implement null type

src/main/java/de/hsrm/compiler/Klang/types/NullType.java

@@ -0,0 +1,36 @@
+package de.hsrm.compiler.Klang.types;
+
+public class NullType extends Type {
+
+    private static NullType instance = null;
+
+    public static NullType getType() {
+        if (instance != null) {
+            return instance;
+        }
+        instance = new NullType();
+        return instance;
+    }
+
+    @Override
+    public String getName() {
+        return "naught";
+    }
+
+    @Override
+    public Type combine(Type that) {
+        // You can not combine null with a primitive type
+        if (that.isPrimitiveType()) {
+            throw new RuntimeException("Type missmatch: cannot combine " + this.getName() + " and " + that.getName());
+        }
+
+        // Everything else combines with null to the type it was before
+        return that;
+    }
+
+    @Override
+    public boolean isPrimitiveType() {
+        return false;
+    }
+
+}

src/main/java/de/hsrm/compiler/Klang/types/StructType.java

@@ -19,6 +19,12 @@ public class StructType extends Type {
             return this;
         }
 
+        // If you combine a null type with a struct type, you
+        // always get the struct type back. 
+        if (that == NullType.getType()) {
+            return this;
+        }
+
         throw new RuntimeException("Type missmatch: cannot combine " + this.getName() + " and " + that.getName());
     }
 

src/main/java/de/hsrm/compiler/Klang/types/Type.java

@@ -16,11 +16,16 @@ public abstract class Type {
     return FloatType.getType();
   }
 
+  public static NullType getNullType() {
+    return NullType.getType();
+  }
+
   public static Type getByName(String name) {
     switch (name) {
       case "bool": return getBooleanType();
       case "int": return getIntegerType();
       case "float": return getFloatType();
+      case "null": return getNullType();
       default: return new StructType(name);
     }
   }