[mercury-users] Exception handling in Mercury.

[Peter Ross]

On 6/28/07, Bartlomiej Szymczak <rhywek at gmail.com> wrote:
> Hi, all.
>
> My mercury code uses the hash_table module. I've read the library
> reference for it, and it says:
>
>          % Lookup the value associated with the given key. An
> exception is raised
>          % if there is no entry for the key.
>          %
>      :- func lookup(hash_table(K, V), K) = V.
>      :- mode lookup(hash_table_ui, in) = out is det.
>
> Guess what - indeed the exception was raised when I was testing my
> program. The program simply terminated. I've got excited, as I didn't
> know there were exceptions in Mercury.
>
> I was trying to find a way of handling this exception, but I couldn't
> find anything like "handle" in SML or "try"+"catch" in C++/Java.
>
> Is there any way of handling this exception?
>
Yes see the exception module in the standard library.

You use the predicate try or try_io which take a higher-order argument
which is the predicate you wish to evaluate and catch any exceptions
thrown by.

eg

X = 7,
exception.try(
    (pred({ResultA, ResultB}::out) is det :-
        ( X > 10 ->
            ResultA = X * 2,
            ResultB = "some_string"
        ;
             throw(X)
        )
    ), TryResult),
(
    TryResult = succeeded({A, B}),
    .... do something ...
;
    TryResult = exception(E),
    % Handle all exceptions which are integers otherwise rethrow the exception
    ( dynamic_cast(E, Integer : int) ->
        .. do something with the Integer ....
    ;
        exception.throw(E)
    )


> But even more useful thing for me would be raising exceptions from my
> own code. Is there such a possibility?
>
Simply call exception.throw(T)
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