Re: [rules-dev] Comparing Prolog
On 08/07/2013, Mark Proctor <mproctor(a)codehaus.org> wrote:
As per the email, we don't' have the nice syntax for list
unification - but
it's just sugar. The same thing can be done, just more verbosely. I've not
checked this compiles, but it shows the intent.
Next time do so, it's quite interesting, see below...
query nextTo(String x, String y, List list)
isRight( x, y, list )
or
isRight( y, x, list )
end
query isRight(String l, String r, List list)
l := String() from list[0]
r := String() from list[1]
tail : List() from list.subList(1, list.size() )
isRight( l, r, tail )
end
Copied your rules into a drl, and 5.5.0 crashes with:
Exception in thread "main" java.lang.NullPointerException
at org.drools.rule.builder.QueryElementBuilder.build(QueryElementBuilder.java:59)
at org.drools.rule.builder.PatternBuilder.build(PatternBuilder.java:180)
at org.drools.rule.builder.PatternBuilder.build(PatternBuilder.java:123)
at org.drools.rule.builder.GroupElementBuilder.build(GroupElementBuilder.java:67)
at org.drools.rule.builder.RuleBuilder.build(RuleBuilder.java:84)
at org.drools.compiler.PackageBuilder.addRule(PackageBuilder.java:2830)
at org.drools.compiler.PackageBuilder.compileRules(PackageBuilder.java:970)
at org.drools.compiler.PackageBuilder.compileAllRules(PackageBuilder.java:879)
at org.drools.compiler.PackageBuilder.addPackage(PackageBuilder.java:871)
at org.drools.compiler.PackageBuilder.addPackageFromDrl(PackageBuilder.java:466)
at org.drools.compiler.PackageBuilder.addKnowledgeResource(PackageBuilder.java:694)
at org.drools.builder.impl.KnowledgeBuilderImpl.add(KnowledgeBuilderImpl.java:51)
at org.drools.builder.impl.KnowledgeBuilderImpl.add(KnowledgeBuilderImpl.java:40)
OK, I must not omit an import, but an NPE is uncalled for. So, I added
import java.util.List
and then I get:
### compilation errors ###
Unable to resolve ObjectType 'isRight' : [Rule name='nextTo']
Query's must use positional or bindings, not field constraints:
This error message ought to be rewritten as
"Queries require positional arguments or bindings, not field constraints"
Next, I added some semicolons:
query nextTo(String x, String y, List list)
isRight( x, y, list; )
or
isRight( y, x, list; )
end
query isRight(String l, String r, List list)
l := String() from list.get(0)
r := String() from list.get(1)
tail : List() from list.subList(1, list.size() )
isRight( l, r, tail; )
end
and then I get:
### compilation errors ###
Unable to resolve ObjectType 'isRight' : [Rule name='nextTo']
I had to swap the queries so that the definition of isRight precedes
its use in nextTo. That's a potentially blocking bug, since queries
can be used mutually recursively.
Quite a haul...
Wolfgang