bug#26058: utf16->string and utf32->string don't conform to R6RS

[taylanbayirli@gmail.com (Taylan Ulrich Bayırlı/Kammer)]

Andy Wingo <wingo@pobox.com> writes:

> Adopting the behavior is more or less fine.  If it can be done while
> relying on the existing behavior, that is better than something ad-hoc
> in a module.

You mean somehow leveraging the existing BOM handling code of Guile
(found in the ports code) would be preferable to reimplementing it like
in this patch, correct?

In that light, I had this attempt:

(define r6rs-utf16->string
  (case-lambda
    ((bv default-endianness)
     (let* ((binary-port (open-bytevector-input-port bv))
            (transcoder (make-transcoder (utf-16-codec)))
            (utf16-port (transcoded-port binary-port transcoder)))
       ;; XXX how to set default-endianness for a port?
       (get-string-all utf16-port)))
    ((bv endianness endianness-mandatory?)
     (if endianness-mandatory?
         (utf16->string bv endianness)
         (r6rs-utf16->string bv endianness)))))

As commented in the first branch of the case-lambda, this does not yet
make use of the 'default-endianness' parameter to tell the port
transcoder (or whoever) what to do in case no BOM is found in the
stream.

From what I can tell, Guile is currently hardcoded to *transparently*
default to big-endian in ports.c, port_clear_stream_start_for_bom_read.

Is there a way to detect when Guile was unable to find a BOM?  (In that
case one could set the endianness explicitly to the desired default.)

Or do you see another way to implement this?

Thanks for the feedback!
Taylan


P.S.: Huge congrats on the big release. :-)