+++ /dev/null
-This function is developed in *The Seasoned Schemer* pp. 55-60. It accepts an atom `a` and a list of atoms `lst`, and returns the part of `lst` following the last occurrence of `a`. If `a` is not in `lst`, it returns `lst` unaltered.
-
- #lang racket
-
- (define (atom? x)
- (and (not (pair? x)) (not (null? x))))
-
- (define alpha
- (lambda (a lst)
- (let/cc k ; calling k with val will immediately return val from the call to alpha
- (letrec ([aux (lambda (l)
- (cond
- [(null? l) '()]
- [(eq? (car l) a)
- ; we abandon any waiting recursive (aux ...) calls, and instead immediately return (aux (cdr l))
- ; ...since Scheme is call-by-value, (aux (cdr l)) will be evaluated first, and
- ; any calls to k therein will come first (and the pending (k ...) here will be abandoned)
- (k (aux (cdr l)))]
- [else (cons (car l) (aux (cdr l)))]))])
- (aux lst)))))
-
-
- (alpha 'a '(a b c a d e f)) ; ~~> '(d e f)
- (alpha 'x '(a b c a d e f)) ; ~~> '(a b c a d e f)
- (alpha 'f '(a b c a d e f)) ; ~~> '()
- (alpha 'a '(a b c x d e f)) ; ~~> '(b c x d e f)
-