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Fix lambda visitor example #2

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Fix lambda visitor example #2

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10 changes: 10 additions & 0 deletions pom.xml
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Original file line number Diff line number Diff line change
Expand Up @@ -9,6 +9,16 @@
<version>1.0-SNAPSHOT</version>


<dependencies>
<dependency>
<groupId>org.derive4j</groupId>
<artifactId>derive4j</artifactId>
<version>0.7</version>
<!-- compile-only dependency (not used at runtime): -->
<optional>true</optional>
</dependency>
</dependencies>

<build>
<plugins>
<plugin>
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63 changes: 27 additions & 36 deletions src/main/java/org/mfusco/fromgoftolambda/examples/visitor/VisitorLambda.java
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Original file line number Diff line number Diff line change
Expand Up @@ -6,35 +6,12 @@
import java.util.Map;
import java.util.function.Function;

public class VisitorLambda {

public static class LambdaVisitor<A> implements Function<Object, A> {
private Map<Class<?>, Function<Object, A>> fMap = new HashMap<>();

public <B> Acceptor<A, B> on(Class<B> clazz) {
return new Acceptor<>(this, clazz);
}

@Override
public A apply( Object o ) {
return fMap.get(o.getClass()).apply( o );
}

static class Acceptor<A, B> {
private final LambdaVisitor visitor;
private final Class<B> clazz;
import static org.mfusco.fromgoftolambda.examples.visitor.Elements.circleElement;
import static org.mfusco.fromgoftolambda.examples.visitor.Elements.rectangleElement;
import static org.mfusco.fromgoftolambda.examples.visitor.Elements.squareElement;

public Acceptor( LambdaVisitor<A> visitor, Class<B> clazz ) {
this.visitor = visitor;
this.clazz = clazz;
}
public class VisitorLambda {

public LambdaVisitor<A> then(Function<B, A> f) {
visitor.fMap.put( clazz, f );
return visitor;
}
}
}

public static class Square {
final double side;
Expand Down Expand Up @@ -62,18 +39,32 @@ public Rectangle( double weidht, double height ) {
}
}

static Function<Object, Double> areaVisitor = new LambdaVisitor<Double>()
.on(Square.class).then( s -> s.side * s.side )
.on(Circle.class).then( c -> Math.PI * c.radius * c.radius )
.on(Rectangle.class).then( r -> r.height * r.weidht );
@org.derive4j.Data
// see the generated "Elements" class for constructors (https://gist.github.com/jbgi/393f621decdeb1374b1f3356b2d140b6)
interface Element {
// we could have used the same visitor interface as in gof visitor... but let's go with an equivalent catamorphism definition:
<T> T match( Function<Square, T> squareElement,
Function<Circle, T> circleElement,
Function<Rectangle, T> rectangleElement );
}


// like with the visitor pattern, pattern matching synthax ensure that all 3 cases are handled:

static Function<Element, Double> areaVisitor = element -> element.match(
s -> s.side * s.side,
c -> Math.PI * c.radius * c.radius,
r -> r.height * r.weidht
);

static Function<Object, Double> perimeterVisitor = new LambdaVisitor<Double>()
.on(Square.class).then( s -> 4 * s.side )
.on(Circle.class).then( c -> 2 * Math.PI * c.radius )
.on(Rectangle.class).then( r -> 2 * r.height + 2 * r.weidht );
// or using the equivalent syntax generated by derive4j:
static Function<Element, Double> perimeterVisitor = Elements.cases()
.squareElement( s -> 4 * s.side )
.circleElement( c -> 2 * Math.PI * c.radius )
.rectangleElement( r -> 2 * r.height + 2 * r.weidht );

public static void main( String[] args ) {
List<Object> figures = Arrays.asList( new Circle( 4 ), new Square( 5 ), new Rectangle( 6, 7 ) );
List<Element> figures = Arrays.asList( circleElement(new Circle( 4 )), squareElement(new Square( 5 )), rectangleElement(new Rectangle( 6, 7 )) );

double totalArea = figures.stream().map( areaVisitor ).reduce( 0.0, (v1, v2) -> v1 + v2 );
System.out.println("Total area = " + totalArea);
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