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Add exception smoke tests, generalize PlUnknownError #104

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Add exception smoke tests, generalize PlUnknownError #104

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10 changes: 7 additions & 3 deletions SWI-cpp2.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -267,11 +267,15 @@ PlResourceError(const std::string& resource)
_SWI_CPP2_CPP_inline
PlException
PlUnknownError(const std::string& description)
{ // For PlWrap()
return PlGeneralError(PlCompound("unknown_error",
PlTermv(PlTerm_atom(description))));
{ return PlUnknownError(PlTerm_atom(description));
}

_SWI_CPP2_CPP_inline
PlException
PlUnknownError(PlTerm description)
{ return PlGeneralError(PlCompound("unknown_error",
PlTermv(description)));
}



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2 changes: 2 additions & 0 deletions SWI-cpp2.h
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Expand Up @@ -1306,6 +1306,8 @@ PlException PlResourceError(const std::string& resource);

PlException PlUnknownError(const std::string& description);

PlException PlUnknownError(PlTerm description);

void PlWrap_fail(qid_t qid);

template<typename C_t> C_t
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8 changes: 8 additions & 0 deletions test_cpp.cpp
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Expand Up @@ -114,6 +114,14 @@ PREDICATE(hello, 2)
return A2.unify_string(buffer.str());
}

PREDICATE(helloex, 0)
{ throw PlUnknownError("an exception");
}

PREDICATE(helloex, 1)
{ throw PlUnknownError(A1);
}

PREDICATE(hello2, 2)
{ PlAtom atom_a1(A1.as_atom());
std::stringstream buffer;
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5 changes: 5 additions & 0 deletions test_cpp.pl
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Expand Up @@ -92,6 +92,11 @@
% hello :- write('hello hello hello')
with_output_to(string(Out), hello).

test(helloex, error(unknown_error('an exception'))) :-
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@JanWielemaker JanWielemaker Jan 7, 2026

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Errors should always be of the shape error(Formal, Context). That is used everywhere in the system to distinguish error exceptions from other exceptions (like timeout, abort, etc.) Typically you leave the 2nd argument unbound in SWI-Prolog. The exception handler adds the context, either as a single predicate or as a stack trace.

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@kamahen kamahen Jan 7, 2026

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There are a few other places where errors are of the wrong shape -- I'll review all the SWI-cpp2.* code and add another commit. This might break backwards compatibility, but I hope not.

If I can, I'll also make PlException's constructor inaccessible, to avoid making this mistake again.

helloex.
test(helloex, error(unknown_error(foo(bar)))) :-
helloex(foo(bar)).

test(hello, Out == "Hello WORLD\nHello WORLD\nHello WORLD\nHello WORLD\nHello WORLD\n") :-
hello("WORLD", Out).
test(hello, error(representation_error(encoding))) :-
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