Re: is_equal of HASHABLE

Dominique Colnet wrote:

> Philippe Ribet has pointed out a possible drawback of your
> proposal: we now have two concrete bodies (one inherited from
> ANY and one in HASHABLE).
> Knowing that HASHABLE is very often used together with multiple
> inheritance... it may bothering for people to undefine the good
> one in order to select the best body.
> So, the idea I have is to propose to redefine is_equal as
> deferred :-)
> 
> Here is the new proposal to be discussed / tested:
> 
> ----------------------------------------
> deferred class HASHABLE
> insert ANY
>       redefine is_equal
>       end
> feature {ANY}
> 
>       is_equal(other: like Current): BOOLEAN is
>           deferred
>           ensure then
>             Result implies hash_code = other.hash_code
>           end
>   ...

Yes, to me that looks more suitable for general purpose programs, i.e.
where it is the intention just to make objects hashable.
BTW, in my G2F3 code a more special situation arises since hundreds of
thousands of game positions may be stored in the transposition table.
Obviuously, I am concerned about memory usage which is why I store only
that part of information which determines whether two positions are
identical or different and who is to move next. This is what the feature
{STORAGE_HASHABLE}.hash_key: HASH_KEY does. Now, in that (admittedly
special) case, HASH_KEY key inherits only from HASHABLE and its
attributes are only expanded types so that the built-in definition of
is_equal will always do. But I agree that Dominque's suggestion (defer
is_equal in HASHABLE and adding the psotcondition) is a good decision:
It guarantees that the assumptions which are made in HASHED_SET and
HASHED_DICTIONARY are checked by a contract and it does not impose much
hassle.

cheers,

Frank


-- 
Dr Frank Boehme                      | Email: [email protected]
National University of Ireland, Cork | phone: +353-21-4903163
Dept of Computer Science             |   fax: +353-21-4903113
Cork, Ireland                        |

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