On Wed, Sep 16, 2015 at 2:50 PM, Eduardo Bellani <ebell...@gmail.com> wrote:
> Hello list
>
> I'm trying to use racket's implementation of datalog as a query language
> for a small database (5 gb). I'm running into some problems that I'd
> appreciate your help with:
>
> * Is there support for negating a subgoal? E.g: [1]
>
> Get the names of employees who do not work on any project.
> temp1(S) :- works_on(S,_,_).
> answer(F,M,L) :- employee(F,M,L,S,_,_,_,_,_,_), not temp1(S).
No, but I have no objection to someone adding this. I believe it will
require a different evaluation technique (bottom-up vs the present
top-down) to run efficiently.
> * Is there support for other comparison operators outside {=, !=}? E.g.
>
> (datalog *db*
> ;; shareholders
> (! (shareholder "Jorge" person))
> (! (shareholder "Bob" person))
> (! (shareholder "Googly" company))
> (! (shareholder "Megasoft" company))
> ;; ownership
> (! (owns-a-share "Jorge" "Construction SA" 60.8))
> (! (owns-a-share "Bob" "Construction SA" 39.2))
> (! (owns-a-share "Bob" "WAKA SA" 100 ))
> (! (owns-a-share "Googly" "Toys SA" 100 )))
>
>
> (let ((majority? (λ (percent)
> (> percent 50))))
> (datalog *db*
> (! (:- (major-stockholder COMPANY)
> (shareholder SHAREHOLDER person)
> (owns-a-share SHAREHOLDER COMPANY PERCENT)
> (majority? PERCENT :- TEMP)
> (= TEMP true)))))
You could write (majority? PERCENT :- #t)
Here's a tiny example
#lang racket/base
(require datalog)
(datalog (make-theory)
(! (number 0))
(! (number 1))
(! (number 2))
(! (number 3))
(! (:- (awesome N)
(number N)
(even? N :- #t)))
(? (awesome N)))
> * A syntax extension difficulty, not very much related to datalog per
> se. I have this syntax extension:
>
>
> (define-syntax (->relations stx)
> ;; point here being getting true relations (sets) out of the results
> (syntax-case stx ()
> [(_ rel-name rel-size)
> (with-syntax ([rel-vars (for/list ([x (in-range (syntax->datum
> #'rel-size))])
> (gensym 'X))])
> #`(map (λ (dic)
> (list->set
> (map (λ (rel-var)
> (hash-ref dic rel-var))
> #'rel-vars)))
> (datalog *db* (? (#,#'rel-name
> #,@#'rel-vars)))))]))
>
> This is an case of an usage, in the context of the previous example:
>
> (->relations major-stockholder 1)
>
> I'm bumping in the following error:
>
> make-variable: contract violation
> expected: (or/c #f (list/c any/c (or/c exact-positive-integer? #f) (or/c
> exact-nonnegative-integer? #f) (or/c exact-nonnegative-integer? #f) (or/c
> exact-positive-integer? #f)))
> given: '(#f #f #f #f 0)
> in: the 1st argument of
> (->
> (or/c
> #f
> (list/c
> any/c
> (or/c exact-positive-integer? #f)
> (or/c exact-nonnegative-integer? #f)
> (or/c exact-nonnegative-integer? #f)
> (or/c exact-positive-integer? #f)))
> symbol?
> variable?)
> contract from: <pkgs>/datalog/ast.rkt
>
> Which I interpreted as the LOC being lost in the syntax generator. I
> haven't being able to resolve that yet. Any light on the subject?
This is an error on my part. It should be POS NONNEG POS NONNEG but I
wrote POS NONNEG NONNEG POS. I just did a push, let me know if that
fixes it (or you could just change it on your own copy.)
Jay
> Thanks in advance for any pointers.
>
> [1] http://tinman.cs.gsu.edu/~raj/8710/f05/datalog.pdf
>
> --
> Eduardo Bellani
>
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--
Jay McCarthy
http://jeapostrophe.github.io
"Wherefore, be not weary in well-doing,
for ye are laying the foundation of a great work.
And out of small things proceedeth that which is great."
- D&C 64:33
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