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copied!<p>I don't think that there's an option in ocamlyacc to do what you want automatically, so let me try to provide below a through description of what could be done to handle syntactic errors and have more useful messages. Maybe it is not what you asked.</p> <p>Errors must actually be separated in lexical and parse errors, depending on which stage of the parsing process the error happens in.</p> <ul> <li>In <code>mll</code> files, a <code>Failure</code> exception will be raised in case of unexpected patterns</li> <li>in <code>mly</code> files, it's a <code>Parsing.Parse_error</code> exception which will be generated</li> </ul> <p>So you have several solutions:</p> <ul> <li>let the lexer and parser code raise their exceptions, and catch them in the code calling them</li> <li>implement the specific cases of errors in the either of them with <ul> <li>a catch all rule for the lexer (or some more specific patterns if necessary)</li> <li>using the <code>error</code> special terminal in the parser rules to catch errors in specific places</li> </ul></li> </ul> <p>In any case, you will have to make functions to get information about the position of the error in the source. <code>Lexing</code> and <code>Parsing</code> both use a <code>location</code> record, defined in <code>Lexing</code>, with the following fields:</p> <ul> <li>pos_fname : the name of the file currently processed</li> <li>pos_lnum : the line number in the file</li> <li>pos_bol : the character number <em>from the start of the file</em> at the beginning of the line</li> <li>pos_cnum : the character number at the current position </li> </ul> <p>The <code>lexbuf</code> variable used by the lexer has two values like that to track the current token being lexed (<code>lexeme_start_p</code> and <code>lexeme_curr_p</code> in <code>Lexing</code> let you access these data). And the parser has four to track the current symbol (or non-terminal) about to be synthetized, and the current rule items, which can be retrieved with <code>Parsing</code> functions (<code>rhs_start_pos</code> and <code>rhs_end_pos</code>, as well as <code>symbol_start_pos</code> and <code>symbol_end_pos</code>). </p> <p>Here's a few functions to generate more detailed exceptions:</p> <pre><code>exception LexErr of string exception ParseErr of string let error msg start finish = Printf.sprintf "(line %d: char %d..%d): %s" start.pos_lnum (start.pos_cnum -start.pos_bol) (finish.pos_cnum - finish.pos_bol) msg let lex_error lexbuf = raise ( LexErr (error (lexeme lexbuf) (lexeme_start_p lexbuf) (lexeme_end_p lexbuf))) let parse_error msg nterm = raise ( ParseErr (error msg (rhs_start_p nterm) (rhs_end_p nterm))) </code></pre> <p>and some basic use case:</p> <p>parser: %token ERR</p> <pre><code>/* ... */ wsorword: WS { $1 } | WORD { $1 } | error { parse_error "wsorword" 1; ERR "" } /* a token needed for typecheck */ ; </code></pre> <p>lexer:</p> <pre><code>rule lexer = parse (* ... *) (* catch all pattern *) | _ { lex_error lexbuf } </code></pre> <p>All that would be left to do is to modify your top level function to catch the exceptions and process them.</p> <p>Finally, for debugging purposes, there is a <code>set_trace</code> function available in <code>Parsing</code> which enable the display messages of the state machine used by the parsing engine: it traces all the internal state changes of the automaton.</p>

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