Re: [Caml-list] Specify the default hash function for a type

[Gabriel Scherer <gabriel.scherer@gmail.com>]

From the C side it is possible to create "custom" values that come
with user-specified comparison and hashing functions. However, for now
the runtime does not allow creating them from OCaml -- this is indeed
a desirable feature, but it's not trivial to implement. I don't really
whether it's actually "really hard", or just that it requires some
effort that nobody has invested yet -- the latter would be my guess.

If you don't mind going through a C stubs to do this, you could thus
create the custom value in C land, and export that in OCaml land.
See the manual:
  Interfacing C with OCaml > custom blocks
  http://caml.inria.fr/pub/docs/manual-ocaml-4.03/intfc.html#sec442

On Fri, May 13, 2016 at 5:26 AM, Soegtrop, Michael
<michael.soegtrop@intel.com> wrote:
> Dear Alain,
>
> this are temptations on Friday morning ... I guess as a beginner I should resist, not be lazy, and write the hash function by hand. But I put it into appendix D of my OCaml notes ;-)
>
> Best regards,
>
> Michael
>
>> -----Original Message-----
>> From: Alain Frisch [mailto:alain.frisch@lexifi.com]
>> Sent: Friday, May 13, 2016 11:06 AM
>> To: Soegtrop, Michael; caml-list@inria.fr
>> Subject: Re: [Caml-list] Specify the default hash function for a type
>>
>> Dear Michael,
>>
>> Don't tell anybody (ahem), but the following might work depending on your
>> exact use case...
>>
>> If you define a type such as:
>>
>>   type t = {
>>     foo: ....;
>>     id: int;
>>   }
>>
>> and if you set the tag of values for this type to Obj.record_tag (= 248) with
>> Obj.set_tag, then polymorphic hash *and* comparison functions will use the
>> "id" field and ignore the "foo" field (and any other extra fields).  It is
>> important to keep "id" as the second field to mimic the layout of real object
>> values (and exception slots).
>>
>> In particular, this lets you easily "forget" sub-parts of a value (by setting "id"
>> to a fixed constant), or implement "references compared with physical
>> equality" (use a counter to assign unique "id" to each such reference).
>>
>>
>> Best regards,
>>
>> Alain
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