Formal comment #204 (defect)

freshness and mutability of quasiquoted structure should be specified
Reported by: 	Kent Dybvig
Version: 	5.92

pages: 55-56
Description:

According to the description of quasiquote, when no unquote or
unquote-splicing forms appear in a <qq template>, (quasiquote <qq
template>) is equivalent to (quote <qq template>). In this case, the
value is a literal. Thus, it is not freshly allocated and is not
mutable.

In all other cases, the mutability is unspecified, so the programmer
has no way of knowing precisely what is freshly allocated and can be
modified and what is literal and cannot be modified.

For example, (let ((a 3)) `((1 2) ,a ,4 ,'five 6)) might be equivalent
to

  (let ((a 3)) (cons '(1 2) (cons a (cons 4 (cons 'five '(6))))))
  (let ((a 3)) (list (list 1 2) a 4 'five 6))
  '((1 2) 3 4 five 6)

among other possibilities.

I would like some guidance both for programmers and implementors.

Proposal:

Specify either:

(A) quasiquote returns fresh, mutable objects for all list and vector
    structure that must be constructed under an assumption that the
    quantities within unquote and unquote-splicing forms are evaluated
    at run time, even if the values can be determined prior to run
    time. Portions that do not need to be rebuilt according to this
    provision are literal. Thus,

(let ((a 3)) `((1 2) ,a ,4 ,'five 6))

is equivalent to

(let ((a 3)) (cons '(1 2) (cons a (cons 4 (cons 'five '(6))))))

(B) quasiquote may return either fresh, mutable objects or literal
structure for any list and vector structure that must be constructed
under an assumption that the quantities within unquote and
unquote-splicing forms are evaluated at run time. Portions that do not
need to be rebuilt according to this provision are literal. This
permits

(let ((a 3)) `((1 2) ,a ,4 ,'five 6))

to be equivalent to a spectrum of expressions with

(let ((a 3)) (cons '(1 2) (cons a (cons 4 (cons 'five '(6))))))

at one end and

'((1 2) 3 4 five 6)

at the other. It disallows

(let ((a 3)) (list (list 1 2) a 4 'five 6))

because (1 2) and (6) must be literals.

I'm sure that the editors can refine the wording.


RESPONSE:

Proposal B will be adopted.