The answer is that arc quasiquotation gets compiles down to scheme quasiquotation, and mzscheme's quasiquote handler understands (... unquote bar) to be equivalent to (... (unquote-splicing bar)). Since the arc compiler doesn't understand unquotes of this style, you'll only get the correct results when the expression unquoted is the same in arc and scheme - ie: it's either a literal, or it's a local variable. For example:
; this works because the local 'x in arc is compiled to 'x in scheme as well
arc> (let x 0 `(a . ,x))
(a . 0)
; this works because '+ in scheme adds numbers just as in arc
arc> (let x 0 `(a . ,(+ x 1)))
(a . 1)
; the following examples do not work
arc> (let x '(b) `(a . ,(join x '(c))))
Error: "reference to undefined identifier: join"
arc> (let x '(b) `(a . ,(+ x '(c))))
Error: "+: expects type <number> as 1st argument, given: (b . nil); other arguments were: (c)"
arc> (= x '(b))
(b)
arc> `(a . ,x)
Error: "reference to undefined identifier: x"
It's funny how a surface bug turns out to lead to something deeper in this way. I feel like Alice down the rabbit hole.
