\section{Error handling}

All syntactic and semantic errors throw exceptions. In particular,
the methods used to match tokens in the parser base class (match
etc) throw §EdpgMismatchedToken§. The methods in the lexer base
class used to match characters (match etc) throw exceptions
similarly.

\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 as follows:

\begin{verbatim}
   EdpgException
      EdpgMismatchedChar
      EdpgMismatchedToken
      EdpgSemantic
\end{verbatim}

\subsection{EdpgException}
The §EdpgException§ is the base class for all DPG exceptions. It
defines the following read-only properties:
\begin{alltt}
   FileName : string;
   Line     : integer;
   Column   : integer;
\end{alltt}
These properties contain information about the location where the exception
occurred.

\subsection{EdpgMismatchedChar}
The §EdpgMismatchedChar§ exception is thrown by the lexer when it
is looking for a character, but finds a different one on the input
stream than expected. It defines the following properties in
addition to those of §EdpgException§.
\begin{alltt}
   FoundChar    : char;
   FoundString  : string;
   CharSet      : TdpgCharSet;
   Str          : string;
   Inverted     : boolean;
\end{alltt}
The §FoundChar§ and §FoundString§ properties contain the character
or string that was found on the input stream. The §CharSet§ and
§Str§ properties contain the values which the lexer expected to
find. The §Inverted§ property is set only if the exception came
from a §MatchNot(...)§ operation. In this case, the §CharSet§
property contains the values, that the lexer must §not§ match. The
validity of pro\-per\-ti\-es are shown in the next table,
depending on the kind of exception.

\begin{table}[H]
  \small
  \begin{center}
      \begin{tabular}{lcc}
                        & Mismatched char   & Mismatched string \\
         \hline
         FoundChar      &  valid            & -                 \\
         FoundString    &   -               & valid             \\
         CharSet        &  valid            & -                 \\
         Str            &   -               & valid             \\
         Inverted       &  valid            & -                 \\
         \hline
      \end{tabular}
  \end{center}
\end{table}

\subsection{EdpgMismatchedToken}
The §EdpgMismatchedToken§ exception is thrown by the parser when
it is looking for a token, but finds a different one on the input
token stream than expected. It defines the following properties in
addition to those of §EdpgException§.
\begin{alltt}
   FoundToken   : IdpgToken;
   TokenSet     : TdpgByteSet;
   Inverted     : boolean;
\end{alltt}
The §FoundToken§ property contains the token the parser received from the
lexer. The §TokenSet§ property contains the vaules the parser expected to
get. The §Inverted§ property is set only if the exception came from a
§MatchNot(...)§ operation. In this case, the §TokenSet§ property contains the
values the parser must §not§ get.

\subsection{EdpgSemantic}
This exception is thrown by a validating semantic predicate. It
defines the following property in addition to those of
§EdpgException§.
\begin{alltt}
   Assert : string;
\end{alltt}
The §Assert§ property contains the validating expression that caused the
exception.