[m-dev.] streams proposal (part 2)

[Ralph Becket]

Ian MacLarty, Friday, 24 February 2006:
> On Tue, Feb 21, 2006 at 05:48:29PM +1100, Julien Fischer wrote:
> > :- typeclass stream.error(Error) where
> > [
> >     % Convert a stream error into a human-readable format.
> >     % e.g. for use in error messages.
> >     %
> >     func error_message(Error) = string
> > ].
> > 
> 
> I don't think this is a particularly useful way to do errors.  In my opinion
> it would be better to add an extra Error argument to the stream typeclass and
> make Error functionally dependent on Stream.  That way you can do more
> with a returned error than simply converting it to a string (since you'll
> know its ground type).

I agree: making the IO errors entirely opaque was probably a mistake.
It could be handy to know whether a file write failed because the stream
was closed or the device was full or whatever.

> >     % The root of the stream class hierarchy.
> >     %
> > :- typeclass stream(Stream, State)
> >     <= (Stream -> State) where [
> >     
> >     % A human readable name describing the stream.
> >     %
> >     pred name(Stream::in, stream.name::out, State::di, State::uo) is det,
> > 
> 
> I would prefer this to be
> 
>     func name(Stream) = string,
> 
> but for the moment it has to have the State, because io.input_stream needs to
> read the I/O state to access the stream info database.

That's odd: why wouldn't a stream carry its name?

> > %
> > % Putback streams
> > %
> > 
> > :- typeclass stream.putback(Stream, Unit, State)
> >         <= stream.input(Stream, Unit, State) where [
> >     
> >     pred unget(Stream::in, Unit::in, State::di, State::uo) is det
> > ]. 
> > 
> 
> How would unget work?  What happens if I get an 'a', but then unget a 'b'?

I believe (I may be wrong) that in the C IO library you can do just that!

> > %
> > % Seekable streams
> > %
> > 
> > :- type stream.whence
> >     --->    set
> >     ;       cur
> >     ;       end.
> 
> Should the set function symbol have an argument?

No, these are relative positions.  They are always given with a separate
argument (e.g., "100 bytes from the start/current point/end of the
stream").

Well, you could store it in a global univ mutable, but 

-- Ralph
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