[m-dev.] for discussion: stream library v2

[Fergus Henderson]

On 25-Sep-2000, Michael Day <mcda at students.cs.mu.oz.au> wrote:
> 
> >     * you have a non-unique handle on a stream which means that you can
> >       still process that stream after an exception.
> 
> Is it really not possible to achieve this by some extension of try_io:
> 
> :- pred try_io(pred(T, S, S), exception_result(T), S, S) <= stream(S).

Oh, now I remember what the problem with that was.
The problem is that when the exception is thrown, the
value currently in the store may be completely invalid --
there is no guarantee in general that it even has the right type.

Consider the following code:

	:- type foo ---> foo(bool, bool).
	:- type bar ---> bar(int, int).

	:- instance stream(foo) where [...].

	:- pred main(state::di, state::uo) is det.
	main -->
		{ Foo0 = foo(no, no) },
		{ try_io(update_foo_stream, R, Foo0, Foo) },
		print("R = "), print(R), nl,
		print("Foo = "), print(Foo), nl.

	:- pred update_foo_stream(foo::di, foo::uo) is det.
	update_foo_stream(Foo0, Foo) :-
		Foo0 = foo(Bool, _),
		Bar0 = bar(42, if Bool = yes then 10 else 20),
		Foo = f(Bar0).

	:- func f(bar::di) = (foo::uo) is det.
	f(_) = _ :- throw("f/2 not yet implemented").

Here the compiler's structure reuse optimization may decide to
reuse the foo/1 cell to hold the bar/1 value in update_foo_stream/2.
So when the exception is thrown, the value of the fields pointed to by
`Foo0' will not even have the correct values for `bool'.  Thus the
call `print(Foo)' may crash.

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
WWW: <http://www.cs.mu.oz.au/~fjh>  |  of excellence is a lethal habit"
PGP: finger fjh at 128.250.37.3        |     -- the last words of T. S. Garp.
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