Add "??=" operator

Author: gamerbross

TestHScript.hx

@@ -137,14 +137,18 @@ class TestHScript extends TestCase {
 			ptnull: null,
 			pt: pt,
 			pt2null: {pt: null},
-			pt2: {pt: pt}
+			pt2: {pt: pt},
+			valuenull: null,
+			value: 10
 		}
 		assertScript("ptnull?.x ?? 5", 5, vars);
 		assertScript("pt?.x ?? 5", 10, vars);
 		assertScript("pt2null?.pt ?? 5", 5, vars);
 		assertScript("pt2null?.pt?.x ?? 5", 5, vars);
 		assertScript("pt2?.pt ?? 5", pt, vars);
 		assertScript("pt2?.pt?.x ?? 5", 10, vars);
+		assertScript("valuenull ??= 5; valuenull", 5, vars);
+		assertScript("value ??= 5; value", 10, vars);
 	}
 
 	function testIsOperator():Void {

hscript/Interp.hx

@@ -122,6 +122,7 @@ class Interp {
 		assignOp("*=",function(v1:Float,v2:Float) return v1 * v2);
 		assignOp("/=",function(v1:Float,v2:Float) return v1 / v2);
 		assignOp("%=",function(v1:Float,v2:Float) return v1 % v2);
+		assignOp("??=",function(v1:Dynamic,v2:Dynamic) if ( v1 == null ) return v1 = v2 else return v1);
 		assignOp("&=",function(v1,v2) return v1 & v2);
 		assignOp("|=",function(v1,v2) return v1 | v2);
 		assignOp("^=",function(v1,v2) return v1 ^ v2);

hscript/Parser.hx

@@ -128,7 +128,7 @@ class Parser {
 			["..."],
 			["&&"],
 			["||"],
-			["=","+=","-=","*=","/=","%=","<<=",">>=",">>>=","|=","&=","^=","=>"],
+			["=","+=","-=","*=","/=","%=","??=","<<=",">>=",">>>=","|=","&=","^=","=>"],
 			["->"]
 		];
 		#if haxe3
@@ -1521,8 +1521,12 @@ class Parser {
 				char = readChar();
 				if( char == ".".code )
 					return TQuestionDot;
-				else if ( char == "?".code )
+				else if ( char == "?".code ) {
+					char = readChar();
+					if ( char == "=".code )
+						return TOp("??=");
 					return TQuestionQuestion;
+				}
 				this.char = char;
 				return TQuestion;
 			case ":".code: return TDoubleDot;