Author: grant

Scheme-OSA stands for Open Scripting Architecture. It provides an abstract interface for applications to compile, execute, and manipulate scripts without needing to know the details of the particular scripting language. Scheme-OSA bridge furnishes a Scheme user with a means of accessing the OS X multimedia and other resources from Apple and the third parties – introducing graphics, scientific plotting, dialog boxes, music, etc. into the world of Scheme language.

(via comp.lang.scheme)

In this post on the PLT discussion list I sleepily wondered how to do something to all elements of a list but the last.

The reasons I asked is because I had used ‘do’ loops in lieu of something better, and from what I can gather, you basically never write ‘do’ loops in Scheme (maybe once every 30 years you do use ‘do’).

I hastily posted a solution using the ‘match’ library as I had just read the documentation while cutting over from ‘mzlib/match’ to ‘match’. Both Robby and Ethan replied with much, much faster solutions. Have a look:

#lang scheme

(require (prefix-in srfi/1: srfi/1))

; Grant's
(define (foo1 args fun last-fun)
  (match args
    [(list arg args ..1)
     (fun arg)
     (foo1 args fun last-fun)]
    [(list arg)
     (last-fun arg)]))

(provide/contract
 [foo2 (-> (cons/c any/c (listof any/c))
           (-> any/c any)
           (-> any/c void?)
           void?)])

; Robby's
(define (foo2 args fun last-fun)
  (let loop ([fst (car args)]
             [rst (cdr args)])
    (cond
      [(empty? rst) (last-fun fst)]
      [else (fun fst) (loop (car rst) (cdr rst))])))

(define data (srfi/1:make-list 10000 "hi"))

; Ethan's
(define (foo3 lst fun funl)
  (begin
    (for-each fun (srfi/1:drop-right lst 1))
    (funl (srfi/1:take-right lst 1))))

(time
 (foo1 data
       (λ (x) (void))
       (λ (x) (void))))

(time
 (foo2 data
       (λ (x) (void))
       (λ (x) (void))))

(time
 (foo3 data
       (λ (x) (void))
       (λ (x) (void))))

> cpu time: 2593 real time: 2625 gc time: 718
> cpu time: 0 real time: 0 gc time: 0
> cpu time: 0 real time: 0 gc time: 0

In the end, I only needed this functionality in two places, and I probably would not have required it if I had designed things better in the first place.