[mercury-users] with_stream

[Michael Day]

> Regarding the with_stream predicate proposed by Fergus in the recent
> stream library debate, it seems that it would unnecessarily complicate
> interactive applications that do not know ahead of time how long they will
> hold a stream open.

Alternatively, if a stream is considered to be a handle to some piece of
state rather than state itself, that would seem to permit the creation of
a change_stream predicate called from within with_stream that closes the
current stream and opens a new one.

with_stream("some filename", predicate, io__states)

predicate(S0, S) :-
	do_stuff(S0, S1),
	( if some_condition_holds then
		change_stream("some other filename", S1, S2),
	else
		S2 = S1
	),
	do_stuff(S2, S3),
	predicate(S3, S).

Presumably change_stream would need to open the new stream *before*
closing the current stream, in case the new stream doesn't open. This
might complicate the implementation of with_stream, though I'm not sure
how much yet. But it or something like it seems necessary if with_stream
is not to drive people crazy.

Any thoughts?

Michael

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