;;;2.3 lisp
;;;Adam nelson
;;;Chet tobrey

(defun mappend (fn list)
  "Append the results of calling fn on each element of the list.
  Like mancon, but uses append instead of nconc."
  (apply #'append (mapcar fn list)))

(defparameter *simple-grammar*
  '((sentence -> (noun-phrase verb-phrase))
    (noun-phrase -> (Article Noun))
    (verb-phrase -> (Verb noun-phrase))
    (Article -> the a)
    (Noun -> man ball woman table)
    (Verb -> hit took saw liked))
  "A grammar for a trivial subset of English.")

(defvar *grammar* *simple-grammar*)

(defun rule-lhs (rule)
  "left hand side of a rule"
  (first rule))

(defun rule-rhs(rule)
  "right hand side of a rule"
  (rest (rest rule)))

(defun rewrites (category)
  "return a list of possible rewrites for this category"
  (rule-rhs (assoc category *grammar*)))

(defun generate (phrase)
  "Generate a random sentence or phrase"
  (cond ((listp phrase)
         (mappend #'generate phrase))
        ((rewrites phrase)
         (generate (random-elt(rewrites phrase))))
        (t (list phrase))))

(egs :2.3
     (eg '(assoc 'noun *grammar*)
         :of "using assoc to take a key and a list of lists to return the
             first element of the list of lists that starts with the key")
     (eg '(generate 'sentence)
         :of "generating a sentence")
     (eg '(generate 'sentence)
         :of "generating a sentence")
     (eg '(generate 'noun-phrase)
         :of "generating a noun-phrase")
     (eg '(generate 'verb-phrase)
         :of "generating a verb-phrase"))