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Re: %default-port-conversion-strategy and string ports

From: David Kastrup
Subject: Re: %default-port-conversion-strategy and string ports
Date: Sat, 02 Jun 2012 16:55:12 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.1.50 (gnu/linux)

address@hidden (Ludovic Courtès) writes:

> Mark H Weaver <address@hidden> skribis:
>> address@hidden (Ludovic Courtès) writes:
>>> Ports in Guile can be used to write characters, or bytes, or both.  In
>>> particular, every port (including string ports, void ports, etc.) has an
>>> “encoding”, which is actually only used for textual I/O.
>>> Conversely, an R6RS port is either textual or binary, but not both.
>>> IMO, one advantage of mixed text/binary ports is to allow things like this:
>>>   scheme@(guile-user)> (define (string->utf16 s)
>>>                          (let ((p (with-fluids ((%default-port-encoding 
>>> "UTF-16BE"))
>>>                                     (open-input-string s))))
>>>                            (get-bytevector-all p)))
>>>   scheme@(guile-user)> (string->utf16 "hello")
>>>   $4 = #vu8(0 104 0 101 0 108 0 108 0 111)
>>>   scheme@(guile-user)> (use-modules(rnrs bytevectors))
>>>   scheme@(guile-user)> (utf16->string $4)
>>>   $5 = "hello"
>> IMHO, this is a bad hack that exposes internal details of our
>> implementation of string ports
> Which details?
> It exposes the fact that ports in general are mixed textual/binary, but
> nothing specific to string ports AFAICS.

If I can't rely on

(string= (with-output-to-string (display x)) x)

then the interface is seriously rotten.  There is lots of code around
that depends on the ability to bounce material between strings and the
default output port.

> We’d have to dig r6rs-discuss, but my recollection is that there were
> arguments both in favor and against separate binary/textual ports.

The question binary/textual concerns ports connected to a file.  String
ports and Scheme ports should be _transparent_: input and output
identical.  They are used for connecting character streams within Scheme
and should not tamper with them.

David Kastrup

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