Question

The function remove-duplicates in lisp only works for simple lists like (a b c d).
My question is that how I can remove duplicate elements from a list with nested lists inside it. So for example

I have a list with nested lists as follows: ( (0 1) (0 2) (0 1) (0 3) )

and I want to remove one of the (0 1) lists which is repeated twice inside the list above to get:

( (0 1) (0 2) (0 3) )

Answer 1

Give :test #'equal to remove-duplicates, and it will do what you want.

As we see in CLHS: Satisfying a Two-Argument Test, :test defaults to #'eql when not given, and two lists like (0 1) are not eql unless they are the same list (eq).

CL-USER> (remove-duplicates '((0 1) (0 2) (0 1) (0 3)) :test #'equal)
((0 2) (0 1) (0 3))