Re: [boost] Interest for a perl6-like-junctions-class?

From: Pavel Vozenilek (pavel_vozenilek_at_[hidden])
Date: 2005-08-02 13:33:09

"Rob Stewart" wrote:

 ________________________________________________________
>> 6. Would it be possible to have other qualifiers,
>>    e.g.: "more_than_half_of"
>>
>>  if (more_than_half_of(a) >= any_of(b)) ...
>>
>> or more complex versions like:
>>
>>   more_than(10).items_of(a) >= ...
>>   more_than(30.1).percents_of(a) == ...
>>
>>  less_than(2).items_of(a) >= ...
>>  less_than(95).percents_of(a) == ...
>>
>>   between(1, 5).items_of(a) >= ...
>>   between(10.5, 20.0).percents_of(a) == ....
>
> Those are possible but are they sufficiently useful?  Do you
> think they would be used enough to justify creating/documenting
> them?
>
IMO yes. Relational databases are almost
all such calculations, for example.

>> Comparison between two "mores" would be
>> disabled.
>
> Why?
>
Because
  if (more_than_half_of(a) >= more_than_half_of(b)) ...
doesn't make sense.

 ________________________________________________________
>> 7. Will it be possible to use other predicates than
>>    >, <=, etc  in following form:
>>
>>   if ( is_true_that(all_of(a), a_binary_functor, one_of(b)) ) ...
>>
>> where the functor could be lambda:
>>
>>   if (is_true_that(all_of(a), _1 >= _2, one_of(b)) ....
>>
>> It feels more natural than the asymetric "do_comparison".
>
> I agree with the asymmetry of do_comparison(), but is it
> necessary to provide the additional predicate support?
>
Yes. Forcing user to define == etc operators doesn't make
good design and there could be many, many possible
predicates for single class.

> Reasoning
> over the result of using none_of() with a user-defined predicate
> gets quite difficult when compared against an all_of() or other
> junction/multivalue objects.  Doing so against a single value is
> much easier.
>
Yes but the documentation should say what combinations
are potentially dangerous.

> I think "evaluate" is better than "is_true_that:"
>
>   if (evaluate(all_of(a), _1 >= _2, one_of(b))) ...
>
People who also code in scripting languages with
an "eval" function would get confused.

 ________________________________________________________
>   if (evaluate(all_of(a).where(filter1)
>      , _1 >= _2
>      , one_of(b).where(filter2)))
>      ...
>
This may be also useful. Either specific filters for each side,
or one common, or both or one or none.

> Nevertheless, the question arises whether all of this is
> warranted.  Indeed, if we support iterator pairs in addition to
> ranges, there are other libraries that one can employ to do the
> filtering, so this one doesn't need to.
>
The filters could be lambdas. Result syntax could be smaller
and easier to read then with another object (a filtering iterator).

/Pavel