(import (rnrs))

(define (compare asc? a b)
  (if asc?
    (cond
      [(and (number? a) (number? b)) (< a b)]
      [(and (char? a) (char? b)) (char<? a b)]
      [(and (string? a) (string? b)) (string<? a b)]
      [else (error 'compare "not supported" a b)])
    (cond
      [(and (number? a) (number? b)) (> a b)]
      [(and (char? a) (char? b)) (char>? a b)]
      [(and (string? a) (string? b)) (string>? a b)]
      [else (error 'compare "not supported" a b)])))

(define (sort asc? l a b)
  (list-sort
    (lambda (a* b*)
      (compare asc? (a a*) (b b*)))
    l))

(define-syntax from
  (lambda (x)
    (define (e? e o)
      (let ((e (syntax->datum e))
            (o (syntax->datum o)))
         (eq? e o))) ; fender takes care of symbol?

    (syntax-case x (in select where orderby asc desc)
      [(from e in l select sel)
        (identifier? #'e)
        (if (e? #'e #'sel)
          #'l
          #'(map (lambda (e) sel) l))]
      [(from e in l where p select sel)
        (if (e? #'e #'p)
          #'(from e in l select sel)
          #'(from e in (filter (lambda (e) p) l)
             select sel))]
      [(from e in l orderby p asc select sel)
        #'(from e in
            (sort #t l (lambda (e) p) (lambda (e) p))
           select sel)]
      [(from e in l orderby p desc select sel)
        #'(from e in
            (sort #f l (lambda (e) p) (lambda (e) p))
           select sel)]
      [(from e in l orderby o select s)
        #'(from e in l orderby o asc select s)]
      [(from e in l where w orderby o select s)
        #'(from e in (from e in l where w select e) orderby o select s)]
      [(from e in l where w orderby o asc select s)
        #'(from e in (from e in l where w select e) orderby o asc select s)]
      [(from e in l where w orderby o desc select s)
        #'(from e in (from e in l where w select e) orderby o desc select s)]
      )))

I have refactored it a bit, added some optimizations on ‘null’ transforms and added a syntax check.

Next, I am going to try to improve the permutations.

I am also not so sure about those (lambda (e) x) transforms.

Your response: