scheme,lisp,r5rs , Append list from assoc to another list in Scheme


Append list from assoc to another list in Scheme

Question:

Tag: scheme,lisp,r5rs

I'm a bit confused on how to append a list that I've gotten from the assoc procedure into another list, here is what I have:

(define new-list (list 'test))
(define source-a (list '(a foo) '(b bar) '(c hello)))
(append new-list (assoc 'a source-a))
(display new-list)

The output is just (test) and I'm not sure why it's not (test a foo). Is it possible to append like this?


Answer:

That's because append is not a mutating function. It returns a new list with its arguments appended together. By convention in Scheme, functions that perform mutation end with an exclamation mark, such as set!.

You can use set! to modify new-list so that it is updated, like this:

(set! new-list (append new-list (assoc 'a source-a)))

However, this is highly discouraged in Scheme. While imperative programming makes heavy use of mutation, functional programming languages (including Scheme) try to avoid mutation and side-effects, since those can make programs harder to reason about.

Ideally, you'd just declare a new binding with the new value instead of updating an existing binding. Something like this would work just fine:

(define original-list (list 'test))
(define source-a (list '(a foo) '(b bar) '(c hello)))
(define new-list (append original-list (assoc 'a source-a)))
(display new-list)

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