Filter IO through an external command

[Tristan Colgate]

Hi,

  I just got this working and thought people mind find it useful. It
passes IO through an external command (I'm using it to get svg output
from dot to include inline in html). I tried using
open-input-output-pipe but hit issues.

  It could probably handle errors nicely. Comments very welcome:

(use-modules (ice-9 rdelim))

(define (filter inproc outproc cmd . args)
  "filter the output of inproc via cmd to outproc through an external cmd"
  (let* ((read-pipe (pipe))
         (read-in  (car read-pipe))
         (read-out  (cdr read-pipe))
         (write-pipe (pipe))
         (write-in  (car write-pipe))
         (write-out  (cdr write-pipe))
         (child-pid    (primitive-fork)))
    (if (= child-pid 0)
        (begin
          (close-output-port read-out)
          (close-input-port write-in)
          (dup2 (fileno read-in) 0)
          (dup2 (fileno write-out) 1)
          (apply execlp (append (list cmd) (append (list cmd) args))))
        (begin
          (close-input-port read-in)
          (close-output-port write-out)
          (parameterize ((current-output-port read-out))
            (inproc))
          (close-output-port read-out)
          (parameterize ((current-input-port write-in))
            (outproc))
          (close-input-port write-in)
          (waitpid child-pid 0)))))

; To use
> (filter
    (lambda()
      (display "Hello World!")(newline))
    (lambda()
      (let loop ((line (read-line)))
        (if (not  (eof-object? line))
          (begin
            (write-line line)
            (loop (read-line))))))
    "sed" "s/World/Planet/")
Hello Planet!
$4 = (3892 . 0)
>

-- 
Tristan Colgate-McFarlane
----
  "You can get all your daily vitamins from 52 pints of guiness, and a
glass of milk"