Add singleOrNull to Listing

pkl-core/src/main/java/org/pkl/core/stdlib/base/ListingNodes.java

@@ -149,6 +149,16 @@ protected Object eval(VmListing self) {
     }
   }
 
+  public abstract static class singleOrNull extends ExternalPropertyNode {
+    @Specialization
+    protected Object eval(VmListing self) {
+      if (self.getLength() != 1) {
+        return VmNull.withoutDefault();
+      }
+      return VmUtils.readMember(self, 0L);
+    }
+  }
+
   public abstract static class distinctBy extends ExternalMethod1Node {
     @Child private ApplyVmFunction1Node applyNode = ApplyVmFunction1Node.create();
 

pkl-core/src/test/files/LanguageSnippetTests/input/api/listing.pkl

@@ -106,6 +106,12 @@ facts {
     module.catch(() -> base.single) == "Expected a single-element Listing."
     new Listing { 42 }.single == 42
   }
+
+  ["singleOrNull"] {
+    empty.singleOrNull == null
+    base.singleOrNull == null
+    new Listing { 42 }.singleOrNull == 42
+  }
 }
 
 examples {

pkl-core/src/test/files/LanguageSnippetTests/output/api/listing.pcf

@@ -70,6 +70,11 @@ facts {
     true
     true
   }
+  ["singleOrNull"] {
+    true
+    true
+    true
+  }
 }
 examples {
   ["length"] {

stdlib/base.pkl

@@ -1903,6 +1903,9 @@ class Listing<out Element> extends Object {
   /// ```
   external single: Element
 
+  /// Same as [single] but returns [null] if this collection is empty or has more than one element.
+  external singleOrNull: Element?
+
   /// Removes elements that are duplicates after applying [selector] from this listing, preserving the first occurrence.
   ///
   /// Facts: