[r6rs-discuss] [Formal] modified interface to list-sort and vector-sort

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This message is a formal comment which was submitted to formal-comment_at_r6rs.org, following the requirements described at: http://www.r6rs.org/process.html
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submitter's name: Daniel Villeneuve
submitter's email address: daniel_villeneuve_at_sympatico.ca
type of issue: enhancement
priority: minor
R6RS component: Sorting
version of the report: 5.92
SUMMARY
Implementations could provide better sorting performance
with a slightly different sorting interface.
DESCRIPTION
This comment is in two related parts.
PART A: prototype for comparison functions
The comparison function provided to list-sort and
vector-sort should return a 3-way result when comparing two
objects a and b:
  - negative integer: a < b
  - 0: a = b
  - positive integer: a > b
This avoids comparing items twice, which can be costly in
some cases (e.g., comparing vectors of numbers).
There is evidence from other languages (C, Java) that the 3-way interface
has advantages, which I presume involve performance as a criterion.
As a useful addition, equality could be returned as #f.
This would enable easy combination of comparison functions,
as in:
(lambda (a b)
  (or (cmp-increasing-number (get-field-1 a) (get-field-1 b))
      (cmp-increasing-number (get-field-2 a) (get-field-2 b))))
Standard comparison functions behaving as described above could be
provided by the implementation:
  - cmp-increasing-number
  - cmp-decreasing-number
  - cmp-increasing-string
  - cmp-decreasing-string
...
The comparison functions on numbers could raise an exception if two numbers
are non-comparable (e.g., NaN, complex numbers).
PART B: prototype of the sorting functions
Sorting can be made more efficient by providing a getter (projection)
function separate from the comparison function:
Both
(list-sort (lambda (a b) (cmp-increasing-number
                           (get-field-1 a)
                           (get-field-1 b)))
           x)
and
(list-sort get-field-1 cmp-increasing-number x)
would behave the same, except that the sorting function could take
advantage of the separation between the getter and the comparator.
PROPOSAL
a) Change the prototype of the comparison functions for list-sort and
   vector-sort so that they are 3-way
b) Optionally allow 0 to be returned as #f.
c) Add 3-arg forms to list-sort and vector-sort, with the first argument
   being a separate getter procedure used to extract the keys passed
   to the comparison function.
--
Daniel Villeneuve