[mercury-users] Using exceptions without cc_multi

[api at gmx.de]

Hello mercury users!

When using try and throw in mercury, I did not find a way to use a try
block within an ordinary clause, say, of a predicate with determinism
declaration semidet instead of cc_multi. So something like

:- module tst_except.

:- implementation.
:- import_module exception.

:- pred q(string::in,string::out) is semidet.

:- pred p(string,string).
:- mode p(in, out) is semidet.
p(A,B) :-
   try(q(A),  Result),
   (Result = succeeded(B)
    ;
    Result = exception(E),
    %% handle E
    fail
   ).

q("A", "B").
q("B", _) :-
    throw("no").


will not work because of the determinism declaration of try to be
cc_multi, but p being semidet. But if p is semidet for very good reasons
(very far down the call tree), then there should be any other way of
using exceptions. Unfortunately, I have not been able to find out how.

Does anyone know how to use exceptions properly?

Thanks

André

P.S. The compiler errors are
tst_except.m:011: Error: call to predicate `exception:try/2' with
determinism `cc_multi'
tst_except.m:011:   occurs in a context which requires all solutions.
tst_except.m:009: In `p(in, out)':
tst_except.m:009:   error: determinism declaration not satisfied.
tst_except.m:009:   Declared `semidet', inferred `nondet'.
tst_except.m:011:   call to `try((pred(out) is semidet), out)' can
succeed more than once.
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