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
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