Debugging in SWI-prolog - unbound variables

Consider the following Prolog code. It edits lines of a particular type in its input and prints out the remaining lines w/o any change. It uses a DCG called rule which isn't included below, since it's not important to the question.

go:-
    prompt(_, ''),
    processInput.

processInput:-
    read_line_to_codes(current_input, Codes),
    processInput(Codes).

processInput(Codes):-
    (Codes \= end_of_file
    ->
        (phrase(rule(Part1, Part2), Codes)
        ->
            format('~s - ~s\n', [ Part1, Part2 ])
        ;
            format('~s\n', [ Codes ])),
        processInput
    ;
        true).

:- go, halt.

This works fine. However, suppose I change processInput/1 to the following, it just says that Warning: /home/asfernan/tmp/tmp.pl:28: Goal (directive) failed: user: (go,halt).

processInput(Codes):-
    (Codes \= end_of_file
    ->
        (\+phrase(rule(Part1, Part2), Codes)
        ->
            format('~s\n', [ Codes ]))
        ;
            format('~s - ~s\n', [ Part1, Part2 ]),
        processInput
    ;
        true).

The if & else parts of the phrase(r开发者_开发问答ule(Part1, Part2), Codes) DCG match have been exchanged. This is obviously a newbie mistake, but the fact that go, halt failed isn't very helpful. What can I do to make the error message indicate that the failure was because Part1 & Part2 were not bound in the format('~s - ~s\n', [ Part1, Part2 ]) line? I was able to track down this error because the code is small, but I may not have been able to do so had the code been big.

---

In Prolog the following is not the same:

..., ( Cond -> Then ; Else ), ...

and

..., ( \+ Cond -> Else ; Then ), ...

In general, a goal \+ Cond will never instantiate its variables. So you have to stick to the original formulation.

In case you are interested to process entire files with DCGs, consider SWI's library(pio).