[m-dev.] for discussion: stream library v2

[Fergus Henderson]

On 25-Sep-2000, Peter Ross <peter.ross at miscrit.be> wrote:
> On Mon, Sep 25, 2000 at 10:49:03AM +1100, Fergus Henderson wrote:
> > On 24-Sep-2000, Peter Ross <petdr at miscrit.be> wrote:
> > > Please find attached my second attempt at a stream library.  This time
> > > I have tried to define a typeclass for an impure lowlevel interface
> > > for streams, and then defined a pure interface on top of that.
> > 
> > I think it's a good idea to build streams using a lowlevel interface
> > for defining streams with a highlevel interface for using streams
> > built on top of that, but it would be nicer if the lowlevel interface
> > didn't need to be impure.
>
> I think that the low level interface has to be impure

Why?

I think a user should be able to define their own stream types without
having to use any impure code or interfaces.

> > Here you're assuming that if the stream is at eof, then the
> > previous operation succeeded.  This assumption is not documented
> > in the lowlevel interface, and I don't think it is likely to hold in
> > general.  Making this assumption may cause some errors to be ignored.
> > 
> You are correct that the assumption needs to be documented.
> 
> I arrived at this interface because fgetc return EOF on error or end of
> file.  So first I check to see whether I am at EOF and if not it must be
> an error.

If fgetc() returns EOF, there are three possibilities:

	(1) end of file
	(2) error
	(3) both of the above.  This can happen, for example, if the file
	    was at eof, and then fseek() was called, but it failed --
	    the failure should be reported, but the file pointer remains
	    at eof.

In case (3), you should return the error, rather than ignoring it and
returning `eof'.

In other words, this code is good

	int c = fgetc(f);
	if (c == EOF) {
		if (ferror(f)) {
			// handle error
		} else {
			// handle EOF
		}
	}

but the alternative below is bad,

	int c = fgetc(f);
	if (c == EOF) {
		if (feof(f)) {
			// handle EOF
		} else {
			// handle error
		}
	}

because it ignores errors in case (3).
Your code is using the bad alternative.

> So I guess the next best step would be for me to submit the stream
> implementation for review with an implementation of stdin/out stream,
> read file stream and write file stream.

OK, that sounds good.

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
WWW: <http://www.cs.mu.oz.au/~fjh>  |  of excellence is a lethal habit"
PGP: finger fjh at 128.250.37.3        |     -- the last words of T. S. Garp.
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