[plt-scheme] Fun with paren-shape

 > [(+ n 3) : n is (< n 10) (even? n)]
(3 5 7 9 11)

 > [(+ n 3) : n is (< n 10)]
(3 4 5 6 7 8 9 10 11 12)

 > [(* n n) : n is (< n 10) (odd? n)]
(1 9 25 49 81)

Is there a better keyword than "is"?

/Jens Axel


(module index mzscheme
   (provide (rename my-app #%app))
   (require-for-syntax (lib "stx.ss" "syntax"))

   (define (interval n m)
     (cond
       [(>= n m) '()]
       [(< n m) (cons n (interval (+ n 1) m))]))

   (define-syntax (my-app stx)
     (define (bracket-shaped? stx)
       (equal? (syntax-property stx 'paren-shape) #\[))
     (define (brackets? parens)
       (syntax-case parens ()
         [(x) (bracket-shaped? parens)]
         [_ #f]))
     (define (range? stx)
       (syntax-case* stx (..) module-or-top-identifier=?
         [(a .. b) (bracket-shaped? stx)]
         [_        #f]))
     (define (handle-brackets exprs)
       ; vector brackets
       (let loop ([exprs      exprs]
                  [prev-exprs '()])
         (cond
           [(null? exprs)
            (reverse prev-exprs)]
           [(null? (cdr exprs))
            (loop (cdr exprs) (cons (car exprs) prev-exprs))]
           [(not (brackets? (cadr exprs)))
            (loop (cdr exprs) (cons (car exprs) prev-exprs))]
           [else
            (loop (cddr exprs)
                  (cons (quasisyntax/loc (cadr exprs)
                          (vector-ref #,(car exprs) #,@(cadr exprs)))
                        prev-exprs))])))
     (define (handle-ranges exprs)
       (let loop ([exprs      exprs]
                  [prev-exprs '()])
         (cond
           [(null? exprs)
            (reverse prev-exprs)]
           [(not (range? (car exprs)))
            (loop (cdr exprs) (cons (car exprs) prev-exprs))]
           [else
            (with-syntax ([[from .. to] (car exprs)])
              (loop (cdr exprs)
                    (cons (syntax/loc (car exprs)
                            (interval from to))
                          prev-exprs)))])))
     (syntax-case* stx (.. : is) module-or-top-identifier=?
       [(_ a .. b)
        (bracket-shaped? stx)
        (quasisyntax/loc stx
          (interval a b))]
       [(_ expr : n is while-pred-expr)
        (bracket-shaped? stx)
        (quasisyntax/loc stx
          (let loop ([n 0])
            (if while-pred-expr
                (cons expr (loop (+ n 1)))
                '())))]
       [(_ expr : n is while-pred-expr filter-expr)
        (bracket-shaped? stx)
        (quasisyntax/loc stx
          (let loop ([n 0])
            (if while-pred-expr
                (if filter-expr
                    (cons expr (loop (+ n 1)))
                    (loop (+ n 1)))
                '())))]
       [(_ n : while-pred-expr)
        (bracket-shaped? stx)
        (quasisyntax/loc stx
          (let loop ([n 0])
            (if while-pred-expr
                (cons n (loop (+ n 1)))
                '())))]
       [(_ n : while-pred-expr filter-expr)
        (bracket-shaped? stx)
        (quasisyntax/loc stx
          (let loop ([n 0])
            (if while-pred-expr
                (if filter-expr
                    (cons n (loop (+ n 1)))
                    (loop (+ n 1)))
                '())))]
       [(_ expr . more )
        (or (ormap brackets? (cddr (syntax->list stx)))
            (ormap range?    (cdr (syntax->list stx))))
        (with-syntax ([(expr ...)
                       (handle-ranges
                        (handle-brackets
                         (cdr (syntax->list stx))))])
          (quasisyntax/loc stx
            (#%app expr ...)))]
       [(_ . more)
        (syntax/loc stx
          (#%app . more))])))