Re: Redefine to make a contract stronger (Was: is_equal ofHASHABLE)

Roger Browne wrote:

> > But what if we redefine an *effective* feature for the sole purpose to
> > make the contract stronger...
> > class C
> > inherit A B
> 
> This works already. Just write:
> 
>    class C
>    inherit
>       A
>       B undefine f end
>    end
> 
> C will get f's postcondition "and-ed" from both A and B, as you would
> expect.

Ah, of course yes. I did not think of that. I am currently writing a
library for binary relations, posets and the like (not actually
trivial). I have a general relation class (lets call it R) and a number
of more specified ones that inherit from R (conforming). These are
REFL_R (reflexive relations), IRR_R (irreflexive), SYM_R (symmetric),
ANTSYM_R (antisymmetric) and TRANS_R (transive relations) to name but a
few. All these classes do not change anything but adding postconditions
to already existing features from R which provide concatanation,
reversion etc. Of course, it is very common to inherit from more than
one of these. For example, LESS_THAN should inherit IRR_R, ANTSYM_R and
TRANS_R. For some other reasons, I do not want to have any of these
classes deferred which means i habe to follow your first suggestion.
Now, it might be a little counterintuitive to have something like

class LESS_THAN
inherit
  IRR_R undefine .... end
  ANTSYM_R undefine .... end
  TRANS_R
...

It does not matter which of the parents has no 'undefine' but there must
be exactly one. A little odd, given the fact that LESS_THAN *is*
irreflexive, antisymmetric and transitive and all of these three
properties are equally significant. But i think I will go this route.
Thanks.

> > But I am not proposing how this could be added to, errrrm...
> > the SmartEiffel.. ahmmm language(?).
> 
> Luckily, it turns out that it's already there.

No no, I strongly advocate the new keyword 'recontract'! We do need
this! :-))

F

-- 
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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