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

["Soegtrop, Michael" <michael.soegtrop@intel.com>]

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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