MIT 6.001                                       Fall, 1997
Instructor: A. Meyer

                 Recitation #7, Notes for Wed., 12/10/97


LAST CLASS.  We discussed final exam topics and then reviewed the desired
behavior and types of a LEXER taking a string to a list of TOKENS, a
PARSER taking a list of tokens to a tree of ATOMS, and a PRINTER taking a
tree of atoms to a string.


                          ;;LEXICAL ANALYSIS

TOKEN = Atom + {*LPARAN*} + {*RPARAN*}
Atom = Symbol + Number + Bool

(define *LPARAN* (LIST '*LPARAN*))
(define *RPARAN* (list '*RPARAN*))

Example:
(string->tokens "a be c#d () 12 s    #t aa  123#")
;Value: (a be c#d (*lparan*) (*rparan*) 12 s #t aa 123#)


                                  ;;PARSING   

;PARSE-TOKENS: List(Token) --> Tree(Atom)

EXAMPLES:
(parse-tokens (list *LPARAN* *LPARAN* 1 'symb *RPARAN* '+ *RPARAN*))
;Value: ((1 symb) +)

(parse-tokens (string->tokens "(+ (- 2x    3yy#t) (if #t 1  0))"))
;Value: (+ (- 2x 3yy#t) (if #t 1 0))


                                       ;;PRINTING

;TREE->STRING: Tree(Atom) --> String

;CONTRACT:
;(equal? TREE (parse-tokens (string->tokens (tree->string TREE)))) ==> #t

;(tree->string '(+ a#2 3 (* 12 x)))
;;Value: "(+ a#2 3 (* 12 x))"


We did not develop code for these since this is not required material.  If
you're interested in seeing and/or running these procedures, look at the
file READ-PRINT.SCM in the section directory.