[r6rs-discuss] [Formal] quasiquote freshness

[R. Kent Dybvig]

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name:      Kent Dybvig
email:     dyb at cs.indiana.edu
type:      defect
priority:  minor
component: base library
version:   5.92
pages:     55-56

Summary: freshness and mutability of quasiquoted structure should be specified

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.