered the bug
>> works fine with 3.01 and with 3.04. Has that bug been
>> identified and fixed (or was I just lucky ..and now unlucky again)? 
>
>
> Hmmm. That is bug #491 in the bug tracker,
> and its categorized "not a bug" which it clearly is. 

OK. I think I have found it. Heh. Or rather, Ocaml 3.01
found it for me. There is a bug in the typing of polymophic variants.

What happens: Ocaml 3.01 reports a type error.
Ocaml 3.04 does not, it crashes at run time instead.
[.. this is confusing but, I think it crashes even when the bug
in my code is fixed .. this is definitely the same problem I
had before and reported in bug #491]

The offending lines of code are:

       match dcl with

        | `DCL_val_typeof e ->
          Hashtbl.add dfns n (id,sr,parent,`SYMDEF_dcl dcl)

The match is matching on values of type dcl_t

The value `DCL_val_typeof _ is NOT a member of the type dcl_t,
so this code will never be executed.

The type of the Hashtable being used here requires the fourth
component of the data part to be of a type which includes
constructor `SYMDEF_dcl of dcl_t.

Were the above code ever executed, a bad variant would be
glued into the hash table, since the type of dcl is infered to
include `DCL_val_typeof _.

The *problem* is that the type of the constructor argument
influences its run-time hash? And so the following code,
which IS executed

        | `DCL_val t ->
          Hashtbl.add dfns n (id,sr,parent,`SYMDEF_dcl dcl)
          ;

is producing the *wrong* hash for the variant `SYMDEF_dcl dcl,
because the inference engine thinks the type of dcl is something
other than dcl_t. When I do a lookup of the hash table later in another 
module,
I get a bad value out, since in *that* module, the correct type of the dcl
is calculated. At least, thats my guess.

I'll try to make a small example,
my initial attempts have failed. Otherwise, I'll have to post the
bogus code to sourceforge where
the developers can download it.

Here is the Ocaml 3.01 error which
Ocaml 3.04 does not report:

This pattern matches values of type
  [< `DCL_module_binding of 'a
   | `DCL_typed_functor of 'b * 'c * 'd
   | `DCL_untyped_functor of 'e * 'f
   | `DCL_function of 'g * 'h * 'i
   | `DCL_match_check of 'j * 'k
   | `DCL_header of 'l
   | `DCL_module of 'm
   | `DCL_interface of 'n
   | `DCL_lazy of 'o
   | `DCL_procedure of 'p * 'q
   | `DCL_if_fun of 'r
   | `DCL_if_proc of 's
   | `DCL_type
   | `DCL_val of 't
   | `DCL_val_typeof of 'u
   | `DCL_var of 'v
   | `DCL_var_typeof of 'w
   | `DCL_type_alias of 'x
   | `DCL_abs of 'y
   | `DCL_const of 'z * 'a1
   | `DCL_fun of 'b1 * 'c1 * 'd1
   | `DCL_proc of 'e1 * 'f1
   | `DCL_union of 'g1
   | `DCL_struct of 'h1]
but is here used to match values of type
  Flx_types.dcl_t =
    [ `DCL_header of string
    | `DCL_function of
        Flx_types.parameter_t list * Flx_types.typecode_t *
        Flx_types.asm_t list
    | `DCL_lazy of Flx_types.expr_t
    | `DCL_procedure of Flx_types.parameter_t list * Flx_types.asm_t list
    | `DCL_union of (Flx_types.id_t * Flx_types.typecode_t) list
    | `DCL_struct of (Flx_types.id_t * Flx_types.typecode_t) list
    | `DCL_match_check of Flx_types.pattern_t * string
    | `DCL_val of Flx_types.typecode_t
    | `DCL_var of Flx_types.typecode_t
    | `DCL_type_alias of Flx_types.typecode_t
    | `DCL_module of Flx_types.asm_t list
    | `DCL_typed_functor of
        Flx_types.parameter_t list * Flx_types.typecode_t *
        Flx_types.asm_t list
    | `DCL_untyped_functor of
        Flx_types.parameter_t list * Flx_types.asm_t list
    | `DCL_module_binding of Flx_types.expr_t
    | `DCL_interface of Flx_types.asm_t list
    | `DCL_if_proc of Flx_types.typecode_t
    | `DCL_if_fun of Flx_types.typecode_t
    | `DCL_type
    | `DCL_abs of Flx_types.c_t
    | `DCL_const of Flx_types.typecode_t * Flx_types.c_t
    | `DCL_fun of
        Flx_types.typecode_t list * Flx_types.typecode_t * Flx_types.c_t
    | `DCL_proc of Flx_types.typecode_t list * Flx_types.c_t]


-- 
John Max Skaller, mailto:skaller@ozemail.com.au
snail:10/1 Toxteth Rd, Glebe, NSW 2037, Australia.
voice:61-2-9660-0850




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