Adding an element to the head of an alist (Associative list) is simple enough:
> (cons '(ding . 53) '((foo . 42) (bar . 27))) ((ding . 53) (foo . 42) (bar . 27)) Appending to the tail of an alist is a bit trickier though. After some experimenting, I produced this:
> (define (alist-append alist pair) `(,@alist ,pair)) > (alist-append '((foo . 42) (bar . 27)) '(ding . 53)) '((foo . 42) (bar . 27) (ding . 53)) However, it seems to me, that this isn’t the idiomatic solution. So how is this usually done in scheme? Or is this in fact the way?
You don’t append to an a-list. You cons onto an a-list.
An a-list is logically a set of associations. You don’t care about the order of elements in a set. All you care about is presence or absence of a particular element. In the case of an a-list, all you care about is whether there exists an association for a given tag (i.e., a pair whose CAR is the specified value), and, given that association, the associated value (i.e., in this implementation, the CDR of the pair).