Call by value but the value is two pointers

With push! from miscmacros you can cons onto that particular name whereas with mutate-cons! defined like this:

(define (mutate-cons! val lis)
  (set-cdr! lis (cons (car lis) (cdr lis)))
  (set-car! lis val))

You can change all instances of that particular list. Super dangerous.♥︎

Here, I’ll show you:

(let* ((abc '(a b c))
       (abc2 abc)
       (fish '(f i s h))
       (fish2 fish))
  (push! 'nope abc)
  (mutate-cons! 'yeah fish)
  (list abc abc2 fish fish2))

Returns this:

((nope a b c) (a b c) (yeah f i s h) (yeah f i s h))

That’s not a slag on push! which is often what you want. It’s a macro modifying what one particular name points to. Whereas mutate-cons! is a procedure that changes the actual underlying object.

Also mutate-cons! doesn’t work on the empty list. You can only push onto lists with at least one element. They need to be actual cons pairs.