sort needs a #:key argument

[Andy Wingo]

Hello,

Do you read guile-devel and are looking for a manageable thing to hack?
Well how about this. `sort!' needs a #:key argument.

I believe you will find more precise specifications in the Common Lisp
specifications, but the basic idea is that the comparison function of
`sort' operates not on elements of the list, but on the result of
applying a function to those elements.

Like this:

  (sort '(1 -2 3 -4) <)
     => (-4 -2 1 3)
  (sort '(1 -2 3 -4) < #:key (lambda (x) (* x x)))
     => (1 -2 3 -4)
  (sort '((a . 1) (b . -2) (c . 3)) < cdr)
     => ((b . -2) (a . 1) (c . 3))

I would do it by exposing scm_sort to Scheme as primitive-sort, and
defining a `sort' in scheme using (ice-9 optargs) and primitive-sort,
customizing the less? procedure if a #:key is given.

Then we'd need to update the documentation, and add a news entry. Any
takers? :)

Andy
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