Re: [racket-users] Apparent Datalog error?

[George Neuner]

On 1/31/2018 2:51 PM, Matthias Felleisen wrote:

> On Jan 31, 2018, at 2:15 PM, George Neuner  wrote:
> 
> 
> On 1/31/2018 2:10 PM, Sam Caldwell wrote:

>> Your definition of `ancestor` is one or two steps of parentage:
>> 
>> > ancestor(A, B) :- parent(A, B).

>> > ancestor(A, B) :-
>>   parent(A, C),
>>   parent(C, B).
>> 
>> I suspect you want one of those lines to appeal to the `ancestor` relation to allow longer chains.
> 
> Knowing nothing about datalog he asks naively:
> 
> Shouldn't the ruleancestor(A, B) :- parent(A, C), parent(C, B).   be applied recursively?



It is — except that there’s no recursion.


> Kevin's definition would work in Prolog.


No it wouldn’t.


Sorry ... brain freeze.   Somehow I misread Kevin's code as being

  ancestor(A, B) :- parent(A, C), ancestor(C, B).

which would work.    You and Sam both are absolutely correct that it 
does not work as written.


Apologies for the noise.
George


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Re: [racket-users] Apparent Datalog error?

[Matthias Felleisen]

> On Jan 31, 2018, at 2:15 PM, George Neuner  wrote:
> 
> 
> On 1/31/2018 2:10 PM, Sam Caldwell wrote:
>> Your definition of `ancestor` is one or two steps of parentage:
>> 
>> > ancestor(A, B) :- parent(A, B).
>> > ancestor(A, B) :-
>>   parent(A, C),
>>   parent(C, B).
>> 
>> I suspect you want one of those lines to appeal to the `ancestor` relation 
>> to allow longer chains.
> 
> Knowing nothing about datalog he asks naively:
> 
> Shouldn't the ruleancestor(A, B) :- parent(A, C), parent(C, B).   be 
> applied recursively?


It is — except that there’s no recursion. 


> Kevin's definition would work in Prolog.


No it wouldn’t. 

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Re: [racket-users] Apparent Datalog error?

[George Neuner]

On 1/31/2018 2:10 PM, Sam Caldwell wrote:

Your definition of `ancestor` is one or two steps of parentage:

> ancestor(A, B) :- parent(A, B).
> ancestor(A, B) :-
  parent(A, C),
  parent(C, B).

I suspect you want one of those lines to appeal to the `ancestor` 
relation to allow longer chains.


Knowing nothing about datalog he asks naively:

Shouldn't the rule ancestor(A, B) :- parent(A, C), parent(C, B).   be 
applied recursively?

Kevin's definition would work in Prolog.

George

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Re: [racket-users] Apparent Datalog error?

[Sam Caldwell]

Your definition of `ancestor` is one or two steps of parentage:

> ancestor(A, B) :- parent(A, B).
> ancestor(A, B) :-
parent(A, C),
parent(C, B).

I suspect you want one of those lines to appeal to the `ancestor` relation
to allow longer chains.

- Sam Caldwell

On Wed, Jan 31, 2018 at 1:53 PM, Kevin Forchione  wrote:

> Walking through the datalog  tutorial I got the following transcript:
>
> Welcome to DrRacket, version 6.12 [3m].
> Language: datalog, with debugging; memory limit: 512 MB.
> > parent(john, douglas).
> > parent(john, douglas)?
> parent(john, douglas).
> > parent(john, evlyn)?
>
> > parent(bob, john).
> > parent(A, B)?
> parent(john, douglas).
> parent(bob, john).
> > parent(ebbon, bob).
> > parent(john, B)?
> parent(john, douglas).
> > parent(A, A)?
>
> > ancestor(A, B) :- parent(A, B).
> > ancestor(A, B) :-
> parent(A, C),
> parent(C, B).
> > ancestor(A, B)?
> ancestor(ebbon, bob).
> ancestor(bob, john).
> ancestor(john, douglas).
> ancestor(bob, douglas).
> ancestor(ebbon, john).
> >
>
> It seems that the correct answer should also include ancestor(ebbon,
> douglas). Am I doing something wrong?
>
> -Kevin
>
>
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