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doc/usersguide/src/lang/lang-err.tex

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+\section{Error handling}
+
+All syntactic and semantic errors cause parser exceptions to be thrown. In
+particular, the methods used to match tokens in the parser base class (match et
+al) throw §EdpgMismatchedToken§. The methods in the lexer base class used to
+match characters (match et al) throw analogous exceptions.
+
+\subsection{DPG exception hierarchy}
+
+DPG-generated parsers throw exceptions to signal recognition errors or other
+stream problems. All exceptions derive from EdpgException. The hierarchy is the
+following:
+
+\begin{verbatim}
+   EdpgException
+      EdpgMismatchedChar
+      EdpgMismatchedToken
+      EdpgSemantic
+\end{verbatim}
+
+\subsubsection{EdpgException} The EdpgException exception class is the base of
+all DPG generated exceptions. User defined exceptions must derive from this
+class.
+
+\subsubsection{EdpgMismatchedChar} This exception is thrown by the lexer when it
+is looking for a character, but finds a different one on the input stream.
+
+\subsubsection{EdpgMismatchedToken} This exception is thrown by the parser when
+it is looking for a token, but finds a different one on the input token stream.
+
+\subsubsection{EdpgSemantic} This exception is thrown by a validating semantic
+predicate.
+
+\subsection{Specifying exception handlers}
+
+DPG allows to specify specific exception handler to a given rule or
+alternative. The general form of an exception handler specification is:
+
+\begin{verbatim}
+   ... except  { code to handle exception }
+   ... finally { code to handle exception }
+\end{verbatim}
+
+\subsubsection{Exception handler for a rule}
+
+The exception handler for a rule must be placed after the terminating
+semicolon. The handler can be either an §except§ block or a §finally§ block.
+The implementation of rule will be surrounded by a try block.
+
+\begin{verbatim}
+   r : ...
+     ;
+     except { handler code }
+\end{verbatim}
+
+\subsubsection{Exception handler for an alternative}
+
+The exception handler of an alternative must be the last element of the
+alternative. Both exception handler blocks can be used. Every alternative that
+have exception block specified, will be surrounded by a §try...except/finally§
+block.
+
+\begin{verbatim}
+   r : alternative_1 ... except  { handler code }
+     | alternative_2 ... finally { handler code }
+       ...
+     | alternative_n
+     ;
+\end{verbatim}
+
+\paragraph{Note:} It is not necessary to define exception handler for each alternative.
+
+\subsubsection{Default error handler in lexer}
+
+To skip every character that isn't recognized by any public lexer rule, specify
+§filter=true§ option for a lexer. That way, the parser doesn't have to deal
+with lexical errors and ask for another token.