[racket-dev] in-file-lines sequence

Thanks for the fix and suggestions.

On Fri, Jan 21, 2011 at 1:37 PM, Matthew Flatt <mflatt at cs.utah.edu> wrote:

> At Fri, 21 Jan 2011 10:53:42 -0700, Doug Williams wrote:
> > What is the best way to perform actions after a sequence terminates? Here
> is
> > a simple example that is like in-lines, but takes a path instead of a
> port.
> > It works, but I was wondering if there is a better way to do it.
> >
> > (define (in-file-lines path)
> >   (let ((port (open-input-file path #:mode 'text)))
> >     (make-do-sequence
> >      (lambda ()
> >        (values
> >         (lambda (_) (read-line port 'any))
> >         void
> >         (void)
> >         void
> >         (lambda _ (if (eof-object? (peek-byte port))
> >                       (begin
> >                         (close-input-port port)
> >                         #f)
> >                       #t))
> >         void)))))
>
> I think you mean
>
>  (define (in-file-lines path)
>   (let ((port (open-input-file path #:mode 'text)))
>     (make-do-sequence
>      (lambda ()
>        (values
>         (lambda (_) (read-line port 'any))
>         void
>         (void)
>         void
>         (lambda (v) (if (eof-object? v) ; <----------
>                         (begin
>                           (close-input-port port)
>                           #f)
>                         #t))
>         void)))))
>
> so that the last line is included in the sequence. Otherwise, I don't
> have any better suggestions.
>
> Beware that if the sequence stops being used before you get to the end
> of the file, then the port won't be closed:
>
>  (for ([line (in-file-lines <file>)]
>       [elem (in-list '(1 2 3))])
>    ...)
>  ; port isn't closed if it has more than 3 lines
>
> But the sequence protocol doesn't give a sequence any way to know that
> it isn't being used, other than GC-based finalization --- which is
> probably a bad idea for file ports, since you can run out of available
> file descriptors much faster than available memory.
>
>
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