试图在Dr. Racket中实现删除*？

Question

(define (removeAll list-a list-b)
  (if (empty? list-b)
    list-a
    (apply
      (removeAll((remove (first list-b) list-a) (rest list-b)))
    )
  )
)

(removeAll '(abbccd) '(aca))

expected output is '(bbd) in this case

I can use remove but I can't use remove* itself. Currently I'm getting this error: application: not a procedure; expected a procedure that can be applied to arguments given: '(bbccd) arguments...: application: not a procedure; expected a procedure that can be applied to arguments given: '(bbccd) arguments...:

Answer 1

If you execute your program in DrRacket you will obtain the error message with a clear evidence of its reason:

You see that the problem is inside:

((remove (first list-b) list-a) (rest list-b)))

The reason is that in scheme you cannot use the parentheses freely like in other languages: (+ 2 (+ 3 4)) is not equal to (+ 2 ((+ 3 4))) : the first expression is evaluated to 9, the second gives an error since ((+ 3 4)) is interpreted in this way:

1. Evaluate the inner form (expression) (+ 3 4) , that is apply the operation + to the arguments 3 and 4, producing 7.
2. Since (+ 3 4) is inside another set of parentheses, than get the result and try to evaluate (7) as a form. But 7 is not an operator (“not a procedure”, as the error message says), so an error is raised.

In your case, the result of remove is a list, and this cannot be used as an operator.

If you remove the double parentheses, in this way:

(removeAll (remove (first list-b) list-a) (rest list-b))

you will remove the first error, and you will see that there is another error, in particular:

apply: arity mismatch;
 the expected number of arguments does not match the given number
  expected: at least 2
  given: 1
  arguments.:

Here is a possible solution:

(define (removeAll list-a list-b)
  (if (empty? list-b)
      list-a
      (removeAll (filter (lambda (x) (not (eq? (first list-b) x))) list-a) (rest list-b))))

The function filter remove all the elements that do not satisfy the predicate.