Add support for null safe field access (#127)

Author: yuxiaomao

TestHScript.hx

@@ -115,6 +115,22 @@ class TestHScript extends TestCase {
 		assertScript("var a = 10; var b = 5; a - b / 2", 7.5);
 	}
 
+	function testNullFieldAccess():Void {
+		var pt = {x : 10};
+		var vars = {
+			ptnull : null,
+			pt: pt,
+			pt2null : {pt : null},
+			pt2: {pt : pt}
+		}
+		assertScript("ptnull?.x", null, vars);
+		assertScript("pt?.x", 10, vars);
+		assertScript("pt2null?.pt", null, vars);
+		assertScript("pt2null?.pt?.x", null, vars);
+		assertScript("pt2?.pt", pt, vars);
+		assertScript("pt2?.pt?.x", 10, vars);
+	}
+
 	function testMap():Void {
 		var objKey = { ok:true };
 		var vars = {

hscript/Parser.hx

@@ -32,6 +32,7 @@ enum Token {
 	TBrOpen;
 	TBrClose;
 	TDot;
+	TQuestionDot;
 	TComma;
 	TSemicolon;
 	TBkOpen;
@@ -806,6 +807,18 @@ class Parser {
 		case TDot:
 			var field = getIdent();
 			return parseExprNext(mk(EField(e1,field),pmin(e1)));
+		case TQuestionDot:
+			var field = getIdent();
+			var tmp = "__a_" + (uid++);
+			var e = mk(EBlock([
+				mk(EVar(tmp, null, e1), pmin(e1), pmax(e1)),
+				mk(ETernary(
+					mk(EBinop("==", mk(EIdent(tmp),pmin(e1),pmax(e1)), mk(EIdent("null"),pmin(e1),pmax(e1)))),
+					mk(EIdent("null"),pmin(e1),pmax(e1)),
+					mk(EField(mk(EIdent(tmp),pmin(e1),pmax(e1)),field),pmin(e1))
+				))
+			]),pmin(e1));
+			return parseExprNext(e);
 		case TPOpen:
 			return parseExprNext(mk(ECall(e1,parseExprList(TPClose)),pmin(e1)));
 		case TBkOpen:
@@ -1489,7 +1502,12 @@ class Parser {
 			case "[".code: return TBkOpen;
 			case "]".code: return TBkClose;
 			case "'".code, '"'.code: return TConst( CString(readString(char)) );
-			case "?".code: return TQuestion;
+			case "?".code:
+				char = readChar();
+				if( char == ".".code )
+					return TQuestionDot;
+				this.char = char;
+				return TQuestion;
 			case ":".code: return TDoubleDot;
 			case '='.code:
 				char = readChar();
@@ -1717,6 +1735,7 @@ class Parser {
 		case TBrOpen: "{";
 		case TBrClose: "}";
 		case TDot: ".";
+		case TQuestionDot: "?.";
 		case TComma: ",";
 		case TSemicolon: ";";
 		case TBkOpen: "[";