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Support placeholders with #external declarations

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This adds support for declaring predicates as placeholders through the
“#external” directive in the input language of clingo.

Placeholders are not subject to completion. This prevents predicates
that represent instance-specific facts from being assumed as universally
false by default negation when translating an encoding.

This stretches clingo’s usual syntax a bit to make the implementation
lightweight. In order to declare a predicate with a specific arity as a
placeholder, the following statement needs to be added to the program:

    #external <predicate name>(<arity>).

Multiple unit tests cover cases where placeholders are used or not as
well as a more complex graph coloring example.
This commit is contained in:
Patrick Lühne
2018-04-05 23:22:25 +02:00
parent c91cbaf58b
commit c294a29cb2
9 changed files with 192 additions and 32 deletions
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62
tests/TestPlaceholders.cpp Normal file
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#include <catch.hpp>
#include <sstream>
#include <anthem/AST.h>
#include <anthem/Context.h>
#include <anthem/Translation.h>
////////////////////////////////////////////////////////////////////////////////////////////////////
TEST_CASE("[placeholders] Programs with placeholders are correctly completed", "[placeholders]")
{
std::stringstream input;
std::stringstream output;
std::stringstream errors;
anthem::output::Logger logger(output, errors);
anthem::Context context(std::move(logger));
context.performSimplification = true;
context.performCompletion = true;
SECTION("no placeholders")
{
input <<
"colored(V, red) :- vertex(V), not colored(V, green), not colored(V, blue).";
anthem::translate("input", input, context);
CHECK(output.str() ==
"forall V1, V2 (colored(V1, V2) <-> (V2 = red and vertex(V1) and not colored(V1, green) and not colored(V1, blue)))\n"
"forall V3 not vertex(V3)\n");
}
SECTION("single placeholder")
{
input <<
"#external vertex(1).\n"
"colored(V, red) :- vertex(V), not colored(V, green), not colored(V, blue).";
anthem::translate("input", input, context);
CHECK(output.str() ==
"forall V1, V2 (colored(V1, V2) <-> (V2 = red and vertex(V1) and not colored(V1, green) and not colored(V1, blue)))\n");
}
SECTION("complex example: graph coloring")
{
input <<
"#external color(1).\n"
"#external edge(2).\n"
"#external vertex(1).\n"
"#show color/2.\n"
"{color(V, C)} :- vertex(V), color(C).\n"
"covered(V) :- color(V, _).\n"
":- vertex(V), not covered(V).\n"
":- color(V1, C), color(V2, C), edge(V1, V2).";
anthem::translate("input", input, context);
CHECK(output.str() ==
"forall V1, V2 (color(V1, V2) <-> (vertex(V1) and color(V2) and color(V1, V2)))\n"
"forall U1 not (vertex(U1) and not exists U2 color(U1, U2))\n"
"forall U3, U4, U5 not (color(U3, U4) and color(U5, U4) and edge(U3, U5))\n");
}
}