Question

Hey, I’m working on a homework question about applying a function to every element in a list, going as deep as needed.

I am getting a error when calling (fun (car l)), that

mcar: expects argument of type <mutable-pair>; given 5

and yet when I just call (fun l) i get the error
+: expects type as 1st argument, given: (5); other arguments were: 1

   (define (map-gen fun l)
    (if (null? fun) l 
        (if (null? l) '()
            (if (list? (car l))
                (append (map-gen fun (cdr l)) (map-gen fun (car l)))
                (append (map-gen fun (cdr l)) (fun (car l)))))))

Any and all help is appreciated!

edit: This is when calling the function like so:

(map-gen (lambda (x) (+ x 1))'(1 (2 (3 4))(((5)))))

Answer 1

You’ve got the arguments for appends backwards, and I think cons would be a better bet. Also, checking to see if fun is null is unnecessary.

(define (map-gen fun l)
   (if (null? l) '()
       (if (list? (car l))
           (cons (map-gen fun (car l)) (map-gen fun (cdr l)))
           (cons (fun (car l)) (map-gen fun (cdr l))))))

Also, as Eli says in his answer, you really should look into cond instead of nested ifs. It’s much more elegant, making your code more readable.