Putting a list in a parameter in scheme

Question

So I am new to using Scheme/Guile and have an assignment where I must take 2 inputs; a simple variable and a list.

I then add the variable to each number in the list and print it out. I have got it to load but when I enter this:

(add 1 (1 2 3))

I get this error:

Backtrace:
  39: 0* [add 1 ...
  39: 1*  [1 2 3]

standard inout:39:8: In expression (1 2 3):
standard input:39:8:wrong type to appy: 1
ABORT: (misc-error)

Here is the code that I currently have.

(define a (list a))
(define (add y a)
    (define x 0)
    (while (< x (length a))
        (display (+ y (car a)))
        (newline)
        (set! a (cdr a))
        (set! x (+ x 1))
    )
)

My question is:
How do I get the list to work in the parameter? I have looked around online but havent found much to fix this problem. Many thanks ahead of time for any help you can give me.

Answer 1

You're getting an error because (1 2 3) tells it to call the function 1 with two arguments, 2 and 3. To create a list containing 1, 2 and 3, use (list 1 2 3).

Answer 2

Do you really intend to write a procedure in such an imperative way? It looks not very Scheme-like. Evaluating your procedures in GNU Guile yields many error messages about unbound variables etc. set! is eval for beginners of Scheme ;) :) (SICP treats assignment extensively in chapter 3). You can do it much easier in functional style without the side-effects of set! and display.

If you are already aware about mapping and high-order functions, you could do something like this:

(define (add x)
  (lambda (y) (+ x y)))

(map (add 1) (list 1 2 3))
===> (2 3 4)

If not, write a recursive procedure yourself:

(define (add-to-list x list)
  (if (null? list)
      '()
      (cons (+ x (car list))
            (add-to-list x (cdr list)))))

(add-to-list 1 '(1 2 3))
===> (2 3 4)

In order to learn thinking recursively, consult books like SICP, HTDP or "The litte Schemer".