Hi, I'm nit-picking here and I've largely written this for documentation reasons, but the following gives a compilation Err:

map := [Int:Int]?[:]
// --> Invalid map type '[sys::Int:sys::Int]?' for map literal

I tried creating one at runtime but got an ArgErr:

map := Map(Map#.parameterize(["K":Int#, "V":Int?#]).toNullable)
// --> sys::ArgErr: Non-nullable map type required: [sys::Int:sys::Int?]?

This patch fixes the ArgErr:

diff -r fe538d44fa4d src/sys/java/fan/sys/Map.java
--- a/src/sys/java/fan/sys/Map.java	Mon Sep 28 13:18:07 2015 -0400
+++ b/src/sys/java/fan/sys/Map.java	Tue Sep 29 10:26:24 2015 +0100
@@ -32,6 +32,11 @@
 
   public static Map make(Type type)
   {
+    if (type instanceof NullableType)
+    {
+      type = ((NullableType) type).root;
+    }
+
     MapType t = null;
     try
     {

But led me to the interesting observation that MapType, and hence Maps in general, can't truly be nullable:

fansh> m2 := Map(Map#.parameterize(["K":Int#, "V":Int#]).toNullable)
[:]
fansh> m2.typeof
[sys::Int:sys::Int]  // --> expected [sys::Int:sys::Int]?

Not that it's an issue - as I can't see why that signature would ever be needed.

But led me to the interesting observation that MapType, and hence Maps in general, can't truly be nullable:

Subtle differences, but you need to separate the parameterized type of a Map instance versus a type signature - they are two separate things. For example this is allowed:

[Int:Int]? someMethod() { return null }

The method returns a map types Int:Int or null. However a given map instance by definition cannot be nullable. So the map type associated with it is non-nullable by definition.

Okay, that makes sense - thanks for the explanation!