[m-users.] Java exceptions and nested classes

[Mark Clements]

I am investigating using JDBC with Mercury. Most of the JDBC classes throw an SQLException, which I want to catch in Java and return a result type with either an ok() value or an error(). I have the following minimal example (not using JDBC):

:- module mve.
:- interface.
:- import_module io, int.
:- type result(T,E) ---> ok(T) ; error(E).
:- type exception.
:- pred some_pred(int::in, mve.result(int,exception)::out, io::di, io::uo) is det.
:- pred main(io::di, io::uo) is det.
:- implementation.
:- pragma foreign_type("Java", exception, "java.lang.Exception").
:- pragma foreign_proc("Java",
    some_pred(A::in, C::out, _IO0::di, _IO1::uo),
    [will_not_call_mercury, thread_safe, promise_pure],
    "try {
         C = new Result_2.Ok_1<Integer,java.lang.Exception>(A);
    } catch(java.lang.Exception e) {
         System.err.println(e.getMessage());
         C = new Result_2.Error_1<Integer,java.lang.Exception>(e);
    }").
main(!IO) :-
    some_pred(1, C, !IO),
    (C = ok(_) ; C = error(_)).

However, when I compile this I get warnings, such as:

mve.m:15: warning: [unchecked] unchecked conversion
         C = new Result_2.Ok_1<Integer,java.lang.Exception>(A);
             ^
  required: Result_2<Integer,Exception>
  found:    Ok_1<Integer,Exception>

From the generated code, I see:

  public static class Result_2<MR_tvar_1, MR_tvar_2>
    implements jmercury.runtime.MercuryType
  {
    public int data_tag;
    public static class Error_1<MR_tvar_1, MR_tvar_2>
      extends mve.Result_2
    {
<snipped>

Should the definition for Error_1 be:

    public static class Error_1<MR_tvar_1, MR_tvar_2>
      extends mve.Result_2<MR_tvar_1, MR_tvar_2>
    {
<snipped>

I note that the mlds_class_inherits type does not include type_param, so that output_generic_tvars() cannot be called for the inherited class.

Alternatively, is there a better way to catch the Java exceptions?

--- Mark.


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