[Chicken-users] Re: packrat example

[Thomas Hafner]

Zbigniew <address@hidden> wrote/schrieb <address@hidden>:

> Try
> 
> (require-extension syntax-case)
> (require-extension packrat)
> (import packrat)

That's sufficient for the original example. But as soon as I played
around with my own first grammar, I had to add also:
(require-extension srfi-1)

It's obviously needed by the packrat egg. Shouldn't srfi-1 then be
provided automatically by just using the packrat egg?

BTW the grammar mentioned above is for a calculator with infix
operators ``+'' and ``-'' with left associativity. I gave myself this
task because a naive solution with a recursive descent parsers
(including a packrat one) is right associative by nature.

Here goes my grammar:
  (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:
(define g (generator '((num . 19) (-) (num . 11) (-) (num . 2))))

Or 10 for that one:
(define g (generator '((num . 19) (-) (num . 11) (+) (num . 2))))

Which is both Ok. The empty tail (last clause of ``tail'' definition
above) helped a lot to simplify the grammar: without it, every
operator would need two variants, one with tail and the other one
without.

Maybe somebody can provide yet a simpler solution?

Regards
  Thomas