宏包含一个循环

Question

So I'm trying to make a lexical analyzer for scheme and when I run JFlex to convert the lever.flex file I get an error similar to this one for example:

Reading "lexer.flex"

Macro definition contains a cycle.
1 error, 0 warnings.

the macro it's referring to is this one:

definition = {variable_definition} 
            | {syntax_definition} 
            | \(begin {definition}*\) 
            | \(let-syntax \({syntax_binding}*\){definition}*\) 
            | \(letrec-syntax \({syntax_binding}*\){definition}*\)

all of the macros defined here have been implemented but fro some reason I can't get rid of this error and I don't know why its happening.

Answer 1

A lex/flex/JFlex style "definition" is a macro expansion, as that error message indicates. Recursive macro expansions are impossible, since macro expansion is not conditional; trying to expand

definition = ... \(begin {definition}*\) ...

will result in an infinitely long regular expression.

Do not mistake a lexical analyser for a general-purpose parser. A lexical analyser does no more than split an input into individual tokens (or "lexemes"), using regular expressions to identify each token. Tokens do not have structure (at least for the purposes of parsing); once a token is identified, it is a single indivisible object. If you find yourself writing lexical descriptions which match structured text, you have almost certainly pushed the lexical analysis beyond its limits.

Parsers use an algorithm which does allow recursive descriptions (but which has very limited forward lookahead) and which can create a recursive description of the input (such as a parse tree).