[m-dev.] stream typeclasses (again)

[Ian MacLarty]

On Wed, Mar 08, 2006 at 03:03:52PM +1100, Julien Fischer wrote:
> 
> Are you reasonably satisified with the rest of the design as it stands?
> 

Not really.  I'm a bit dissapointed I couldn't implement the rot13
example the way I wanted, because of the current restrictions on
instance declarations.  It seems that making the state and the stream
arguments polymorphic is going to be more trouble than it's worth (at
least with the current type and mode systems).

What do you think about doing one of the following:

1. Make the state updated by a stream always be io.state.  The only
   example I could think of where it wouldn't be an io.state is with
   string buffers, but then you showed it's probably better to use the
   io.state anyway.

2. Do away with the Stream argument and just have a di, uo pair of polymorphic
   states.  Something like:

	:- typeclass stream.input(Unit, State) where [
   		pred get(Unit::out, State::di, State::uo) is det
	].

   File streams could be implemented by embedding the IO state, together
   with the file handle and name in the State type.

   We'd also then be able to define forwarding streams like my version
   of rot13 nicely:

	:- type encryption_stream_state(S)
		--->	encryption_stream_state(
				stream_state_to_forward_to	:: S,
				internal_encryption_state 	:: ...
			).

	:- instance stream.output(string, encryption_stream_state(S)).

  This instance declaration conforms to the current restrictions.

  With such a scheme we could also ensure that closed streams are never
  used, by clobbering the stream when it's closed.

  Do you think the mode system is up to the task?  I don't see why it
  wouldn't be, since it handles di, uo pairs for IO just fine.

Ian.

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