Fixed parser error in fn def

Src/AST/Myc.Ast.Script.pas

@@ -348,7 +348,14 @@ begin
                 Result := TAst.Assign(IIdentifierNode(tailNodes[0]), tailNodes[1]);
             end
             else if SameText(head.Token.Text, 'fn') then
-                Result := TAst.LambdaExpr(elements[High(tailNodes)].Params, tailNodes[1])
+            begin
+                // Corrected handling for lambda expressions
+                if Length(tailNodes) <> 2 then
+                    raise Exception.Create('Syntax Error: ''fn'' requires a parameter list and a body.');
+                if elements[1].Node <> nil then
+                    raise Exception.Create('Syntax Error: Expected a parameter list [...] after ''fn''.');
+                Result := TAst.LambdaExpr(elements[1].Params, tailNodes[1]);
+            end
             else if SameText(head.Token.Text, 'do') then
                 Result := TAst.Block(tailNodes)
             else if SameText(head.Token.Text, 'recur') then