Use JDK 17 syntax

ast/src/main/java/net/hamnaberg/json/Folder.java

@@ -0,0 +1,87 @@
+package net.hamnaberg.json;
+
+import java.util.function.Function;
+import java.util.function.Supplier;
+
+public interface Folder<A> {
+    A onNull();
+
+    A onBoolean(Json.JBoolean b);
+
+    A onNumber(Json.JNumber n);
+
+    A onString(Json.JString s);
+
+    A onArray(Json.JArray a);
+
+    A onObject(Json.JObject o);
+
+    default VoidFolder toVoid() {
+        var self = this;
+        return new VoidFolder() {
+            @Override
+            public void onNull() {
+                self.onNull();
+            }
+
+            @Override
+            public void onBoolean(Json.JBoolean b) {
+                self.onBoolean(b);
+            }
+
+            @Override
+            public void onNumber(Json.JNumber n) {
+                self.onNumber(n);
+            }
+
+            @Override
+            public void onString(Json.JString s) {
+                self.onString(s);
+            }
+
+            @Override
+            public void onArray(Json.JArray a) {
+                self.onArray(a);
+            }
+
+            @Override
+            public void onObject(Json.JObject o) {
+                self.onObject(o);
+            }
+        };
+    }
+
+    static <X> Folder<X> from(Function<Json.JString, X> fString, Function<Json.JBoolean, X> fBoolean, Function<Json.JNumber, X> fNumber, Function<Json.JObject, X> fObject, Function<Json.JArray, X> fArray, Supplier<X> fNull) {
+        return new Folder<>() {
+            @Override
+            public X onNull() {
+                return fNull.get();
+            }
+
+            @Override
+            public X onBoolean(Json.JBoolean b) {
+                return fBoolean.apply(b);
+            }
+
+            @Override
+            public X onNumber(Json.JNumber n) {
+                return fNumber.apply(n);
+            }
+
+            @Override
+            public X onString(Json.JString s) {
+                return fString.apply(s);
+            }
+
+            @Override
+            public X onArray(Json.JArray a) {
+                return fArray.apply(a);
+            }
+
+            @Override
+            public X onObject(Json.JObject o) {
+                return fObject.apply(o);
+            }
+        };
+    }
+}