Question

I’m working on a Scheme assignment for school and there’s a question involving us defining a record “type” (implemented as a list) (which represents a music record).

The question I’m having trouble with is I’m asked to create a procedure which creates a list of these records, and then a function to add a record to this list. This is pretty simple, but I feel like I might be doing something wrong.

I know how to append an element to a list (i.e. the record shelf in this example), but I’m not sure how to properly maintain this list across invocations of this add function. Here’s what I’ve got:

(define (add-record record lst)
  (append lst (list record)))

Which works as I’d expect, but my problem is when I invoke this procedure.

(define record-self '())

Was my first attempt, but of course every time I add a record with the add-record procedure, passing in that just-defined record-shelf list, well, my add function returns a brand new list (i.e. a copy, with the appended record). This makes sense, but I’m not sure if that’s what I want.

So if I wanted to add a bunch of records to the list like so:

(add-record highway61 record-shelf)
(add-record sgtPepper record-shelf)

Of course it doesn’t result in what I want, because record-shelf doesn’t get updated. And I don’t think at this point in the course we’re supposed to use set! or assignments.

Should I just be grabbing a copy of the returned list (from add-record) every time, and then use that returned list in the next invocation?

Answer 1

You are wishing for stateful behavior. As Enrique says, the answer to your last question is Yes.

Is there a reason you can’t just cons the new record onto the head of the list? That is the idiomatic way to add items to a list in Lisp. Or in any functional language that uses singly linked lists.

(define (add-record record lst)
  (cons record lst))

(define newshelf (add-record 36chambers oldshelf))

There is no way to make record-shelf contain a new item after you invoke add-record without using set! in some form.