Re: [PATCH] Re: where to put useful lists in the docs?

[Nicolas Sceaux]

Le 30 mai 09 à 10:20, Mark Polesky a écrit :

> > Here's another useful set of procedures that is not in
> > lily-library.scm. These functions make quick work of calculating
> > offsets, extents, bounding-boxes, etc.
>
> Here's a new patch that also incorporates conditionals.
>
> ;; defines 14 functions:
> ;; pair_+    pair_-     pair_*     pair_/
> ;; pair_=    pair_<     pair_>     pair_<=    pair_>=
> ;; pair_gcd  pair_lcm   pair_min   pair_max   pair_average.
> ;; eg. (pair_+ '(1 . 1) '(2 . 3) '(5 . 8)) ==> (8 . 12)
> ;;     (pair_<= '(1 . 8) '(1 . 5) '(2 . 3)) ==> (#t . #f)
> ;;     (pair_min '(1 . 8) '(1 . 5) '(2 . 3)) ==> (1 . 3)
> (map primitive-eval
> (map
>  (lambda (f)
>   `(define-public
>      (,(symbol-append 'pair_ f) . pairs)
>      (apply (lambda (f . pairs)
>               ((lambda (lst) (cons (car lst) (cadr lst)))
>                (primitive-eval
>                 (append `(map ,f)
>                          (map (lambda (x) `(list ,(car x) ,(cdr x)))
>                               pairs)))))
>             ,f pairs)))
> '(+ - * / = < > <= >= gcd lcm min max average)))

This is not the way to do it in guile scheme. When you find yourself
using primitive-eval, ask yourself if there is not a better way.
To write code that writes codes, use macros: read about defmacro.
Also note that you usually don't use underscores in function names.

But in that particular case, why not define a function that takes
a function and a list of pairs? It's usual to have procedure arguments:

(define (map-on-pairs fn pairs)
  (cons (apply fn (map car pairs))
        (apply fn (map cdr pairs))))

(map-on-pairs + '((1 . 1) (2 . 3) (5 . 8))) => (8 . 12)
(map-on-pairs min '((1 . 8) (1 . 5) (2 . 3))) ==> (1 . 3)

nicolas