I am trying to produce the list (1 unquote 2) using quasiquote. I have tried this:
`(1 unquote 2)
However, in Racket, MIT Scheme and Chez Scheme, I get a dotted list: '(1 . 2).
So I tried this:
`(1 'unquote 2)
However, I get (1 'unquote 2).
I finally got the list I wanted using this technique:
`(1 unquote 2 ,((lambda (x) x) 'unquote) 2) ; Returns: '(1 unquote 2)
Why do I get a dotted list out of a quasiquoted proper list when unquote is the second to last element in the quasiquoted list?
Actually, it does not always produce a dotted list. For example:
`(1 unquote (list 2 3 4)) ; Returns: '(1 2 3 4)
Please explain this strange behavior of unquote when it is the second to last element in a quasiquoted list.
(a b c) is a shorthand for (a . (b . (c . ()))).
So (quasiquote (1 unquote 2)) is really (quasiquote (1 . (unquote 2))) which is '(1 . 2).
(Or if you want to fully expand it, (quasiquote (1 unquote 2)) is (quasiquote . ((1 . (unquote . (2 . ()))) . ())))
Similarly, (quasiquote (1 unquote (list 2 3 4))) is really (quasiquote (1 . (unquote (list 2 3 4)))) = (quasiquote (1 . (2 3 4))) = '(1 2 3 4).
By the way, an easier way to produce '(1 unquote 2) using quasiquote is:
`(1 ,'unquote 2)