Re: [Ocaml-lib-devel] Purpose of IO.nread

[Nicolas C. <war...@fr...>]

> > Actually IO does not work maybe well with buffered streams : the
> > specification is that when you're reading n "elements" (characters or
any)
> > from an input, the result will be exactly the n elements you needed. Not
> > more, and not less. This might actually be questionable : if you have
spare
> > time, could you suggest what parts of IO module should be modified in
order
> > to support buffered streams?
>
> I attach a patch that (IMO) improves things, but it does involve
> changing the interface somewhat - maybe that's still okay at this stage?
>
> It changes the semantics of nread to return anything up to the requested
> number items.  Unlike Pervasives.input, the only situation in which
> fewer are returned is if the end of the stream is reached first.  The
> return value is an (int, 'b) pair where the int is the number actually
> returned.  A new read_exactly function is added that behaves like nread
> did before, i.e. it calls nread and raises No_more_input if the returned
> item count does not match that requested.
>
> A side-effect of the implementation is that a failed call to
> read_exactly is guaranteed to consume the remainder of the stream.  This
> may or may not be desirable, but it was already the case for streams
> wrapping enums, so this merely defines a previously undefined behaviour.
>
> There's one problem, which is input_bits/output_bits, where the old
> behaviour makes more sense.  For now all I've done is changed the return
> value of nread for input_bits so it compiles.
>
> Of course, I won't be offended if you don't think it's worth making
> these changes.

I'm sorry but that's not exactly what I was thinking of.
I don't like so much the idea of having nread returning a pair.
For example if we have an (char,string) input , the int returned is already
stored into the String.length value. Same for a ('a, 'a list) input. I have
been looking a little and it looks ok to modify the behavior of IO without
actually modifying the interface. We just have to replace really_input by
input - followed by a String resize if the readed number of chars is lower
than the requested one. Same for the Enum's IOs : actually we're throwing an
exception if we're requesting more than available : we will now just return
the partial enum (not an empty one).
Please tell me if it's ok for you this way, I'll then make the changes.

Regards,
Nicolas Cannasse