added IValue.getPatternMatchFingerprint() that simulates rascal-core:TopLevelType.getFingerprint(v) exactly for the value kinds we have in vallang

src/main/java/io/usethesource/vallang/IConstructor.java

@@ -25,6 +25,14 @@
  */
 public interface IConstructor extends INode {
 
+	@Override
+	default int getPatternMatchFingerprint() {
+		return getName().hashCode() << 2 + arity();
+
+		// TODO: this would distinguish more constructors based on incomparable argument types:
+		// return getUninstantiatedConstructorType().hashCode();
+	}
+
 	/**
 	 * @return the specific ConstructorType of this constructor
 	 */

src/main/java/io/usethesource/vallang/IExternalValue.java

@@ -37,6 +37,12 @@
  * Note that NORMAL USE OF THE PDB DOES NOT REQUIRE IMPLEMENTING THIS INTERFACE
  */
 public interface IExternalValue extends IValue {
+    /** 
+     * External values must re-think their pattern match fingerprint,
+     * instead of returning `IValue.hashCode()` automatically.
+     */
+    @Override
+    int getPatternMatchFingerprint();
 
 	/**
 	 * @return an ExternalType

src/main/java/io/usethesource/vallang/IList.java

@@ -22,6 +22,12 @@
 import io.usethesource.vallang.visitors.IValueVisitor;
 
 public interface IList extends ICollection<IList> {
+
+    @Override
+    default int getPatternMatchFingerprint() {
+        return 3322014; // "list".hashCode()
+    }
+
     /**
      * @return the number of elements in the list
      */

src/main/java/io/usethesource/vallang/IMap.java

@@ -26,6 +26,11 @@
 
 public interface IMap extends ICollection<IMap> {
 
+    @Override
+    default int getPatternMatchFingerprint() {
+        return 107868; // "map".hashCode()
+    }
+
     /**
      * Adds a new entry to the map, mapping the key to value. If the
      * key existed before, the old value will be lost.

src/main/java/io/usethesource/vallang/INode.java

@@ -30,6 +30,12 @@
  * it recursively.
  */
 public interface INode extends IValue, Iterable<IValue> {
+
+	@Override
+	default int getPatternMatchFingerprint() {
+		return getName().hashCode() << 2 + arity();
+	}
+
 	/**
 	 * Get a child
 	 * @param i the zero based index of the child

src/main/java/io/usethesource/vallang/ISet.java

@@ -21,6 +21,11 @@
 
 public interface ISet extends ICollection<ISet> {
 
+    @Override
+    default int getPatternMatchFingerprint() {
+        return 113762; // "set".hashCode()
+    }
+
     /**
      * Add an element to the set. 
      * @param element

src/main/java/io/usethesource/vallang/IString.java

@@ -19,6 +19,7 @@
 import io.usethesource.vallang.visitors.IValueVisitor;
 
 public interface IString extends IValue, Iterable<Integer> {
+
 	/**
 	 * @return the Java string that this string represents
 	 */

src/main/java/io/usethesource/vallang/ITuple.java

@@ -16,6 +16,11 @@
 import io.usethesource.vallang.visitors.IValueVisitor;
 
 public interface ITuple extends Iterable<IValue>, IValue {
+    @Override
+    default int getPatternMatchFingerprint() {
+        return 442900256 /* "tuple".hashCode() << 2 */ + arity();
+    }
+
     /**
      * Retrieve the given field at the given index.
      * 

src/main/java/io/usethesource/vallang/IValue.java

@@ -28,6 +28,22 @@ public interface IValue {
 	 * @return the {@link Type} of a value
 	 */
     public Type getType();
+
+    /**
+     * This method is used exclusively by code generated by the Rascal compiler,
+     * or by the Rascal interpreter. The returned integer codes are opaque, although stable.
+     * If you need to know what kind of value you have, use the IValueVisitor or the ITypeVisitor
+     * interfaces and the `accept` methods on IValue and Type.
+     * 
+     * @return an integer code that:
+     *    * accurate reflects the identity of the top-level structure of this value
+     *    * such that if pattern.match(this) ===> pattern.getPatternMatchFingerprint() == this.getPatternMatchFingerprint()
+     *    * distinguishes maximally between different kinds of values
+     *    * never makes the same or similar value have a different fingerprint
+     */
+    default int getPatternMatchFingerprint() {
+        return hashCode();
+    }
 
     /**
      * Execute the {@link IValueVisitor} on the current node