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copied!<p>Think about your lexer from the perspective of preconditions and postconditions. Here are two ways to design a lexer :</p> <ol> <li><p>A lexer is a device which takes in a well-formed input string and outputs a lexical analysis. The input string is required to be well-formed; if it is not then the lexical analyzer fails with an exception.</p></li> <li><p>A lexer is a device which takes an input string and produces a lexical analysis. Lexical analysis includes identification of and analysis of error conditions.</p></li> </ol> <p>Do you <em>know</em> that the string you're going to pass in is correct, and you want an exception in the exceptional circumstance that it is incorrect? Or do you <em>not know</em> that the string is correct, and you wish the lexer to determine that?</p> <p>In the former case, throw an exception. In the latter case, make the error analysis part of the output of the lexer.</p> <p>Basically what I'm saying is <a href="http://ericlippert.com/2008/09/10/vexing-exceptions/" rel="nofollow">do not produce vexing exceptions</a>. Vexing exceptions are extremely irritating. If the caller of your code is calling it to <em>determine</em> whether the string is correct then they don't want that information in the form of an exception, they want it in the form of a data object that represents the error analysis. If the purpose of the code is to produce an error analysis then <em>produce such an analysis as the normal operation of the method</em>. Do not embed the analysis in an exception and make someone trap the exception in order to get it.</p>

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