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DCG compilation: Unexpected warning when using unsupported grammar control construct #2675

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triska opened this issue Dec 3, 2024 · 1 comment · May be fixed by #2681
Open

DCG compilation: Unexpected warning when using unsupported grammar control construct #2675

triska opened this issue Dec 3, 2024 · 1 comment · May be fixed by #2681

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@triska
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triska commented Dec 3, 2024

With discontiguous_dcg_error.pl comprising:

:- use_module(library(dcgs)).

d -->
        (   { true } -> []
        ;   { true } -> []
        ).

I get:

$ scryer-prolog -f discontiguous_dcg_error.pl
   error(representation_error(dcg_body),[culprit-({true}->[])]).
% Warning: overwriting -->/2 because the clauses are discontiguous

The error is expected. The warning is unexpected, because the clauses are not discontiguous.

Low priority.
@hurufu
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hurufu commented Dec 4, 2024 •
edited
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I think the root cause is that throwing an exception during term expansion is treated as false by the loader.pl. So when exception is thrown then term is passed to compiler as-is, which then causes the warning. I don't know how to fix it correctly. One solution would be either to remove the offending term, so it wouldn't be compiled at all or change its principal functor to something different...

hurufu added a commit to hurufu/scryer-prolog that referenced this issue Dec 5, 2024
@hurufu
Remove DCGs that have thrown an exception during term expansion
a172650 
Some DCG constructs aren't supported and can't be expanded, here we
remove offending DCG rule and don't compile it at all – in a similar
fashion to what we do when incorrect goal was found – whole predicate
isn't getting compiled.

Fixes mthom#2675
@hurufu hurufu linked a pull request Dec 5, 2024 that will close this issue
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Remove DCGs that have thrown an exception during term expansion hurufu/scryer-prolog
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@hurufu @triska