Consult parse errors are not excpetions?

Hi,

I originally noticed this when using the C API, but it can also be seen
by using the swipl interactive interpreter. It is probably easier to
explain using the interpreter.

Look at a normal exception, for example, we call a non-existing
predicate:

---8<---
?- blah(a), writeln(hi).
ERROR: toplevel: Undefined procedure: blah/1 (DWIM could not correct goal)
---8<---

The predicate blah/1 does not exist, and so an exception fires, thus
preventing  the writeln/1 predicate from executing. Fine.

Now consider a theory file with a syntax error in it -- call it a.pl.

---8<---
?- consult(a), writeln(hi).
ERROR: /tmp/a.pl:1:11: Syntax error: Unexpected end of file
% a compiled 0.00 sec, 1 clauses
hi
true.
---8<---

A syntax error occurred, but an exception was not thrown. We can see this
is the case since writeln/1 was executed.

How comes? Should a syntax error not be an exception? There may well be
a good reason for this, but I am curious.

As I said, this can be seen at the C level too. When I use PL_open_query()
followed by PL_next_solution() to initiate 'consult(a)', where a.pl has a
syntax error, I see the syntax error reported on the terminal, but
PL_exception() reports that everything is OK. I have no means by which to
detect the parse error.

Any thoughts on this behaviour?

Thanks

-- 
Best Regards
Edd Barrett

http://www.theunixzoo.co.uk