Implement support for handling exceptions in user scripts

doc/17-language-reference.md

@@ -851,7 +851,7 @@ Example:
 
     var s = String(3) /* Sets s to "3". */
 
-## <a id="throw"></a> Exceptions
+## <a id="throw"></a> Throwing Exceptions
 
 Built-in commands may throw exceptions to signal errors such as invalid arguments. User scripts can throw exceptions
 using the `throw` keyword.
@@ -860,7 +860,20 @@ Example:
 
     throw "An error occurred."
 
-There is currently no way for scripts to catch exceptions.
+## <a id="try-except"></a> Handling Exceptions
+
+Exceptions can be handled using the `try` and `except` keywords. When an exception occurs while executing code in the
+`try` clause no further statements in the `try` clause are evaluated and the `except` clause is executed instead.
+
+Example:
+
+    try {
+        throw "Test"
+
+        log("This statement won't get executed.")
+    } except {
+        log("An error occurred in the try clause.")
+    }
 
 ## <a id="breakpoints"></a> Breakpoints
 
@@ -920,7 +933,7 @@ These keywords are reserved and must not be used as constants or custom attribut
     template
     include
     include_recursive
-    ignore_on_error
+    include_zones
     library
     null
     true
@@ -928,7 +941,13 @@ These keywords are reserved and must not be used as constants or custom attribut
     const
     var
     this
+    globals
+    locals
     use
+    default
+    ignore_on_error
+    current_filename
+    current_line
     apply
     to
     where
@@ -937,12 +956,16 @@ These keywords are reserved and must not be used as constants or custom attribut
     ignore
     function
     return
+    break
+    continue
     for
     if
     else
+    while
+    throw
+    try
+    except
     in
-    current_filename
-    current_line
 
 You can escape reserved keywords using the `@` character. The following example
 tries to set `vars.include` which references a reserved keyword and generates

lib/base/configwriter.cpp

@@ -262,6 +262,8 @@ const std::vector<String>& ConfigWriter::GetKeywords(void)
 		keywords.push_back("else");
 		keywords.push_back("while");
 		keywords.push_back("throw");
+		keywords.push_back("try");
+		keywords.push_back("except");
 	}
 
 	return keywords;

lib/config/config_lexer.ll

@@ -197,6 +197,8 @@ if				return T_IF;
 else				return T_ELSE;
 while				return T_WHILE;
 throw				return T_THROW;
+try				return T_TRY;
+except				return T_EXCEPT;
 ignore_on_error			return T_IGNORE_ON_ERROR;
 current_filename		return T_CURRENT_FILENAME;
 current_line			return T_CURRENT_LINE;

lib/config/config_parser.yy

@@ -172,6 +172,8 @@ static void MakeRBinaryOp(Expression** result, Expression *left, Expression *rig
 %token T_ELSE "else (T_ELSE)"
 %token T_WHILE "while (T_WHILE)"
 %token T_THROW "throw (T_THROW)"
+%token T_TRY "try (T_TRY)"
+%token T_EXCEPT "except (T_EXCEPT)"
 %token T_FOLLOWS "=> (T_FOLLOWS)"
 %token T_NULLARY_LAMBDA_BEGIN "{{ (T_NULLARY_LAMBDA_BEGIN)"
 %token T_NULLARY_LAMBDA_END "}} (T_NULLARY_LAMBDA_END)"
@@ -666,6 +668,10 @@ lterm: T_LIBRARY rterm
 	{
 		$$ = new ThrowExpression($2, false, @$);
 	}
+	| T_TRY rterm_scope T_EXCEPT rterm_scope
+	{
+		$$ = new TryExceptExpression($2, $4, @$);
+	}
 	| rterm_side_effect
 	;
 	

lib/config/expression.cpp

@@ -941,3 +941,16 @@ ExpressionResult UsingExpression::DoEvaluate(ScriptFrame& frame, DebugHint *dhin
 	return Empty;
 }
 
+ExpressionResult TryExceptExpression::DoEvaluate(ScriptFrame& frame, DebugHint *dhint) const
+{
+	try {
+		ExpressionResult tryResult = m_TryBody->Evaluate(frame, dhint);
+		CHECK_RESULT(tryResult);
+	} catch (const std::exception& ex) {
+		ExpressionResult exceptResult = m_ExceptBody->Evaluate(frame, dhint);
+		CHECK_RESULT(exceptResult);
+	}
+
+	return Empty;
+}
+

lib/config/expression.hpp

@@ -1027,6 +1027,27 @@ private:
 	Expression *m_Name;
 };
 
+class I2_CONFIG_API TryExceptExpression : public DebuggableExpression
+{
+public:
+	TryExceptExpression(Expression *tryBody, Expression *exceptBody, const DebugInfo& debugInfo = DebugInfo())
+		: DebuggableExpression(debugInfo), m_TryBody(tryBody), m_ExceptBody(exceptBody)
+	{ }
+
+	~TryExceptExpression(void)
+	{
+		delete m_TryBody;
+		delete m_ExceptBody;
+	}
+
+protected:
+	virtual ExpressionResult DoEvaluate(ScriptFrame& frame, DebugHint *dhint) const override;
+
+private:
+	Expression *m_TryBody;
+	Expression *m_ExceptBody;
+};
+
 }
 
 #endif /* EXPRESSION_H */