stream interfaces - with ranges

Steven Schveighoffer schveiguy at
Thu May 17 07:02:09 PDT 2012

OK, so I had a couple partially written replies on the 'deprecating etc' thread, then I had to go home.

But I thought about this a lot last night, and some of the things Andrei
and others are saying is starting to make sense (I know!).  Now I've
scrapped those replies and am thinking about redesigning my i/o package
(most of the code can stay intact).

I'm a little undecided on some of the details, but here is what I think
makes sense:

1. We need a buffering input stream type.  This must have additional
methods besides the range primitives, because doing one-at-a-time byte
reads is not going to cut it.
2. I realized, buffering input stream of type T is actually an input range
of type T[].  Observe:

struct /*or class*/ buffer(T)
      T[] buf;
      InputStream input;
      @property T[] front() { return buf; }
      void popFront() {;} // flush existing buffer, read  
      @property bool empty() { return buf.length == 0;}

Roughly speaking, not all the details are handled, but this makes a
feasible input range that will perform quite nicely for things like
std.algorithm.copy.  I haven't checked, but copy should be able to handle
transferring a range of type T[] to an output range with element type T,
if it's not able to, it should be made to work.  I know at least, an
output stream with element type T supports putting T or T[].  What I think
really makes sense is to support:

buffer!ubyte b;
outputStream o;

o.put(b); // uses range primitives to put all the data to o, one element
(i.e. ubyte[]) of b at a time

3. An ultimate goal of the i/o streaming package should be to be able to
do this:

auto x = new XmlParser("<rootElement></rootElement>");

or at least

auto x = new XmlParser(buffered("<rootElement></rootElement>"));

So I think arrays need to be able to be treated as a buffering streams.  I
tried really hard to think of some way to make this work with my existing
system, but I don't think it will without unnecessary baggage, and losing
interoperability with existing range functions.

Where does this leave us?

1. I think we need, as Andrei says, an unbuffered streaming abstraction.
I think I have this down pretty solidly in my current
2. A definition of a buffering range, in terms of what additional
primitives the range should have.  The primitives should support buffered
input and buffered output (these are two separate algorithms), but
independently (possibly allowing switching for rw files).
3. An implementation of the above definition hooked to the unbuffered
stream abstraction, to be utilized in more specific ranges.  But by
itself, can be used as an input range or directly by code.
4. Specialization ranges for each type of input you want (i.e. byLine,
byChunk, textStream).
5. Full replacement option of File backend.  File will start out with
C-supported calls, but any "promotion" to using a more D-like range type
will result in switching to a D-based stream using the above mechanisms.
Of course, all existing code should compile that does not try to assume
the File always has a valid FILE *.

What do you all think?  I'm going to work out what the definition of 2
should be, based on what I've written and what makes sense.

Have I started to design something feasible or unworkable? :)


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