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Re: C++ STL

From: Eric E Moore
Subject: Re: C++ STL
Date: Mon, 24 Jun 2002 22:22:01 +0100
User-agent: Gnus/5.090004 (Oort Gnus v0.04) Emacs/21.2

"Maurício" <address@hidden> writes:

>     Yes, I think you are right. However, I think there are some ideas that
> could be interesting to use. For instance, it would be nice if we could do
> things like this:
> (for_each_c++ (i (iota 1 10)) (line (file_lines "file_name.txt)) (in
> stdinput)
>     (some_function i line in))
> meaning that some_function would be executed with i going from 1 to 9, line
> would be consecutive lines from a file and in would be lines got from user
> input (and the "loop" would finish when i or line became #f). In this
> example, for_each_c++ itself could be a generator that would return the
> consecutive values of some_function, and so it could be used inside another
> for_each_c++ (or other iterator-aware algorithm).

Oh.  Well, we have streams, and stream-for-each.

The following code is untested (so probably doesn't work :), but
should be close enough to what would that it makes the point we can do

(use-modules (ice-9 streams))

(define (counting-stream n)
  (lambda (x)
    (if (>= x n) #f (cons (1+ x) (1+ x))))

(stream-for-each some-function
                 (make-stream (countto 10) 1)
                 (port->stream (open-input-file "file_name.txt") read-line)
                 (port->stream (current-input-port) read-line))

Also, read-line seems to not be in CVS guile (well, the oldish one I
have installed), but it's not too long to write (if one is unfussy
about it):

; Why do we no longer have read-line?

(define (read-line port)
  (let f ((chars ()))
    (let ((c (read-char port)))
       ((equal? c #\newline)
        (list->string (reverse chars)))
       ((eof-object? c) c)
        (f (cons c chars)))))))

Eric E. Moore

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