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

[Peter J. <pe...@jo...>]

> 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.