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From: Niklas Holsti <niklas.holsti@tidorum.invalid>
Newsgroups: comp.lang.ada
Subject: Re: How to get Ada to ?cross the chasm??
Date: Tue, 8 May 2018 23:53:45 +0300
Organization: Tidorum Ltd
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Xref: reader02.eternal-september.org comp.lang.ada:52126
Date: 2018-05-08T23:53:45+03:00
List-Id: <comp.lang.ada>

On 18-05-06 21:38 , Paul Rubin wrote:
> Niklas Holsti <niklas.holsti@tidorum.invalid> writes:
>> The Ada programs for satellites on which I am currently working often
>> meet with such problems, and either work slower (mutual exclusion
>> between readers and writers), use more memory (dual physical copies of
>> the data structure) and/or pass the problem to the satellite operators
>> (commands to mutate the on-board data structure are delayed or failed
>> if the data structure is in use by some reader). Better solutions are
>> needed.
>
> I think the natural Ada-flavored solution is use more memory.  You're
> describing a classic transactional database update which has
> traditionally been implemented using shadow pages.  Basically you write
> the record in two places, then commit the transaction by flipping a
> pointer.

Yes, something like that is sometimes used.

> Because I can't resist, here's a beautiful Haskell implementation of
> red-black trees using zippers, with the RB-tree invariants guaranteed by
> the type signature:
>
>   https://gist.github.com/2660297 (based on https://redd.it/ti5il)

The ideas behind the implementation may be beautiful, but the code is 
painful to my eyes; for example:

    insertAt x (BC pb pa py (EC gb ga (R ul ua ur) gc)) = insertAt g gc
      where p = if pb then B x pa py else B py pa x
            u = B ul ua ur
            g = if gb then R p ga u else R u ga p

One of the things I really like in Ada, and in the Ada programming 
style, is the emphasis on readability, and the ability to provide 
meaningful names for operation parameters, both in the operation 
declaration _and_ in calls. Quick, what is the meaning of the argument 
"py" to insertAt?

The concepts of functional programming are attractive to me, but the 
practical problems or long, position-specific argument lists, and the 
style of using very short identifiers, scare me.

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
       .      @       .