[Caml-list] Hash over ref

[Caml-list] Hash over ref

[Jon Harrop]

Is it possible to hash a ref itself, rather than the object it references?

Cheers,
Jon.

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Re: [Caml-list] Hash over ref

[David Monniaux]

On Thu, 2004-05-20 at 22:37, Jon Harrop wrote:
> Is it possible to hash a ref itself, rather than the object it references?
Yes. References are actually implemented as mutable records with one
field.

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Re: [Caml-list] Hash over ref

[Damien Doligez]

On May 20, 2004, at 22:37, Jon Harrop wrote:

> Is it possible to hash a ref itself, rather than the object it 
> references?

If I understand the question correctly, you want the hash to remain
constant even when you update the reference.  Then the answer is no.

The normal technique for this case is to replace your reference with
a record of two fields: a mutable field for the contents, and an int
field which you initialize with a random number (or a sequential
counter) when you create the record.  Then your hash code is this
number.

-- Damien

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Re: [Caml-list] Hash over ref

[Alain Frisch]

On Thu, 20 May 2004, Jon Harrop wrote:

> Is it possible to hash a ref itself, rather than the object it references?

If you really need to use values of type 'a ref (that you create yourself,
but need to store in existing data structures), you can cheat and
implement Damien suggestion like this:

module HashedRef : sig
  val ref: 'a -> 'a ref
  val hash: 'a ref -> int
end = struct
  class c = object end
  type 'a my_ref = { mutable value: 'a; uid : int }

  let ref x = Obj.magic { value = x; uid = Oo.id (new c) }

  let hash x =
    if Obj.size (Obj.repr x) = 2 then (Obj.magic x).uid
    else failwith "HashedRef.hash: this reference is not hashable"
end

(Don't do this at home, of course.)

-- Alain

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Re: [Caml-list] Hash over ref

[Jon Harrop]

Ok, thanks for all the advice from everyone. :-)

Essentially, I wish to hash the pointer value of the reference itself. So, if 
a reference were reassigned, then the hash would be different. Two identical 
copies of an object would have different references and, consequently, would 
give different hashes (i.e. a != b is true where a and b are 'a ref).

The reason I want to do this is to spot duplicate uses of a single object 
(multiple references to it) but without confusing this with different uses of 
identical objects (i.e. where "=" would give true but "==" would not).

I can think of a couple of alternative approaches. I could implement a naff 
kind of set as a list which used "==" to test for equality. Or, I could loop 
through all of the references to the objects, marking unmarked objects with a 
new identifier (an int, say) and then insert them all into a Hashtbl on the 
basis of their identifiers. I just thought hashing refs themselves would be 
simpler.

I think I can do this given Alain's code though, thanks!

Cheers,
Jon.

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