Fix Function.toString (#1411)

Fixes an issue where the underlying default Java toString() is leaking
through

docs/modules/release-notes/pages/0.31.adoc

@@ -138,7 +138,7 @@ Things to watch out for when upgrading.
 
 The following bugs have been fixed.
 
-* XXX (https://github.com/apple/pkl/issues/XXX[XXX])
+* Incorrect Function.toString() (https://github.com/apple/pkl/issues/1410[#1410])
 
 == Contributors [small]#🙏#
 

pkl-core/src/main/java/org/pkl/core/runtime/VmFunction.java

@@ -181,4 +181,10 @@ public final class VmFunction extends VmObjectLike {
   public int hashCode() {
     return System.identityHashCode(this);
   }
+
+  @Override
+  @TruffleBoundary
+  public String toString() {
+    return VmValueRenderer.singleLine(Integer.MAX_VALUE).render(this);
+  }
 }

pkl-core/src/main/java/org/pkl/core/runtime/VmValueRenderer.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -34,8 +34,13 @@ public final class VmValueRenderer {
   private final String interiorNewline;
   private final String indent;
   private String currIndent = "";
+  private static final VmValueRenderer maxSingleLine =
+      new VmValueRenderer(Integer.MAX_VALUE, " ", "; ", "");
 
   public static VmValueRenderer singleLine(int lengthLimit) {
+    if (lengthLimit == Integer.MAX_VALUE) {
+      return maxSingleLine;
+    }
     return new VmValueRenderer(lengthLimit, " ", "; ", "");
   }
 

pkl-core/src/test/files/LanguageSnippetTests/input/basic/string.pkl

@@ -43,6 +43,10 @@ examples {
     "\(new Person2 { name = "Pigeon"; age = 42 })"
     "\(null)"
     "\(Null(new Person { name = "Pigeon"; age = 42 }))"
+    "\(() -> null)"
+    "\((p1) -> null)"
+    "\((p1, p2) -> null)"
+    "\((p1, p2, p3) -> null)"
   }
 
   ["escaping"] {

pkl-core/src/test/files/LanguageSnippetTests/input/lambdas/lambda1.pkl

@@ -9,4 +9,7 @@ res3 = even.apply(11)
 
 local mult = (x, y) -> x * y
 
-res4 = mult.apply(2, 3)
+res4 = mult.apply(2, 3)
+
+res5 = even.toString()
+res6 = mult.toString()

pkl-core/src/test/files/LanguageSnippetTests/output/basic/string.pcf

@@ -34,6 +34,10 @@ examples {
     "My name is Pigeon and I'm 42 years old."
     "null"
     "null"
+    "new Function0 {}"
+    "new Function1 {}"
+    "new Function2 {}"
+    "new Function3 {}"
   }
   ["escaping"] {
     "\\\"\\("

pkl-core/src/test/files/LanguageSnippetTests/output/lambdas/lambda1.pcf

@@ -2,3 +2,5 @@ res1 = "abc"
 res2 = true
 res3 = false
 res4 = 6
+res5 = "new Function1 {}"
+res6 = "new Function2 {}"