I am trying to write a Definite Clause Grammar syntax for Racklog, analogous to Prolog's DCG extension. Basically, I have created a syntax definition %rule so that:
(define %sentence (%rule () [() (%noun-phrase) (%verb-phrase)])) would be redefined as: (define %sentence (%rel (s0 s1 s) [(s0 s) (%noun-phrase s0 s1) (%verb-phrase s1 s)])) This code that I have so far works: #lang racket (require racklog (for-syntax racket)) ;;; Define a rule using DCG notation. (define-syntax (%rule stx) (syntax-case stx () [(_ (v ...) ((a ...) subgoal ...) ...) (with-syntax ([gls (for/list ([g (syntax->list #'(((a ...) subgoal ...) ...))]) (length (syntax->list g)))]) (with-syntax ([rule (append '(%rel) (list (append (for/list ([i (in-range (sub1 (apply max (syntax->datum #'gls))))]) (string->symbol (~a "s" i))) '(s) #'(v ...))) (for/list ([g (syntax->datum #'(((a ...) subgoal ...) ...))]) (cons (append '(s0 s) (first g)) (for/list ([sg (rest g)] [i (in-range (length (rest g)))]) (append sg (list (string->symbol (~a "s" i)) (if (= i (sub1 (length (rest g)))) 's (string->symbol (~a "s" (add1 i))))))))))]) #'rule))])) ;;; Define a terminal using DCG notation. (define-syntax (%term stx) (syntax-case stx () [(%term (t ...) ...) #'(%rel (x) [((append '(t ...) x) x)] ...)])) ;;; A simple English grammar in DCG notation. (define %sentence (%rule () [() (%noun-phrase) (%verb-phrase)])) (define %noun-phrase (%rule () [() (%proper-noun)] [() (%det) (%noun)] [() (%det) (%noun) (%rel-clause)])) (define %verb-phrase (%rule () [() (%trans-verb) (%noun-phrase)] [() (%intrans-verb)])) (define %rel-clause (%rule () [() (%dem-pronoun) (%verb-phrase)])) (define %det (%term [the] [every] [a])) (define %noun (%term [cat] [bat])) (define %proper-noun (%term [john] [mary])) (define %dem-pronoun (%term [that])) (define %trans-verb (%term [eats])) (define %intrans-verb (%term [lives])) ;;; Tests (%which (x) (%sentence x null)) (%which () (%sentence '(a cat eats the bat) null)) (%which (x) (%noun x null)) The problem I am having is that while this works when %rule is included in the same module (as above), when I move %rule and %term into a different module, I get the error "%noun-phrase: unbound identifier in module in: %noun-phrase". I suspect that it is some kind of a namespace issue, or possibly a phase issue. I have tried rewriting %rule a couple of different ways (one using syntax-rules rather than syntax-case, and another using quasiquote/unquote rather than with-syntax). Neither of which worked. Anyhow, I've spent a lot of time trying to figure this out, and I finally am at the point where I need some help. -Scott ____________________ Racket Users list: http://lists.racket-lang.org/users