PAIP grammar interpreter

first-grammar.clj

(ns org.enrico_franchi.paip.simplegrammar.grammar
  (:use clojure.core))
  
(def *simple-grammar*
     '((sentence -> (noun-phrase verb-phrase))
       (noun-phrase -> (Article Noun))
       (verb-phrase -> (Verb noun-phrase))
       (Article -> the a)
       (Noun -> ball man woman table)
       (Verb -> hit took saw liked)))

(def *grammar* *simple-grammar*)
  
(defn rule-lhs [rule]
  "The left hand side of a rule."
  (first rule))

(defn rule-rhs [rule]
  "The right hand side of a rule."
  (rest (rest rule)))
  
(defn get-rule [grammar category]
  "Get the selected rule from the grammar"
  (letfn [(key [rule]
               (let [lhs (rule-lhs rule)]
                 (when (= lhs category) rule)))]
  (some key grammar)))

(defn rewrites [category]
  "Return a list of the possible rewrites for this category"
  (rule-rhs (get-rule *grammar* category)))

(defn generate [phrase]
  "Generate a random sentence or phrase."
  (cond
   (list? phrase) (mapcat generate phrase)
   (seq (rewrites phrase)) (recur (rand-nth (rewrites phrase)))
   :default (list phrase)))