Phoenix Network Forums

Hi, my first simple packrat parser according to <http://www.lshift.net/~tonyg/packrat.pdf> works, using Chicken Scheme. It's a calculator with infix operators ``+'' and ``-'' with left associativity. I gave myself this task because a naive solution with a recursive descent parser (including a packrat one) is right associative by nature.

Here goes the grammar:

Code: Select all

  (define calc
    (packrat-parser
     expr
     (expr ((a <- simple b <- tail)
            (b a)))
     (tail (('+ a <- simple b <- tail)
            (lambda (x) (b (+ x a))))
           (('- a <- simple b <- tail)
            (lambda (x) (b (- x a))))
           (()
            values))
     (simple ((a <- 'num)
              a))))

E.g. the resulting value is 6 for this generator:

Code: Select all

(define g (generator '((num . 19) (-) (num . 11) (-) (num . 2))))

Or 10 for that one:

Code: Select all

(define g (generator '((num . 19) (-) (num . 11) (+) (num . 2))))

Regards
Thomas