+++ /dev/null
-This function is developed in *The Seasoned Schemer* pp. 84-89. It accepts an atom `a` and a list `lst` and returns `lst` with the leftmost occurrence of `a`, if any, removed. Occurrences of `a` will be found no matter how deeply embedded.
-
- #lang racket
-
- (define (atom? x)
- (and (not (pair? x)) (not (null? x))))
-
- (define gamma
- (lambda (a lst)
- (letrec ([aux (lambda (l k)
- (cond
- [(null? l) (k 'notfound)]
- [(eq? (car l) a) (cdr l)]
- [(atom? (car l)) (cons (car l) (aux (cdr l) k))]
- [else
- ; when (car l) exists but isn't an atom, we try to remove a from (car l)
- ; if we succeed we prepend the result to (cdr l) and stop
- (let ([car2 (let/cc k2
- ; calling k2 with val will bind car2 to val and continue with the (cond ...) block below
- (aux (car l) k2))])
- (cond
- ; if a wasn't found in (car l) then prepend (car l) to the result of removing a from (cdr l)
- [(eq? car2 'notfound) (cons (car l) (aux (cdr l) k))]
- ; else a was found in (car l)
- [else (cons car2 (cdr l))]))]))]
- [lst2 (let/cc k1
- ; calling k1 with val will bind lst2 to val and continue with the (cond ...) block below
- (aux lst k1))])
- (cond
- ; was no atom found in lst?
- [(eq? lst2 'notfound) lst]
- [else lst2]))))
-
- (gamma 'a '(((a b) ()) (c (d ())))) ; ~~> '(((b) ()) (c (d ())))
- (gamma 'a '((() (a b) ()) (c (d ())))) ; ~~> '((() (b) ()) (c (d ())))
- (gamma 'a '(() (() (a b) ()) (c (d ())))) ; ~~> '(() (() (b) ()) (c (d ())))
- (gamma 'c '((() (a b) ()) (c (d ())))) ; ~~> '((() (a b) ()) ((d ())))
- (gamma 'c '(() (() (a b) ()) (c (d ())))) ; ~~> '(() (() (a b) ()) ((d ())))
- (gamma 'x '((() (a b) ()) (c (d ())))) ; ~~> '((() (a b) ()) (c (d ())))
-