[r6rs-discuss] [Formal] Implementation-dependent transcoders considered harmful

[John Cowan]

William D Clinger scripsit:

> I concur.  I would, however, warn the editors against
> the parallel overspecification that would come of
> requiring port-transcoder to return #f when passed
> a port created by open-string-{input,output}-port
> or make-custom-textual-{input,output,input/output}-port.
> The specification of port-transcoder needs to be changed
> in recognition of the fact that textual ports may or may
> not have transcoders.

+1

> > 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.

I misspoke on this one: it's only the with-*-from-file
and open-*-file procedures that are relevant here; naturally the
others would use the transcoder (if any) that belongs to the port
being accessed.

> I would recommend against that, because implementations
> may have esoteric knowledge that lets them pick different
> transcoders based on the file's type or content.  

An excellent point.

> That would be consistent with John's suggestion only if the
> native-transcoder were nonstandard (e.g. 'we-pick-em),

Which really won't work, because a transcoder is a tripartite
data structure (codec, eol-style, and error-handling).

> but requiring native-transcoder to return nonstandard
> values in implementations of unusually high quality
> seems like a mistake.

Quite so.

I withdraw this part of my comment, therefore, and instead propose
that the procedures with-{input,output}-from-port be added to the
port I/O library.  These accept a port and a thunk and execute the
thunk, ensuring that the port is closed afterwards, in the manner of
with-{input,output}-from-file.  With-*-from-file would then just be a
simple variant of these.  Writing a portable version of with-*-from-port
would not be easy, particularly in the presence of various threading
models, so it should be provided by the implementation.

> The simple procedures are supposed to be simple; if a
> programmer needs to supply an optional transcoder,
> I think the programmer should open the file using the
> io ports library instead.

With-*-from-port makes the combination straightforward: open 
with the port I/O library, but do transput with the simple I/O
library.

-- 
Andrew Watt on Microsoft:                       John Cowan
Never in the field of human computing           cowan at ccil.org
has so much been paid by so many                http://www.ccil.org/~cowan
to so few! (pace Winston Churchill)