; Notes on Pattern Matching
; and Assignment #1
; 4701 - Artificial Intelligence
;Recall that we wrote and anlyzed
the "equal" function in class.
;A generalization of equality is computing the "similarity" of
;two objects. This we call pattern matching.
;Here is the basic pattern matching function that
;the real pattern matcher (rpm). Note that it is called by
;our users without specifying a third argument. rpm, however,
;requires an association list as its third argument. The
;association list encodes the "pattern variables and their bindings".
;Recall that our data can be ANY LISP s-expression
;atoms. Patterns may include pattern-variables (symbols that
;have been defined by some means to be variables) and Kleene
;star represented by the symbol "*".
; In a nutshell, rather than simply allowing ;"?" pattern matching,
;we would also like to use variables to match data
;objects and be bound to them. These pattern variables are stored on an
;"association list" we cart around thru the recursion. Read on.
(defun match (p d)
(rpm p d nil))
;ok, now the real pattern matcher
;This function returns "nil" if it fails to match, "t" if it
;succeeds but finds NO VARIABLES in the pattern, and "an association
;list of variable bindings" in the case where it succeeds
;finds variables in the pattern.
;An association list is a list of pairs. Each pair
is a list of
;two LISP values. The first LISP value is a symbol, the name of
;a pattern variable. The second LISP value is the data element
;found to match the variable during the course of pattern matching.
;This bound value can be any LISP value.
; (match '(a ? ?x b ) '(a 1 2 b)) matches and the returned value would
;be ( (?x 2) ), i.e. it matched and the variable ?x was bound to 2.
; Note that a ? matches anything (a single unit (atom or list)) without
; binding data to a variable.
(defun rpm (p d a)
( (and (null p) (null d)) ;we've exhausted both successfully
(cond (a a)(t t))) ;return association list or T
( (or (null p) (null d)) ;we've exhausted one and
not the other
nil ) ;return failure
( (is-vbl (first p)) ;we've encountered a pattern
((boundp (first p) a) ;its a bound variable:
;if its bound value is equal to the
;first data element, we return the
;real pattern match of the rest of the
;pattern with the rest of the data
((eql (first d) (assoc (first p) a))
(rpm (rest p)(rest d) a))
;here we know its bound value is unequal
;so we fail in the match.
;if its unbound, we bind it on the
;association list and similarly call
(t (rpm (rest p)(rest d) (cons (list (first p) (first d)) a)))))
;Ok, here is Kleene Star
(for free!) You can
;also include ? if you like preceeding this.
( (eq '* (first p))
;Kleene Star matches 0 or more elements, so...
(let ((newa (rpm (rest p) d a))) ;See if we match 0 elements.
;Note how this is accomplished. We advance
;p to the cdr of p in the recursion BUT DO
;NOT ADVANCE d.
;if so we return the new association list
(cond (newa newa)
;otherwise we try to match one data element
(t (rpm p (rest d) a)))))
;Note how this is accomplished. p
;ADVANCED, but d is. In the recursive call
;p will still have kleene star as its head
;but d will be its cdr. Think about that!
;The entire "let" can be simply replaced!
;HOW? Well, here's how:
; (rpm (rest p) d a)
; (rpm p (rest d) a) )
;Ok, now that that is done...
;Now we check to see if p starts with
;a symbol that is not a variable, nor a
;a Kleene Star.
( (atom (first p)) ;if so, check to see if it is
;the first element of the data
(cond ((eql (first p)(first d)) (rpm (rest p)(rest d) a))
(t nil))) ;otherwise we failed
;Now we know we have a list at the head of p
;This is a little tricky so watch carefully:
;I'm repairing a bug left behind in class.
(let (( newa (rpm (first p)(first d) a))) ;pattern match the sublists
(cond ( (null newa) nil) ;but it failed, so we fail
;we succeeded, but newa
could be "t" or
;a binding list, so....
( (listp newa) (rpm (rest p)(rest d) newa))
;Here newa is "t" so rpm with the association
;list of nil
(t (rpm (rest p)(rest d) nil)))))
;and that's all folks!
; Lastly we have to define our variable definition functions, ie. a means
;of defining variables so our pattern matcher will know when its encountered
;a pattern variable to distinguish it from an ordinary symbol. Well,
;that's left to you as an exercise. I'll write the defun's just to
;remind you that they have to be written. PLEASE NOTE that I used
;"assoc" in rpm. Look it up. And here's a hint: pattern variables may
;have a new property associated with their symbol names, or may be collected
;together in one master list of pattern variable symbols.....I guess that's
;more than a hint.
;(defun is-vbl (x) ( ... ) )
;(defun assoc ( x a-list
) ( ... ) )
;hmmmm: I wonder if there is a built in function like this?
; Well I'm feeling like I'm in a real good mood, so there aren't
;very many bugs left in this code. There may be one...check it out...
; There are very many more ways to write this function. It can be
;written smaller, cleaner and faster. If you like puzzles, try working
;out a better solution.
; By the way, did you note how to write comments in LISP code?
;Any indeed, if you would like to run this code although its sitting
;out on a file, you need to use the "load" function in lisp. "load"
;does what it says: it loads a file of LISP definitions and expressions
;and evaluates each one in turn. After loading a file, you can
;call any function that you have defined in that file. Try it and see
;for yourself AFTER defining the is-vbl and assoc functions.
;Ok, now here is your First Assignment due in about two weeks.
;Extend the (possibly buggy) solution written above to enumerate
;Produces the “left most” first match and binding of variables.
;You assignment is to extend this so that the function generates
;For example, as it now stands,
(match ( ‘(A * ?x *) ‘(A B C D)) ;will produce
;( ?x.B) as the ONLY output.
;Rather, your new matching function should produce
( ( (?x B) ) ( (?x C) ) ( (?x D) ) )
;HERE IS A SAMPLE TEST SUITE YOU OUGHT TO INPUT INTO YOUR FINISHED PROJECT:
;For brevity, the output of the commands (function
evaluations) is indicated
;after the sign "=>". The output of the function similar depends on the
;implementation. I implemented the similarity with an association list, but it
;could be done differently.
(match '() '()) ;=>T
(match '(ai) '(ai)) ;=>T
(match '(ai cs) '(ai cs)) ;=>T
(match '(cs ai) '(ai cs)) ;=>NIL
(match '(1 2 3 0) '(1 2 3 4 0)) ; => NIL
(match '(? mudd) '(seely mudd)) ;=>T
(match '(?first ?middle mudd) '(seely w mudd)) ;=> ((?MIDDLE
W) (?FIRST SEELY))
(match '(? ?x ? ?y ?) '(Warren Buffet Is A Good Man)) ;=> NIL
(match '(School Of Engineering and Applied Science) '(School Of
Engineering)) ;=> NIL
(match '(* School Of Engineering and Applied Science) '(The Fu Foundation School Of Engineering and Applied Science)) ;=> T
(match '(The * School Of Engineering and Applied Science) '(The Fu Foundation School Of Engineering and Applied Science)) ; => T
(match '(The * School Of Engineering and Applied Science) '(The School Of Engineering and Applied Science)) ;=> T
(match '(* 3 ?x 4 *) '(3 5 4)) ;=> ( (?x 5))
(match '( ?x (1 2) ?y (4 5)) '(c (1 2) d (4 5))) ;=> ((?Y D) (?X C))
(match '(?y ?z (c v)) '(8 gh (c v) )) ;=> ((?Z GH) (?Y 8))
(match '(((get) me) out) '(get (me (out)))) ;=> NIL
(match ‘( A * B) ‘(A A A A A B) ) ;=> T
(match ‘(?x * ?y) ‘(A A A A A B) ) ;=> ((?Y B) (?X A))
(match ‘(?x * ?y *) ‘(A A A A A A A A A A A A A A A A A A) ) ;=> uh oh…what happened?