How I can force scheme to return #f explicitly instead of just void?

(define every-aux
  (lambda(status predicate lst)
    (cond((null? lst) status)
         ((cond((equal? (predicate (car lst)) #t)
                (set! status #t)
                (every-aux status predicate (cdr lst)))
               (else (set! status #f) status))))))

开发者_JAVA百科Above Procedure returns void if predicate does not match with every element in lst?

It does not have any problem is returning #t though if predicate matches every element of lst.

Changing the last line to

(else (set! status #f) status))))))

to

 (else (set! status "#f") status))))))

returns "#f" so procedure is correct.

How I can force scheme to return #f explicitly instead of just void?

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Your code is very messy:

• You have a cond inside another, but cond is intended for multiple tests/results.

• There is no reason to have that status modified -- Scheme uses call-by-value, so this is likely not doing whatever you think it does.

• Specifically, there is no reason to use (else (set! status #f) status) -- you could just return #f directly.

• The actual reason for your confusion is the weird cond nesting -- the second cond is actually used as a test, so if you make it (the inner cond) return #f, then the whole test of the outer cond is getting #f, which means that it (the outer cond) didn't get any true result, and resorts to returning an unspecified value (and if this is Racket, then that value is shown as #<void>). So if you flatten the two conds into one, your problem will go away.

• Finally, if you're having problems at such a level, then you should consider using some textbook to familiarize yourself with the language. Specifically HtDP is intended to give you an easy path into getting familiar with the syntax.

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@Eli Barzilay

After some I deliberation I could see the solution. Thanks for the pointers.

(define every?
  (lambda (predicate list)
    (if(null? list) "Empty List not allowed."
       (every-aux? predicate list))))

(define every-aux?
  (lambda (predicate lst)
    (cond ((null? lst) #t)
          ((equal? (predicate (car lst)) #t) (every-aux? predicate (cdr lst)))
          (else #f))))