[m-users.] Self-improvement subjects.

[Sean Charles (emacstheviking)]

I decided to ask Chat-GPT Volker, it has been most helpful too!

	In terms of logic programming, what is an existential variable? Can you show me an example in a language called Mercury?

And what came back was clear, and explained and so for the rest of the evening, some cheese and  biscuits will accompany on my Chat-GPT education about such matters!

Thanks!

:D


> On 23 Oct 2023, at 17:37, Volker Wysk <post at volker-wysk.de> wrote:
> 
> Am Montag, dem 23.10.2023 um 14:37 +0100 schrieb Sean Charles
> (emacstheviking):
>> What areas of maths / logic do I need to study to better understand the
>> Mercury compiler?
>> 
>> I have a brain, so I have a chance.
>> 
>> What I find mostly confusing is all the talk of 'existential' things, from
>> philosophy I know what Existentialism means but not when applied to a
>> language like Mercury. 
> 
> Existential types (more precisely: types with existentially quantified type
> variables in them) gave me a headache, too, at first. I recommend this page:
> 
> https://github.com/Mercury-Language/mercury/wiki/Existential-types
> 
> "Existential" refers to the existential quantifier in such types. As opposed
> to the usual implicit universal quantifier (which can be made explicit, but
> doesn't need to).
> 
> What that page doesn't say, is that for an existential type you will
> probably also need a type class, so you can do more with it. 
> 
> You can think of an existential data constructor as a container for a value
> that can have any type (possibly restricted to the members of a type class).
> That type, encapsulated inside of it, isn't give out to the outside. This
> means, it isn't restricted to anything from the outside.
> 
> 
>> Also I still struggle with modes/inst and 'and-or' trees. I read the
>> Wikipedia page on those until it stopped making sense (didn't take long),
>> I have rough idea of what the Mercury manual is trying to tell me but I am
>> sick and tired of feeling like a semi-educated simpleton and need to up my
>> game.
>> 
>> When I learned Prolog, I did some study about Horne clauses, and some
>> other related subjects but only enough really to understand what I was
>> doing at the time time, and it got me further down the road.
>> 
>> But...Mercury. What a beast!   
> 
> Yes, insts are beasty.  :-))
> 
>> I truly want to get to know it's inner dialogue as it once again is forced
>> to eat my probably horrendous code over and over.
>> 
>> So... any topic for self-study would be most appreciated.
> 
> Good luck,
> Volker

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