bug#22901: drain-input doesn't decode

[Taylan Kammer]

Closing this since it's 5 years old and fixed in Guile 2.1 and higher.

-- 
Taylan

bug#22901: drain-input doesn't decode

[Taylan Kammer]

Are we still maintaining 2.0, or can this issue be closed?

-- 
Taylan

bug#22901: drain-input doesn't decode

[Matt Wette]

> On Feb 26, 2017, at 9:46 AM, Matt Wette  wrote:
> 
> I put together a test and tried on 2.1.7 - my test fails.  See attached.
> 
>  (pass-if "encoded input"
>(let ((fn (test-file))
> (nc "utf-8")
> (st "\u03b2\u03b1\u03b4 \u03b1\u03c3\u03c3 am I.")
> ;;(st "hello, world\n")
> )
>  (let ((p1 (open-output-file fn #:encoding nc)))
>   ;;(display st p1)
>   (string-for-each (lambda (ch) (write-char ch p1)) st)
>   (close p1))
>  (let* ((p0 (open-input-file fn #:encoding nc))
>(s0 (begin (unread-char (read-char p0) p0) (drain-input p0
>   (simple-format #t "~S\n" s0)
>   (equal? s0 st
> 

My bad.  The failure was on guile-2.0.13.  It seems to work on guile-2.1.7:

mwette$ guile-2.1.7-dev3/meta/guile port-di.test
"βαδ ασσ am I."
PASS: drain-input: encoded input

bug#22901: drain-input doesn't decode

[Matt Wette]

I put together a test and tried on 2.1.7 - my test fails.  See attached.

  (pass-if "encoded input"
(let ((fn (test-file))
  (nc "utf-8")
  (st "\u03b2\u03b1\u03b4 \u03b1\u03c3\u03c3 am I.")
  ;;(st "hello, world\n")
  )
  (let ((p1 (open-output-file fn #:encoding nc)))
;;(display st p1)
(string-for-each (lambda (ch) (write-char ch p1)) st)
(close p1))
  (let* ((p0 (open-input-file fn #:encoding nc))
 (s0 (begin (unread-char (read-char p0) p0) (drain-input p0
(simple-format #t "~S\n" s0)
(equal? s0 st



port-di.test
Description: Binary data

bug#22901: drain-input doesn't decode

[Andy Wingo]

On Fri 04 Mar 2016 04:09, Zefram  writes:

> The documentation for drain-input says that it returns a string of
> characters, implying that the result is equivalent to what you'd get
> from calling read-char some number of times.  In fact it differs in a
> significant respect: whereas read-char decodes input octets according to
> the port's selected encoding, drain-input ignores the selected encoding
> and always decodes according to ISO-8859-1 (thus preserving the octet
> values in character form).
>
> $ echo -n $'\1a\2b\3c' | guile-2.0 -c '(set-port-encoding!
> (current-input-port) "UCS-2BE") (write (port-encoding
> (current-input-port))) (newline) (write (map char->integer (let r ((l
> '\''())) (let ((c (read-char (current-input-port (if (eof-object?
> c) (reverse l) (r (cons c l))) (newline)'
> "UCS-2BE"
> (353 610 867)
> $ echo -n $'\1a\2b\3c' | guile-2.0 -c '(set-port-encoding!
> (current-input-port) "UCS-2BE") (write (port-encoding
> (current-input-port))) (newline) (peek-char (current-input-port))
> (write (map char->integer (string->list (drain-input
> (current-input-port) (newline)'
> "UCS-2BE"
> (1 97 2 98 3 99)

Thanks for the test case!  FWIW, this is fixed in Guile 2.1.3.  I am not
sure what we should do about Guile 2.0.  I guess we should make it do
the documented thing though!

Andy

bug#22901: drain-input doesn't decode

[Zefram]

The documentation for drain-input says that it returns a string of
characters, implying that the result is equivalent to what you'd get
from calling read-char some number of times.  In fact it differs in a
significant respect: whereas read-char decodes input octets according to
the port's selected encoding, drain-input ignores the selected encoding
and always decodes according to ISO-8859-1 (thus preserving the octet
values in character form).

$ echo -n $'\1a\2b\3c' | guile-2.0 -c '(set-port-encoding! (current-input-port) 
"UCS-2BE") (write (port-encoding (current-input-port))) (newline) (write (map 
char->integer (let r ((l '\''())) (let ((c (read-char (current-input-port 
(if (eof-object? c) (reverse l) (r (cons c l))) (newline)'
"UCS-2BE"
(353 610 867)
$ echo -n $'\1a\2b\3c' | guile-2.0 -c '(set-port-encoding! (current-input-port) 
"UCS-2BE") (write (port-encoding (current-input-port))) (newline) (peek-char 
(current-input-port)) (write (map char->integer (string->list (drain-input 
(current-input-port) (newline)'
"UCS-2BE"
(1 97 2 98 3 99)

The practical upshot is that the input returned by drain-input can't
be used in the same way as regular input from read-char.  It can still
be used if the code doing the reading is totally aware of the encoding,
so that it can perform the decoding manually, but this seems a failure
of abstraction.  The value returned by drain-input ought to be coherent
with the abstraction level at which it is specified.

I can see that there is a reason for drain-input to avoid performing
decoding: the problem that occurs if the buffer ends in the middle
of a character.  If drain-input is to return decoded characters then
presumably in this case it would have to read further octets beyond the
buffer contents, in an unbuffered manner, until it reaches a character
boundary.  If this is too unpalatable, perhaps drain-input should be
permitted only on ports configured for single-octet character encodings.

If, on the other hand, it is decided to endorse the current non-decoding
behaviour, then the break of abstraction needs to be documented.

-zefram