Question

I’m trying to create a list by adding a number to an existing list. The problem is that the existing list is not necessarily actually a list. It could be either an empty list ((list )), just a number, or an actual list.

Basically, I need something like append but it has to be able to handle this case:

(append 1 2) and generate a list (list 1 2)

In addition to the typical case:

(append (list 1 2) 3)

Using append for the first case gives me the error append: expected argument of type <proper list>; given 1

Is there something like append that can handle both of these cases? Or is there some other way to do this?

Thanks!

Answer 1

Try this simple program and tell me if it solve your issue:

#lang racket
(define (apnd a b)
   (flatten (cons a b)) 
)

#test
(apnd 1 2)
(apnd (list 1 2) (list 3 4))
(apnd '() 1)
(apnd 1 '())
(apnd '() '())
(apnd (list 1 2) 3)
(apnd 1 (list 2 3))

References: flatten