▼Datalog: Deductive Database Programming
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4 Racket Interoperability

 (require datalog)

The Datalog database can be directly used by Racket programs through this API.

Examples:

> (define family (make-theory))
> (datalog family
           (! (parent joseph2 joseph1))
           (! (parent joseph2 lucy))
           (! (parent joseph3 joseph2)))

'()

> (datalog family
           (? (parent X joseph2)))

'(       #hasheq((X . joseph3)))

> (datalog family
           (? (parent joseph2 X)))

'(       #hasheq((X . joseph1)) #hasheq((X . lucy)))

> (datalog family
           (? (parent joseph2 X))
           (? (parent X joseph2)))

'(       #hasheq((X . joseph3)))

> (datalog family
           (! (:- (ancestor A B)
                  (parent A B)))
           (! (:- (ancestor A B)
                  (parent A C)
                  (= D C)
                  (ancestor D B))))

'()

> (datalog family
           (? (ancestor A B)))

'(       #hasheq((A . joseph3) (B . joseph2)) #hasheq((A . joseph2) (B . lucy)) #hasheq((A . joseph2) (B . joseph1)) #hasheq((A . joseph3) (B . lucy)) #hasheq((A . joseph3) (B . joseph1)))

> (let ([x 'joseph2])
    (datalog family
             (? (parent x X))))

'(       #hasheq((X . joseph1)) #hasheq((X . lucy)))

> (datalog family
           (? (add1 1 :- X)))

'(       #hasheq((X . 2)))

theory/c : contract?
A contract for Datalog theories.

(make-theory) → theory/c
Creates a theory for use with datalog.

(write-theory t) → void
  t : theory/c
Writes a theory to the current output port. Source location information is lost.
(read-theory) → theory/c
Reads a theory from the current input port.

(datalog thy-expr
         stmt ...)
 
  thy-expr : theory/c
Executes the statements on the theory given by thy-expr. Returns the answers to the final query as a list of substitution dictionaries or returns empty.

(datalog! thy-expr
         stmt ...)
 
  thy-expr : theory/c
Executes the statements on the theory given by thy-expr. Prints the answers to every query in the list of statements. Returns (void).

Statements are either assertions, retractions, or queries.

(! clause)
Asserts the clause.
(~ clause)
Retracts the literal.

(:- literal question ...)
A conditional clause.

(? question)
Queries the literal and prints the result literals.

Questions are either literals or external queries. Literals are represented as identifier or (identifier term ...). External queries are represented as (identifier term ... :- term ...), where identifier is bound to a procedure that when given the first set of terms as arguments returns the second set of terms as values. A term is either a non-capitalized identifiers for a constant symbol, a Racket datum for a constant datum, or a capitalized identifier for a variable symbol. Bound identifiers in terms are treated as datums.

External queries invalidate Datalog’s guaranteed termination. For example, this program does not terminate:
(datalog (make-theory)
         (! (:- (loop X)
                (add1 X :- Z)
                (loop Z)))
         (? (loop 1)))

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