Re: Community Poll: What do you think about qi supporting n-ary operators?

Von meinem iPhone gesendet
> Am 09.08.2016 um 00:12 schrieb Joel de Guzman <[email protected]>:
>
>> On 09/08/2016 1:00 AM, Frank Hein wrote:
>>
>>
>>
>>
>> Frank Hein
>>
>> Geschäftsführer
>> maxence business consulting gmbh
>>
>> phone +49-(0)-2133-2599-0
>> fax +49-(0)-2133-2599-29
>> cell +49-(0)-151-54 74 29 89
>> internet [email protected] <mailto:[email protected]>
>> skype fhmaxence
>>
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>> Geschäftsführer: Dipl.-Inform. Frank Hein
>> www.maxence.de <http://www.maxence.de/>
>>
>> Von meinem iPhone gesendet
>>> Am 08.08.2016 um 07:33 schrieb Joel de Guzman <[email protected]
<mailto:[email protected]>>:
>>>
>>>> On 08/08/2016 8:24 AM, Frank Hein wrote:
>>>>
>>>> Am 08.08.2016 um 02:05 schrieb Frank Hein <[email protected]
>>>> <mailto:[email protected]>
>>>> <mailto:[email protected]>>:
>>>>
>>>>>
>>>>>
>>>>> Von meinem iPad gesendet
>>>>>
>>>>> Am 07.08.2016 um 22:40 schrieb Joel de Guzman <[email protected]
>>>>> <mailto:[email protected]> <mailto:[email protected]>>:
>>>>>
>>>>>>> On 07/08/2016 12:29 PM, Frank Hein wrote:
>>>>>>> I reworked the description of the Pull Request on Github. (
>>>>>>> https://github.com/boostorg/spirit/pull/200)
>>>>>>>
>>>>>>> For your convenience all the information from the initial post are
now
>>>>>>> part of the description. I even added a bunch of new arguments and
>>>>>>> considerations for your review.
>>>>>>>
>>>>>>> I would highly appreciate any feedback. Did I miss something
important? Do
>>>>>>> you want that feature? Or is it not needed or superfluos?
>>>>>>> Is my writing understandable (I am obviously not a native
speaker ;). Is
>>>>>>> there something you want me to explain in more detail?
>>>>>>>
>>>>>>> Is there anybody out there?
>>>>>>
>>>>>> How about we start with a use-case? What would this facility provide
that
>>>>>> the current code does not, or does so inelegantly?
>>>>>
>>>>> Points 1, 3, 5, 8 and 9 of my argument list describe what
capabilities qi currently
>>>>> does not provide.
>>>>>
>>>>> This PR enables the implementation of a new class of parsers, n-ary
operators.
>>>>> It does not introduce any particular parser of that class. So talking
about particular
>>>>> use cases is somewhat difficult.
>>>>>
>>>>> Anyway, I provided a link to sehe's implementation of an n-ary
operator for X3,
>>>>> longest_match. http://coliru.stacked-crooked.com/a/cb13c1d357cdc464.
This can't be done
>>>>> with qi currently.
>>>>>
>>>>> In other words: Implement a parser which works like qi alternative.
It takes a variadic
>>>>> number of alternative parsers and delivers a boost::variant made from
these parser's
>>>>> attribute types. Different to qi alternative this parser should not
return the first
>>>>> match. It should return that match, where parsing consumed most of
the input,
>>>>> independent of the ordering of the alternative parsers.
>>>>>
>>>>> You can not solve the whole class of problems which this particular
exercise represents
>>>>> with qi currently. With the PR you can.
>>>>
>>>> It is not completely true, that something working like that was
completely impossible. But
>>>> a solution would need a binary operator, which would persistently
store info about it's
>>>> operands. Maybe with an additional directive which takes the operator
chain as an argument
>>>> (a wrapper to know where the operator chain begins and ends) which has
also access to the
>>>> persistent context store, this could be done somehow.
>>>>
>>>> But that would introduce a directive and an operator which must be
used together always.
>>>> Not very clean.
>>>>
>>>>
>>>>
>>>>> Please let us not discuss if this particular example parser is a good
idea or not. That
>>>>> would be a different topic.
>>>
>>> I'm not sure about that. I always preferred:
>>>
>>>   longest[a | b | c]
>> That would would not work with the qi alternative operator, because it
returns the first
>> match. You'd have to implement a new one working together with longest.
>>
>> And isn't
>>
>>    longest[a | b | c]
>>
>> somewhat sophisticated to understand compared to
>>
>>   longest(a, b, c)?
>>
>> What does the operator in your example do what's not already expected to
be done by longest?
>>
>> Is max[a+b+c+d] ( or max[a|b|c|d] if you like) a better expression then
max(a,b,c,d), if
>> you want the maximum of these four values? Just to illustrate.
>
> Well, the directive *is* the syntax of choice used by Spirit for such
things from the
> very start. Consistency matters. max(a, b) is consistent in its domain.
One can also
> argue that alternative(a, b, c) is consistent in the FP domain.
>

As far as I understand it, a directive is somerhing that modifies the
environment a parser runs in. It can be used to preprocess input,
postprocess output, force consecutive runs and so on. The parser implements
the parsing logic/algorithm. The directive apapts a parser for a particular
application. Is that understanding wrong? Doc explained: " directives
(are)... usable to augment and parameterize other parsers."

My notion of "clean" means, that all qi components should be
self-contained. The common interface is parser member function. Qi follows
this paradigm strictly and I love that. The user so can define a parser
knowing exactly what he gets, because there are no hidden side effects from
components having private talks.

For me it was important to maintain that paradigm (for consistency reasons
to aasige predictable behaviour under all ciecumstances.


FP = Functional Programming?

As far as I can see, there are no "such things" (n-ary operators) in qi so
far.


> Regards,
> --
> Joel de Guzman
> http://www.ciere.com
> http://boost-spirit.com
> http://www.cycfi.com/
>
>
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