Equivalent in Dart - dart

Question

I've just started exploring Dart language and I wanted to test an existing code that I wrote in Java as given:
public interface Condition {
Condition FALSE = facts->false;
Boolean evaluate(Fact<?> fact);
default Condition and(Condition other) {
return fact-> this.evaluate(fact) && other.evaluate(fact);
}
default Condition or(Condition other) {
return fact-> this.evaluate(fact) || other.evaluate(fact);
}
}
And the caller calls it as:
#Test
public void testCondition() {
String str = "A String";
Condition a = fact -> !str.isBlank();
Condition b = fact -> str.contains("A");
a.and(b);
}
A complete test class that makes use of this is :
public class AnonymousLoopTest {
#Test
public void test() {
RulesEngine rulesEngine = new InferenceRuleEngine();
List<Name> names = NamesFactory.fetchNames();
Rules rules = new Rules();
Facts facts = new Facts();
AtomicReference<Integer> countRef = new AtomicReference<>(1);
names.forEach(personName -> {
facts.put("name-" + countRef.get(), personName);
countRef.set(countRef.get()+1);
Condition condition = fact -> !personName.name().isEmpty();
//Hack the comparator logic of DefaultRule/BasicRule in order to override its internal logic as below.
//This is needed to register our Rule with Rules which uses a Set<Rule> to register new Rules
//with the comparator logic written in BasicRule.
Rule nameRule = new RuleBuilder((o1, o2) -> personName.name().compareTo(o1.getName()))
.when(condition).then(action -> System.out.println("In Action:" + personName)).build();
rules.register(nameRule);
});
rulesEngine.fire(rules, facts);
}
}
record Name(Integer id, String name){}
class NamesFactory{
static List<Name> fetchNames(){
return List.of(new Name(10, "Sara"), new Name(20, "Zara"), new Name(30, ""),new Name(40, "Lara"));
}
}
The condition is used by when() method.
In the given example the blank name will be filtered out. The other three names will be printed.
I was trying to write and equivalent in Dart but I'm just stuck. What is the way to write this code in Dart?

Answer 1

That looks like something I'd do by:
typedef Condition = bool Function(Fact);
bool falseCondition(Fact _) => false;
extension ConditionComposition on Condition {
Condition operator &(Condition other) => (Fact fact) => this(fact) && other(fact);
Condition operator |(Condition other) => (Fact fact) => this(fact) || other(fact);
Condition operator ~() => (Fact fact) => !this(fact);
}
If you insist on having a wrapper class for the function objects, I'd do it as:
class Condition {
static const Condition falseCondition = Condition(_kFalse);
final bool Function(Fact) _test;
const Condition(bool test(Fact fact)) : _test = test;
bool evaluate(Fact fact) => _test(fact);
Condition operator &(Condition other) => Condition((fact) =>
this.evaluate(fact) && other.evaluate(fact));
Condition operator |(Condition other) => Condition((fact) =>
this.evaluate(fact) || other.evaluate(fact));
static bool _kFalse(_) => false;
}
but a class seems overkill for something which really is just a simple function. Dart has first class functions.
You can use the former version as:
test("test Condition", () {
var str = "A String";
Condition a = (fact) => str.isNotEmpty();
Condition b = (fact) => str.contains("A");
var both = a & b;
expect(both(someDefaultFact), true);
}