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

[Alain Frisch <alain.frisch@lexifi.com>]

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



On 13/05/2016 09:52, Soegtrop, Michael wrote:
> Dear Ocaml Users,
>
>
>
> I wonder if there is a way to specify a default hash function for a
> type, which is then used by the automated hash functions e.g. created
> for records containing this type. I have e.g. a record with about 10
> fields and I am happy with the default hash functions for most fields
> and for the record, but for just one field I would like to use a custom
> hash function, but I don’t want to write a hash function for the record.
>
>
>
> I am currently using the standard library, but if there is a solution
> with Batteries or Core, I would also be interested.
>
>
>
> Best regards,
>
>
>
> Michael
>
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