From: John Cowan
Date: Tue, 5 Jun 2007 18:09:38 -0400
Subject: [Formal] Implementation-dependent transcoders considered harmful

Submitter: John Cowan
Issue type: Defect
Priority: Major
Component:  Port I/O
Report version: 5.93
Summary: Implementation-defined transcoders considered harmful

There are three kinds of references to implementation-defined transcoders
in R5.93RS.  One is in the description of native-transcoder,
which is appropriate: the native transcoder is inherently an
implementation-specific notion.  The other two kinds are respectively
unnecessary and vague, and should be removed.

The unnecessary ones are those which arise from the false
assumption that a textual port necessarily has a transcoder.
In fact, a textual port should have a transcoder if and only if
it is derived (explicitly or implicitly) from a binary port of
some sort.  These transcoders don't do anything and should simply
be removed.  This involves changing port-transcoder to return #f for
textual ports without transcoders, and removing the references to
implementation-defined transcoders in open-string-{input,output}-port
and make-custom-textual-{input,output,input/output}-port.

The other kind of implementation-dependent transcoder applies to the
current {input,output,error} ports and to the simple I/O procedures
that depend on them.  These procedures are very convenient to use,
particularly with-{input,output}-from-file, but one simply has to
hope that the implementer has done the right thing with respect to
transcoders.  I suggest at least the minimal change of saying that the
procedures with-{input,output}-from file, open-{input,output}-file,
{read,peek,write}-char, read, write, newline and display be specified
to use the transcoder returned by native-transcoder.  However, I
would greatly prefer providing for each of them to accept an optional
final argument specifying an explicit transcoder.  Doing so allows the
simple procedures to be used with textual files using any reasonable
representation, a necessity in modern programming environments.


RESPONSE:

The report has been changed so that a textual port need not have a
transcoder.  Extending the "simple procedures" seemed too big a change,
given that the requested functionality is already available through
the regular ports library.