Re: [m-users.] Self-improvement subjects.

That Chat GPT answer doesn't make much sense to me. I've tried to compile
it, but it doesn't. I know the "^" operator only for field access
predicates. And so does the compiler, it seems.

Am Montag, dem 23.10.2023 um 19:03 +0200 schrieb Volker Wysk:
> Sorry, I sent my mail to Sean directly, instead of to the list, by
> accident. And he replied to me, off list as well. So I'm forwarding his
> answer here. My mail just said "For real? What did it say?".
> 
> -------- Ursprüngliche Nachricht --------
> Von: Sean Charles (emacstheviking) <[email protected]>
> An: Volker Wysk <[email protected]>
> Betreff: Re: [m-users.] Self-improvement subjects.
> Datum: Mon, 23 Oct 2023 17:52:53 +0100
> 
> > 
> Here you go!
> 
> ---------[start chatgpt content]
> 
> In terms of logic programming, what is an existential variable? Can you
> show me an example in a language called Mercury?
> ChatGPT
> 
> 
> In logic programming, an existential variable is a variable that is
> introduced to represent an unknown value or an existentially quantified
> variable. Existential variables are used in predicate logic to indicate
> that there exists some value that satisfies a certain condition without
> specifying the value itself. These variables are typically denoted using
> unique symbols, and their values are determined by the program's inference
> engine when solving a logical query.
> 
> Mercury is a logic programming language that is closely related to Prolog.
> In Mercury, existential variables are represented using the ^ symbol
> followed by a unique identifier. Here's an example of how you might use an
> existential variable in Mercury:
> 
> mercury
> 
> :- type person ---> john ; mary.
> 
> :- pred likes(person, person).
> :- mode likes(in, in) is semidet.
> 
> % Rule 1: John likes someone (existential variable Y).
> likes(john, Y) :-
>     % Here, ^ is used to introduce an existential variable.
>     Y ^ (mary ; john).
> 
> % Rule 2: Mary likes someone (existential variable X).
> likes(mary, X) :-
>     X ^ (john ; mary).
> 
> % Query: Who does John like?
> :- func who_does_john_like = person is nondet.
> who_does_john_like = X :-
>     likes(john, X).
> 
> % Query: Who does Mary like?
> :- func who_does_mary_like = person is nondet.
> who_does_mary_like = Y :-
>     likes(mary, Y).
> 
> In the example above, ^ is used to introduce existential variables, such
> as X ^ (john ; mary), indicating that there exists a value X (an unknown
> person) that Mary likes. The actual value of X is determined by the
> inference engine when solving queries.
> 
> When you query who_does_john_like or who_does_mary_like, the Mercury
> system will provide the answers based on the rules and the values of the
> existential variables introduced in the rules. The use of existential
> variables allows you to express existentially quantified statements and
> perform logical inference in Mercury.
> 
> ---------[end of chatgpt content]
> 
> 
> 
> 
> > On 23 Oct 2023, at 17:48, Volker Wysk <[email protected]> wrote:
> > 
> > Am Montag, dem 23.10.2023 um 17:39 +0100 schrieb Sean Charles
> > (emacstheviking):
> > > I decided to ask Chat-GPT Volker, it has been most helpful too!
> > 
> > For real? What did it say?
> > 
> > > 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 <[email protected]> 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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