null-coalescing

jael/lib/src/ast/error.dart

@@ -1,6 +1,6 @@
 import 'package:source_span/source_span.dart';
 
-class JaelError {
+class JaelError extends Error {
   final JaelErrorSeverity severity;
   final String message;
   final FileSpan span;

jael/lib/src/ast/expression.dart

@@ -1,8 +1,27 @@
+import 'package:source_span/source_span.dart';
 import 'package:symbol_table/symbol_table.dart';
 import 'ast_node.dart';
+import 'token.dart';
 
 abstract class Expression extends AstNode {
   compute(SymbolTable scope);
 }
 
-abstract class Literal extends Expression {}
+abstract class Literal extends Expression {}
+
+class Negation extends Expression {
+  final Token exclamation;
+  final Expression expression;
+
+  Negation(this.exclamation, this.expression);
+
+  @override
+  FileSpan get span {
+    return exclamation.span.expand(expression.span);
+  }
+
+  @override
+  compute(SymbolTable scope) {
+    return !(expression.compute(scope));
+  }
+}

jael/lib/src/ast/identifier.dart

@@ -20,6 +20,8 @@ class Identifier extends Expression {
       default:
         var symbol = scope.resolve(name);
         if (symbol == null) {
+          if (scope.resolve('!strict!')?.value == false)
+            return null;
           throw new ArgumentError(
               'The name "$name" does not exist in this scope.');
         }

jael/lib/src/ast/token.dart

@@ -43,6 +43,7 @@ enum TokenType {
   colon,
   comma,
   dot,
+  exclamation,
   percent,
   plus,
   minus,

jael/lib/src/renderer.dart

@@ -27,7 +27,13 @@ Document parseDocument(String text,
 class Renderer {
   const Renderer();
 
-  void render(Document document, CodeBuffer buffer, SymbolTable scope) {
+  /// Renders a [document] into the [buffer] as HTML.
+  ///
+  /// If [strictResolution] is `false` (default: `true`), then undefined identifiers will return `null`
+  /// instead of throwing.
+  void render(Document document, CodeBuffer buffer, SymbolTable scope, {bool strictResolution: true}) {
+    scope.add('!strict!', value: strictResolution != false);
+
     if (document.doctype != null) buffer.writeln(document.doctype.span.text);
     renderElement(
         document.root, buffer, scope, document.doctype?.public == null);

jael/lib/src/text/parselet/infix.dart

@@ -18,6 +18,7 @@ const Map<TokenType, InfixParselet> infixParselets = const {
   TokenType.nequ: const BinaryParselet(10),
   TokenType.question: const ConditionalParselet(),
   TokenType.equals: const BinaryParselet(3),
+  TokenType.elvis: const BinaryParselet(3),
 };
 
 class ConditionalParselet implements InfixParselet {

jael/lib/src/text/parselet/prefix.dart

@@ -1,6 +1,7 @@
 part of jael.src.text.parselet;
 
 const Map<TokenType, PrefixParselet> prefixParselets = const {
+  TokenType.exclamation: const NotParselet(),
   TokenType.$new: const NewParselet(),
   TokenType.number: const NumberParselet(),
   TokenType.hex: const HexParselet(),
@@ -11,6 +12,22 @@ const Map<TokenType, PrefixParselet> prefixParselets = const {
   TokenType.lParen: const ParenthesisParselet(),
 };
 
+class NotParselet implements PrefixParselet {
+  const NotParselet();
+
+  @override
+  Expression parse(Parser parser, Token token) {
+    var expression = parser.parseExpression(0);
+
+    if (expression == null) {
+      parser.errors.add(new JaelError(JaelErrorSeverity.error,
+          'Missing expression after "!" in negation expression.', token.span));
+    }
+
+    return new Negation(token, expression);
+  }
+}
+
 class NewParselet implements PrefixParselet {
   const NewParselet();
 

jael/lib/src/text/scanner.dart

@@ -3,6 +3,7 @@ import '../ast/ast.dart';
 
 final RegExp _whitespace = new RegExp(r'[ \n\r\t]+');
 
+final RegExp _id = new RegExp(r'(([A-Za-z][A-Za-z0-9]*-)*([A-Za-z][A-Za-z0-9]*))');
 final RegExp _string1 = new RegExp(
     r"'((\\(['\\/bfnrt]|(u[0-9a-fA-F][0-9a-fA-F][0-9a-fA-F][0-9a-fA-F])))|([^'\\]))*'");
 final RegExp _string2 = new RegExp(
@@ -31,7 +32,7 @@ final Map<Pattern, TokenType> _htmlPatterns = {
   '!=': TokenType.nequ,
   _string1: TokenType.string,
   _string2: TokenType.string,
-  new RegExp(r'(([A-Za-z][A-Za-z0-9]*-)*([A-Za-z][A-Za-z0-9]*))'): TokenType.id,
+  _id: TokenType.id,
 };
 
 final Map<Pattern, TokenType> _expressionPatterns = {
@@ -45,7 +46,10 @@ final Map<Pattern, TokenType> _expressionPatterns = {
   ':': TokenType.colon,
   ',': TokenType.comma,
   '.': TokenType.dot,
+  '??': TokenType.elvis,
+  '?.': TokenType.elvis_dot,
   '=': TokenType.equals,
+  '!': TokenType.exclamation,
   '-': TokenType.minus,
   '%': TokenType.percent,
   '+': TokenType.plus,
@@ -67,7 +71,7 @@ final Map<Pattern, TokenType> _expressionPatterns = {
   new RegExp(r'0x[A-Fa-f0-9]+'): TokenType.hex,
   _string1: TokenType.string,
   _string2: TokenType.string,
-  new RegExp('[A-Za-z_\\\$][A-Za-z0-9_\\\$]*'): TokenType.id,
+  _id: TokenType.id,
 };
 
 class _Scanner implements Scanner {

jael/pubspec.yaml

@@ -1,5 +1,5 @@
 name: jael
-version: 1.0.0-beta+1
+version: 1.0.0-beta+2
 description:  A simple server-side HTML templating engine for Dart.
 author: Tobe O <thosakwe@gmail.com>
 homepage: https://github.com/angel-dart/jael/tree/master/jael

jael/test/render/render_test.dart

@@ -9,19 +9,28 @@ main() {
 <html>
   <body>
     <h1>Hello</h1>
-    <img src=profile['avatar'] />
+    <img ready="always" data-img-src=profile['avatar'] />
   </body>
 </html>
 ''';
 
     var buf = new CodeBuffer();
-    var document = jael.parseDocument(template, sourceUrl: 'test.jl');
-    var scope = new SymbolTable(values: {
-      'profile': {
-        'avatar': 'thosakwe.png',
-      }
-    });
+    jael.Document document;
+    SymbolTable scope;
 
+    try {
+      document = jael.parseDocument(template, sourceUrl: 'test.jl');
+      scope = new SymbolTable(values: {
+        'profile': {
+          'avatar': 'thosakwe.png',
+        }
+      });
+    } on jael.JaelError catch(e) {
+      print(e);
+      print(e.stackTrace);
+    }
+
+    expect(document, isNotNull);
     const jael.Renderer().render(document, buf, scope);
     print(buf);
 
@@ -33,7 +42,7 @@ main() {
     <h1>
       Hello
     </h1>
-    <img src="thosakwe.png">
+    <img ready="always" data-img-src="thosakwe.png">
   </body>
 </html>
     '''