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copied!<p>In general <strong>the more complex the error, the more complex the code for asserting the error</strong>. Lexers and parsers are fairly simple (in practice), and therefore catch simple errors.</p> <ol> <li><strong>Lexers</strong> catch invalid sequences of characters supposed to make up a <strong>token</strong></li> <li><strong>syntax analysis</strong> tools such as Bison/Yacc catch invalid sequences of tokens that make up <strong>syntax</strong> and statements.</li> </ol> <p><strong>Complex</strong> errors usually happen elsewhere at runtime or in various translations at compile-time. Examples might include referencing a function/method that doesn't exist. Closure scopes / binding, object and reference identifiers, argument validity, overloading, and tons of other language dependent stuff.</p> <p>Anything outside that very narrow token/syntax scope is (or should be handled) well outside those tools in AST analysis or intermediate code generation.</p> <p>Consider:</p> <pre><code>a.b(); ab(); </code></pre> <p>Both <em>should</em> make it through the lexer/parser for an object oriented language where both statements are valid. <strong>is there an error?</strong></p> <ol> <li><p>Your compiler might assert this at <strong>compile time</strong> if the language is a fairly static one and identifiers can be resolved at compile time. </p></li> <li><p>You might replace both statements with ID resolution code to be run at <strong>runtime</strong> to produce a runtime error rather than a compiler error.</p></li> </ol> <p>The differences between runtime and compile-time resolution and semantics can be subtle and widely different from language to language.</p> <p>Errors are usually caught when they can be <strong>known to be errors</strong> and where you have the most information. This varies between languages and implementations.</p>

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