Make module nullable in custom type

compiler/NubLang/Syntax/Parser.cs

@@ -7,7 +7,6 @@ public sealed class Parser
 {
     private List<Token> _tokens = [];
     private int _tokenIndex;
-    private string _moduleName = string.Empty;
 
     private Token? CurrentToken => _tokenIndex < _tokens.Count ? _tokens[_tokenIndex] : null;
     private bool HasToken => CurrentToken != null;
@@ -19,34 +18,9 @@ public sealed class Parser
         Diagnostics.Clear();
         _tokens = tokens;
         _tokenIndex = 0;
-        _moduleName = string.Empty;
 
+        string? moduleName = null;
         var imports = new List<string>();
-
-        try
-        {
-            while (TryExpectSymbol(Symbol.Import))
-            {
-                imports.Add(ExpectStringLiteral().Value);
-            }
-
-            ExpectSymbol(Symbol.Module);
-            _moduleName = ExpectStringLiteral().Value;
-        }
-        catch (ParseException e)
-        {
-            Diagnostics.Add(e.Diagnostic);
-            while (HasToken)
-            {
-                if (CurrentToken is SymbolToken { Symbol: Symbol.Module or Symbol.Import })
-                {
-                    break;
-                }
-
-                Next();
-            }
-        }
-
         var definitions = new List<DefinitionSyntax>();
 
         while (HasToken)
@@ -54,6 +28,41 @@ public sealed class Parser
             try
             {
                 var startIndex = _tokenIndex;
+
+                if (TryExpectSymbol(Symbol.Import))
+                {
+                    var name = ExpectStringLiteral();
+                    if (imports.Contains(name.Value))
+                    {
+                        Diagnostics.Add(Diagnostic
+                            .Warning($"Module {name.Value} is imported twice")
+                            .At(name)
+                            .WithHelp($"Remove duplicate import \"{name.Value}\"")
+                            .Build());
+                    }
+                    else
+                    {
+                        imports.Add(name.Value);
+                    }
+
+                    continue;
+                }
+
+                if (TryExpectSymbol(Symbol.Module))
+                {
+                    if (moduleName != null)
+                    {
+                        throw new ParseException(Diagnostic
+                            .Error("Module is declared more than once")
+                            .At(CurrentToken)
+                            .WithHelp("Remove duplicate module declaration")
+                            .Build());
+                    }
+
+                    moduleName = ExpectStringLiteral().Value;
+                    continue;
+                }
+
                 var exported = TryExpectSymbol(Symbol.Export);
 
                 if (TryExpectSymbol(Symbol.Extern))
@@ -93,7 +102,7 @@ public sealed class Parser
             }
         }
 
-        return new SyntaxTree(definitions, _moduleName, imports);
+        return new SyntaxTree(definitions, moduleName ?? "default", imports);
     }
 
     private FuncParameterSyntax ParseFuncParameter()
@@ -659,7 +668,7 @@ public sealed class Parser
                     return new BoolTypeSyntax(GetTokens(startIndex));
                 default:
                 {
-                    var module = _moduleName;
+                    string? module = null;
 
                     if (TryExpectSymbol(Symbol.DoubleColon))
                     {