[mercury-users] Exception handling documentation.
pbone at csse.unimelb.edu.au
Thu Jul 17 15:20:03 AEST 2008
On Thu, Jul 17, 2008 at 02:30:30PM +1000, Julien Fischer wrote:
> On Thu, 17 Jul 2008, Paul Bone wrote:
> >io.close_output throws an exception if bad stuff happens. I don't see
> >any information in the language reference guide about exceptions, this
> >is probably a bug.
> exceptions are a library feature so they are documented in the library
> reference guide. (The reference manual of course documents the
> determinism erroneous).
Ah, Found it :-).
> >Alot of the io package returns errors as part of their return value.
> >Should this be consitant and always return a type that can be used to
> >check for errors?
> It is fairly consistent isn't it. Those that throw an exception
> put the error inside of the exception.
Some throw exceptions and others return values. This is the bit that's
When I use try_io to try a closure and catch exceptions it requires that
it's parent be cc_multi or multi, and that that predicate's parent is
cc_multi or multi. I thought that the idea of cc_multi was that it
behaved like det but semantically there could be more solutions that
would _never_ be found.
/* ### In `write2'(in, in, in, out, di, uo): */
/* ### error: determinism declaration not satisfied. */
/* ### Declared `det', inferred `multi'. */
:- pred write2(output_stream::in, string::in, feedback_state::in,
feedback_write_result::out, io::di, io::uo) is det.
write2(Stream, ProgName, Feedback, Res, !IO) :-
/* ### call to `exception.try_io'((pred(out, di, uo) is det), */
/* ### out((exception.cannot_fail)), di, uo) can succeed more than */
/* ### once. */
/* ### Error: call to predicate `exception.try_io'/4 with determinism */
/* ### `cc_multi' occurs in a context which requires all solutions. */
try_io(write3(Stream, ProgName, Feedback), ExcpRes, !IO),
ExcpRes = succeeded(_),
Res = ok
ExcpRes = exception(ExcpUniv),
% If the exception is not a type we exected then re-throw it.
Res = write_error(Excp)
Mercury Compiler, version rotd-2008-07-07, configured for i686-pc-linux-gnu
Copyright (C) 1993-2008 The University of Melbourne
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